Finally some results! But lets start with what I have done so far. Since my initial experiments with my scanner were somewhat successful and I had my new QFH antenna made and ready to go I just needed my Minikits weather satellite receiver to arrive and then I could try getting some decent images.

The kit arrived after just over a week and ended up costing me $NZ125 including shipping. This is far cheaper than any other kit I could find online, all of which you have to pay in Euros or US dollars.

The Minikits kit.

The kit is rather nice. It comes with a tinned double sided plate through hole circuit board. Several little bags of components including all the coils and ICs you need. It also came with an F connector to solder onto the board but I have chosen to use BNC connectors instead. It also comes with the instructions and the description of how the circuit works.

The kit uses an MC13135P receiver chip which is a complete FM receiver on a chip basically. There is also a TL072 op-amp and an MC34119 audio amplifier. All of the components are standard leaded types except for a surface mount NTC thermistor used as part of the automatic frequency control circuit to account for temperature variations.

You do need a few other things so I added the speaker, the signal meter, a AFC switch (the kit comes with a jumper) and the audio out connector. I also used full sized pots for the volume and squelch control since I will mount them off the board. The kit comes with trimmers to use on the board though. The kit runs off a 12 volt supply which is regulated down to 5 volts on the board for most of the circuit to use. I made a small power supply board with a 12 volt regulator on it and powered that from an old power supply from a set of old computer speakers. The used speaker I am recycling came from the same set of old speakers. The signal meter was a nice find. The first place I look for odd electronic stuff, Surplustronics, didn’t have anything so I headed up the road to the top of Queen Street to a real electronics junk shop. I have no idea what it’s called but it’s been there for years now. It’s cram packed with junk. Most of it is useless but the odd interesting thing here and there. Nothing is sorted. It’s all just in piles. I dug about and came up with this nice signal meter with a 0-5 scale on it. He wanted $3 for it. I had $2.20 in change (good thing I hadn’t bought a Mars bar that day) so he said close enough. It was unused and still in the bag!

Assembled kit set. The red thing is a plastic tuning tool.

Building the kit is easy. The instructions say it takes about four hours but I did it in two. All those years of working as an electronics tech to pay for my way through university came in handy! Once you have built it you need to set it up.

The way the receiver works is the incoming signal, which is around 137MHz, first goes though three bandpass filters. These eliminate any extra signals from getting through and just pass the weather satellite transmission frequencies that we want to receive. This is important in some places where there are strong pager frequencies nearby (people still use pagers – probably more reliable that Telecom’s XT network).

We then want to take the incoming signal and convert it down to a much lower frequency. This is called an intermediate frequency.  An intermediate frequency, or IF, is used because it is easier to work with and  filter on the lower frequency and because if a receiver converts all the frequencies you want to receive down to the same IF then all your circuitry from that point on can be optimised for just that IF frequency. This receiver uses the same IF most FM receivers use, 10.7MHz.

To get to the IF the receiver chip uses a mixer. The mixer takes the received signal and mixes it with another signal that we generate on the board. This signal needs to be 10.7MHz higher than the signal we wish to receiver. The mixer takes our signal and subtracts from it the received signal to get the 10.7MHz IF signal. This process is called Hetrodyning (which is probably what my friend Fing tells his friends he is doing when he comes to visit Auckland and we all go to the pub for lunch). The on board frequency is generated by a thing called a voltage controlled oscillator or VCO.

In this circuit the VCO is controlled by a coil, which gives a rough setting, and the tuning resistors, to fine tune to for each frequency.  The kit uses 25 turn trimmers for tuning. Each channel uses one trimmer. The kit only comes with one but you can simply add more and use a multi-pole switch to switch in the appropriate ones. For now I just have the one which I adjust for each different frequency when I need to.

The signal passes though various stages in the chip and eventually gets spat out as audio. There are two audio paths. One is to the audio amplifier chip which is controlled by the squelch circuit. The other is a line output to the PC sound card.

To set the receiver up you need to adjust the VCO to be 10.7MHz above the frequency you want to receive. The receiver needs to receive from 137MHz to 138MHz approximately. The tuning resistor needs to tune across that range.  If you set the tuning resistor to half way in it’s travel that would correspond to 137.500MHz. We therefore want the VCO to be oscillating at 148.200MHZ (148.2 – 10.7 = 137.5).

To set the VCO frequency you need to adjust one of the coils on the board. These coils have adjustable ferrite slugs in them that you screw up and down to set the frequency. They are fairly delicate and also metal near them will upset the frequency so you need to use a red plastic tool for adjusting these. This was available as an extra from Minikits cheaply.

The best way to know you have the right frequency is with a frequency counter. Unfortunately I don’t have one. So the next best thing is to use my scanner again. Because the coil is oscillating it is also radiating radio waves. If you hold the scanner close to the circuit board set to 148.200MHz when you adjust the coil and it hits that frequency you’ll hear it on the scanner.

With the VCO set this way you know the receiver should be receiving across the 137 to 138MHz frequencies over the 25 turn range of the trimmer tuning resistors.

Next the instructions say to attach a RF signal generator set to 137.500MHz to the antenna input and adjust the filters and output demodulator for the cleanest signal. Again, I don’t have a signal generator so instead I had to wait until I knew a satellite was overhead. With the antenna sitting out on the front deck railing I used the WXtoImg software to know when to listen and on which frequency. I used my scanner to be able to hear when the signal was there.  I then connected the antenna to the receiver and tuned the receiver to the right frequency and quickly adjusted the rest of the circuit while I still had a signal. This can take some time given the low number of passes per day and the short time the passes last. You have about 15 minutes in which to tune things.

To do this tuning I used my ocsilloscope on the audio output to see how clean the signal looked as well as by watching the signal meter and adjust it to give the highest signal as suggested in the instructions. This worked very well.

Initial setup and testing.

One interesting part of the circuit is the automatic frequency control, or AFC. This is quite clever and it will allow the receiver to account for the Doppler shift in the satellite as it passes towards then away from the receiver by adjusting the VCO.

You will see that the EeePC is displaying an image here but I found that it can’t actually decode the pictures properly. I think this is because I was using the mic input instead a a line in (which the EeePC doesn’t have) and so I wasn’t getting a proper signal.

The next thing to do was transfer everything over to my main PC and to put the antenna in a better position (on top of the guttering on the edge of my roof for now). With everything set up like that I finally got my first real picture.

The first image!

And this is what I got. The noise and blank part at the bottom of the image was me trying to tune things in as the satellite passed (from south to north). The horizontal lines were when I was adjusting things. In between though you can see a good image.

With more tweaking and tuning I am now getting nicer images. I am just using the basic settings in the software for now. I think there are a number of different settings to show you different things but I haven’t played with them must so far. These images are using MCIR map colour IR and an Equidistant Cylindrical  settings.

First good image.

I still need to manually tune hence the missing bottom to the image but this was a imagine received from NOAA 18 on 137.9125 MHz (that’s why so much of the bottom is missing – I had to turn that 25 turn trimmer a lot) at about 2pm.

The same satellite did another pass about 1 hour 40 minutes later and I got the image below.

Second pass by the same satellite showing Australia.

You can see how the satellite follows different paths on different orbits so you end up receiving differing pictures. Using some (free) software such as WXTrack you can actually see the path the satellite will take overhead. If the satellite is very low on the horizon you will get noisy images. The antenna needs a clear line to the satellite so obstructions in the way will degrade the signal.

So, what’s next? Well, I am still only using the one tuning resistor so for each different satellite frequency I need to re-tune the receiver. This won’t do as a long term solution. Instead of the idea that is given in the kit of using 5 tuning resistors all switched in via a rotary switch I want to try something a bit more automatic.

The kit does provide connection points for adding a phase locked loop or PLL for short. This would allow scanning across the whole frequency range then have it stop once it detects the appropriate signals. You can buy suitable experimenters PLLs that are micro controlled but these are rather expensive and I am trying to do this on the cheap!

I considered using a micro controller to switch between five or so pre-tuned frequencies. Because there are only a few satellites transmitting on fixed frequencies (the status of the satellites and information about them is available on the NOAA POES status site) you don’t actually need to scan the whole range. You can just step through fixed channels looking for a signal.  I could use a micro to switch each channel in for a brief period, say 2 or 3 seconds, and if a suitable signal was detected stay on that channel until the signal drops out again.

Switching in a set of preset tuning resistors can be done using a bilateral switching chip. A micro could control the switching. I then though that’s perhaps over complicating things so perhaps I can do it entirely using normal ICs.

My current plan is to use a 555 timer providing a slow clock pulse. These I feed into a 4017 decade counter (one of the first ICs I ever experimented with so a favourite). The outputs of the counter feed into a 4066 bilateral switch to switch in each tuning resistor in turn. They also turn on various LEDs to show what channel is currently selected. When a signal is detected the counter stops on that signal until the whole signal is received. This part I haven’t worked out yet. I was thinking of using the squelch signal but I don’t think this will work too well. Instead I might make a tone detector to listen for the tone you hear when the signal is coming in. The APT system used to transmit the images used a 2400Hz sub-carrier it should be possible to detect easily.

Progress so far.

So far I have got the clock, counter and switching working. You can see those on the breadboard in the picture above. I need to start on the tone detector. Perhaps using a LM567. My backup plan if I can’t get that working is to use an Arduino perhaps. Everyone loves Arduino!

Also I need a more permanent antenna installation. Perhaps a small mast mounted on the roof of my garage so I can put it there and get a nice, clear view of the sky.

And of course once done everything needs to be put into some nice housing.

Will update this with progress soon.

Now I will describe how I made a much neater QFH antenna to receive the satellite signals. My wooden one worked fine but it was not waterproof or particularly pretty or accurately made. So I decided to make a waterproof, prettier and more accurate one.

Again I followed the directions given here: http://www.g4ilo.com/qfh.html

I started off with the raw materials which I got from one of my most frequented places – Bunnings Warehouse. There are probably cheaper places to buy this stuff but Bunnings is convenient. I bought a 1m length of 32mm diamater PVC pipe and three lengths of black 15mm diameter Enduroflex pipe. I have no idea what that stuff is used for. Water piping I think. The PVC pipe cost NZ$15.51 and the black pipe was $11.16 for the three lengths although I only ended up using two of them. That black pipe incidentally is the perfect size for the barrel for a home made Nerf gun firing suction darts (a project for another day)!

I also bought some RG-58 (50 ohm) co-axial cable from Jaycar rather than the RG-6U I used on the wooden antenna. The The problem I had with the RG-6U is that the braiding isn’t copper but rather aluminium so it was impossible for me to solder the connection easily. The RG-58 is somewhat thinner diameter than the RG-6U which does mean the antenna is more easily bent out of shape but as my antenna will only be used occasionally it should be fine for now. The 15 metres of co-ax was $28.50.I cut the PVC down to about 800mm long and marked it up ready to drill the holes for the cross arms. I was very careful with the dimensions on this antenna. An easy way to mark around the pipe is to wrap a piece of printer paper around it ensuring the paper is square to the pipe by making it overlap itself perfectly as it wraps over itself. You can then mark off the place the sheet overlaps. You unroll the paper and the distance form the edge of the paper to the mark you a made is exactly the circumference of the pipe. It is then easy to mark off the quarter and half way points on the flat paper. Then you wrap the paper around the pipe once more to transfer those marks to it.

I then put the pipe on a nice flat surface and use the ended of a long, straight piece of wood to transfer the lines down the entire length of the pipe.

After carefully marking the pipe and measuring it several times I drilled the holes to hold the cross members. I first drilled a pilot hole with a 3mm bit then I used the spade bit to drill out a larger hole (sorry for the badly focused photo).

To make the holes a nice, tight fit I drilled slightly undersized then used a tapered reamer to enlarge the holes to the exact right diameter.

Once done you end up with mast finished. The positioning of the middle set of holes holes isn’t very critical as those arms just hold the ends of the cables in the correct place. Obviously you want them close to the mid point though.

I next carefully cut the six cross arms from the black pipe. Note the two different sizes for the two different sized loops in the antenna. I trimmed the ends square in the lathe (because I could, not for any technical reason) then carefully marked and drilled the holes in the ends. Looking at the plans you can see these holes need to be a fixed width apart to ensure the loops of the antenna are the correct size. They also need to be drilled at 45 degree angles to the mast so the cable has the correct spiral to it.

To drill the holes so they are at 45 degrees to the mast when installed you actually need to drill the holes in each cross member 90 degrees apart, one at either end of the support. Since the drill press drills down vertically all I did was drill one hole then push a piece of aluminium tube through it temporarily. Then, turning the tube around I simply adjusted it in the vice until the aluminium tube was parallel to my drill table. With it parallel when I drilled the vertical hole I could be sure the two were 90 degrees apart. When you push the crossmember through the mast you simply twist it so that one of the holes is at 45 degrees to the mast and the other will be aligned at 45 degrees too. You do have to twist it the right way though so that the cable is spiraling down anti-clockwise when viewed from above.

Because I might want to experiment with this antenna a little I didn’t want to glue the crossmembers fully in place yet so I am temporarily using some pieces of split ribbed tubing to stop the crossmembers moving in the mast.

In the top and bottom sets of holes I needed to file slots so that the cable could pass through into the mast. The holes for the arms at the ends of the mast I drilled so that the centre of the co-ax cable would be sitting at the exact right measurement given in the plans with the cable running along the outside of the crossmember , not inside it. Running the cable inside the crossmembers would of course be neater. But there is then no way to make sure the cable lies flat in the correct place. Does this make much difference? I have no idea! But the dimensions are given down to fractions of a mm accuracy so I decided to get things as close as possible.

I started putting the crossmembers in place and also soldering the connections. Shown here is the bottom most connection. That is the feed wire soldered to the shield of the bottom of one of the loops. I insulated the connection with waterproof tape then carefully pushed it into the mast and then pushed the crossmember fully home.

Similarly at the top I carefully soldered the connection inside the top of the mast once the loops and the crossmembers were all in place.  Notice the anti-clockwise direction of the spirals. I used lots of cable ties to hold the co-ax down flat across the crossmembers. Eventually I will remove the split tube and just glue the crossmembers in place once I know everything is correctly positioned.

To finish off the antenna I put a PVC cap over the top of the mast and I also made a quick and dirty stand from a scrap piece of wood and another off-cut of pipe that the mast happens to slide into perfectly. Eventually my plan is to hang it from a line suspended above my garage roof where it will have a clear view of the sky. The antenna might be connected to a line going around a pulley so I can hook the antenna onto the line and pull it up into position but still bring it inside if the weather is too bad. Either that or I will make a third, more robust version to permanently mount on my house roof.

With the antenna on it’s temporary stand I was able to move it about outside to see how well it would work. It works well when it has a clear line to the satellite down to very low on the horizon. The signal is very easily blocked though so building and even trees will cause you to lose the signal.

Now I just need my receiver kit to arrive so I can build that and really see how this works!

Update: The kit arrived! Read about it in Part 3.

I should point out that I am not a radio amateur, or HAM, as they are known. I am just an electronics tinkerer. Anyone with good, general electronics skills can do this.

Bender showing off my scanner.

One thing you can do is pick up pager frequencies. I pick up 157.950MHz easily at home which is a Telecom pager frequency. It sounds like a random collection of bleeps and bloops – R2D2 with his knickers in a twist. Apparently with a simple mod you can break out the unfiltered audio (called the discriminator output) in the scanner and then, using some software called PDW, decode the pager signals.

Even though the mod is easy to do (if you are electronically inclined) looking at other peoples pager messages strikes me as being terribly uninteresting, kind of like having to overhear other peoples one sided phone calls on the bus, so I haven’t tried this modification myself.

One other thing I could try (after doing the discriminator mod above) is receiving AIS signals. These are the identification signals used by ships. Since I work in the city and it’s not too far down to the port I should really try seeing if I can receive them. Perhaps the Northern Steamship Company pub might be a good place to try!

The frequencies used are 161.9750 for AIS1 and 162.0250 for AIS2. Apparently you can then feed the signals into a sound card on a PC and using some software like Shipplotter you can track the movements of shipping in your area. Something to try another day.

One thing I did want to try, being interested in all things to do with space and space technology, is receiving weather satellite transmissions. Now, before I start I should say that you cannot actually decode the weather satellite transmission using this particular scanner. You can HEAR the signals but you cannot generate any kind of useful pictures with them. Well, I have been unable to. But given that you can hear them was enough to start me off on this project.

Lets start with some background first.  Up there, in space (well, low earth orbit 850km up), there are some weather satellites. Both the Americans and the Russians have them up there, the NOAA and the METEOR satellites respectively. There are others of course but the NOAA are the ones you can easily receive signals from so they are the ones I am listening to. These satellites are continuously transmitting signals down to earth using a system called APT. The Wikipedia Automatic Picture Transmission article gives you all the background you need but here are the basics.

The NOAA satellites are in a polar orbit around the earth and as they orbit they are continuously scanning a line beneath them 3000km wide and transmitting this line back to earth. Being in a polar orbit means each satellite will pass over the entire planet as the earth rotates beneath them. As a satellite comes up over the horizon and passes overhead a receiver on the ground can start receiving these lines, decode them in sequence and build up a picture. Due to their low altitude these satellites orbit rapidly and each satellite will make several passes over your location each day.

The signals are transmitted on a group of frequencies around 137MHz.  The actual picture received  contains two views of the earth below the satellite – one in visible wavelengths and another in infrared. A line is scanned twice every second and each line contains the image data as well as non image data. This system has been used since the 1960s so it is quite remarkable it is still in use today. You can get up to date information about the NOAA weather satellites from this status page.

To receive the pictures you need three things: a receiver, an antenna and a PC (with a sound card and some software to do the decoding).

First, the receiver. As I mentioned above my particular scanner (or indeed most scanners) isn’t suited for doing more than hearing the signals but it is enough to let me work on the second piece, the antenna.

The antenna obviously needs to pick up the 137MHz signal the satellites transmit on. It also needs a wide receiving pattern as it has to be able to pick up the signal from horizon to horizon. The signal from the satellites is circularly polarised so the antenna must be designed in a particular way to receive the signal. Two suitable antenna designs are the turnstile antenna or the quadrifilar helix antenna (QFH).

Looking around online I came across several references to the QFH antennas with very good instructions on how to make them specifically for receiving weather satellite pictures. The design I decided to use was from here: http://www.g4ilo.com/qfh.html

Because I didn’t know if I was going to be able to receive anything I made my first QFH using materials I had on hand, namely wooden dowels. I made an antenna using the instructions and dimensions given above. I used RG-6U coaxial cable as that was easy for me to get quickly in order to try things out. I first tried using the antenna indoors until a friend pointed out that the metal roof would block the signal (a duh moment) so instead I simply took the antenna outside and hand held it in order to try it out. To my amazement it worked! I could hear, albeit with a lot of noise and static, the satellites transmitting their signals.

It’s worth mentioning that because the satellites are constantly orbiting you need to know exactly when a satellite will be passing over. This brings us neatly to the third thing you require, a PC with a sound card and some software running on it.

The software I am using is called WXtoImg and it is available to download and use for free (you can also pay to register it for additional features). Interestingly it seems to be written here in New Zealand. This software does two things. It decodes the received audio signal and formats that into usable images and it also tells you when (and where) the next satellite will appear.

Another useful piece of free software is WXTrack by David Taylor. This software lets you see exactly where satellites are over the earth at any given time.

Both of these programs require you to download up to date satellite tracking information called Keplers but this is well explained and easy to set up.

Since I could now hear the signal and I had downloaded and installed the appropriate software I rigged up a temporary support to hold the antenna up outside my window were it would have a (reasonably) clear view of the sky and attempted to feed the signals into my PC.

My quick and dirty wooden QFH antenna and dodgy mounting method.

The results were not good (it wooden work)! This I expected and the problem isn’t the antenna itself but the limitations of my scanner.

There are various problems with using scanners for receiving weather satellites. Other weather satellite sites explain the problems better than me but to summarise the main problem is due to bandwidth. Now bandwidth, as I understand it (and I could be wrong!), when talking about receiving weather satellite transmissions is the difference between the lowest frequency and the highest frequency that you need to be receiving signals over to get the full signal. A scanner like mine is designed for receiving voice signals. Voice signals don’t need a very wide frequency range so we say the scanner has a narrow bandwidth. The bandwidth of a scanner like mine is about 15kHz so the scanner will filter out any signal outside that range around the main frequency.

The satellites transmit their signal over a much wider range spread around the main frequency, i.e. is has a wider bandwidth. To accurately receive the entire weather satellite signal you need a receiver with a 30kHz to 50 kHz bandwidth. So in effect my scanner is blocking out part of the signal. We basically lose information from that lost signal so the software can’t build a good picture from it.

Other issues with a normal scanner are sensitivity, how strongly it picks up the weak satellite signal, and selectivity, how well it picks up only the frequency you want. Another, more subtle problem, is the doppler shift caused by the movement of the satellite as it passes overhead. This causes the frequency to shift as the satellite passes overhead.

All these issues mean my little scanner isn’t up to the job. To really demonstrate this here is what a properly recorded signal of a weather satellite should sound like: http://n8imo.com/APT/images/N14.WAV (from http://n8imo.com/wefax4.html).

What I get is this: http://www.asciimation.co.nz/misc/06250521.wav\

Image produced by the scanner.

My friend Dave wanted to see a picture of what the scanner produced. This is the best that I ever managed to get. Mainly noise. With some vague picture in the middle as the satellite passed right overhead.

So when people say a normal scanner won’t work, they are right!

The next thing to do is sort out a proper receiver. There are a number of options here. You can buy a scanner that does have the correct bandwidth but these are very expensive. Or you can buy a ready made receiver just for receiving weather satellites. These are also expensive. Finally you have kit receivers you buy and assemble yourself. Being an electronics tinkerer this is what I was after. There are various kits on offer but I chose the Minikits weather satellite receiver.

This receiver is very reasonably priced, should do everything I want and the company is more or less local (well, Australia) so I was able to pay in Australian dollars rather than US dollars or Euros which tend to work against someone earning NZ dollars. Shipping was also cheaper and faster.

I ordered the kit last week and it should arrive any day now so I will be building that as soon as possible.

In the mean time, having proved that a QFH antenna would indeed work I set about making a more accurate, more weather proof version.

Will describe that in part 2 here.

Miele S5210.

It’s a Miele S5210. It’s a very, very good vacuum cleaner. Well, it was until the point that it broke! Actually I was somewhat responsible for that. For a little while now I have been restoring a car (OK, almost 6 years but I am nearly done – www.asciimaton.co.nz/pics). After weeks of filling and sanding of filler I finally sent it off to the panel beaters to be painted. This left me with a garage full of sanding dust. I swept up what I could the used the vacuum to clean up the rest. Unfortunately your average house vacuum isn’t really designed to handle lots of very, very fine filler dust. I ended up clogging it up and the motor stopped running smoothly and instead started stuttering. I needed to take the vacuum cleaner apart to clean it and remove all the dust so it would run properly again. What follows is the procedure I used to take the vacuum cleaner apart. I imagine the process is probably similar to other Miele vacuum cleaner models.

I was inspired to do this page after I found the following page online for a different Miele model (a Miele s300): http://www.sannerud.com/house/miele.html

You don’t need many tools to take the vacuum cleaner apart. Just a Torx T20 driver and a small flat screwdriver to push on the plastic clips that holds the parts together. All the screws used to hold it together are the same. The Torx bit shown here is actually a tamper proof Torx bit with a hole in the middle but it works fine on the screws. Click on any of the pictures for a larger view.

Torx screw and T20 Torx driver bit.

First unplug the vacuum cleaner and remove the bag and all the filters. The small silver honeycomb filter just clips in place. Remove this so you can then remove the lid.

The clips holding the lid on.

The lid just slides onto the hinges and two small square clips hold it in place as shown above. Depress the small squares and then slide the lid off the hinges.

Large honeycomb filter.

The large honeycomb filter is also just clipped in place. Carefully push back the two clips shown circled above and the filter should come out.

Rear plastic piece.

The plastic piece at the rear between the two buttons is also just held in place by clips. Brute force will remove this. Just yank it upwards and it will pop loose.

Removing the speed selector.

With the rear cover removed you should see two screws holding the speed selector part in place. Remove these.

Clips holding speed selector in place.

With the two screws removed the speed selector can be removed by pushing in the small clips that hold the front of it in place and lifting it off. This piece just contains the knob that controls the speed. The knob has a stalk that sticks down underneath it that fits into a selector switch on the electronics board.

Top cover screws.

The top cover is held in place with four screws shown, two at the front and two down deep holes in front of each button. Undo these then the top cover should lift off.

Top cover removed.

Electronics board plug.

With the top cover removed you can see the electronics board. It’s pretty simple really and doesn’t have much on it. The board should be free to pull off now. The only thing holding it in place is the connector shown above. Simply unplug this connector and the board will lift off.

Inner cover.

With the electronics board removed you should be able to see the screws holding the inner cover in place. There are three at the back and one in the centre as shown above. Remove all these screws.

Clips holding inner cover.

As well as the four screws there is a clip either side of the cover on the sides of the vacuum. You can simply pop these apart by hand then the inner cover should lift off. There is a small rubber hose that goes between the cover and the cord retractor mechanism which you also need to disconnect from the cover (it will probably just fall off anyway).

Inner cover removed.

With the inner cover removed you can now remove the motor (which has a foam pad over it) and the cord retracting mechanism. The only trick here is to unplug the connector that joins the two together.

Motor connector.

The motor and cord retractor will simply lift out. I gave everything a good cleaning to get all the dust out. I used my air compressor to blow it all clean. With all the dust removed from the motor I sprayed it’s brushes with electrical contact cleaner. I didn’t go as far as dismantling the motor itself (March 2010 – OK, I did eventually See below!).

Contact cleaner for the brushes.

The brushes are either side of the motor and I simply sprayed cleaner into the hole at back of them.

After letting the contact cleaner dry I put the motor, cord retractor and electronics boards temporarily back in place the tested the vacuum. You need to be VERY careful doing this as nothing is properly attached and there are exposed mains connections that will bit you it you touch them (don’t ask how I know). Also the vacuum motor is extremely loud when not encased in plastic!

Once everything was cleaned and working again reassembling the vacuum cleaner is basically the revers of taking it apart. Make sure you reattach the small rubber hose and also make sure the cord and plug are free and don’t get caught when screwing all the pieces of the case back together.

After my cleaning and spraying the motor with contact cleaner the vacuum is working nicely again. I know now I should really get a nice shop vac for cleaning the garage and leave the Miele for purely domestic duties!

I can really recommend these vacuums. They are good value for money and very powerful. And now, having seen how they look inside, I can say they are very nice quality too.

Update March 2010.

I have had a few people comment that this page was useful so I decided to post the second part of my vacuum cleaning story in case people find this further detail helpful.

My cleaned up vacuum worked well for a little while but then the motor started stuttering again until eventually it stopped running altogether. Another tear down was in order. This time right down to the motor itself. Again the nice design of the Miele made this an easy job to tackle.

First you need to remove the motor from the vacuum as described above. Then carefully tap off the metal shield on the end of the motor exposing the blower fan. Next remove the nut holding the blower fan in place. Now it was a few months ago that I did this but from memory the nut is a reverse threaded one, i.e. turn it clockwise to undo it. This allows you to pull off the aluminium blower and the flat spacer washer.

Nut and blower removed.

Next you can lift out the two carbon motor brushes. These are simply held in with spade connectors so you can just pull them straight out. In the picture below you can see the female spade socket on the face of the stator housing.

One brush already removed. The other still in place.

The brushes are nice and long so should last a very long time.You can see the long male spade connector on the bottom of the brass housing. You can also see how despite my previous cleaning this brush is still covered in sanding dust. If I didn’t mention it above I should say don’t sand filler off a car (http://asciimation.co.nz/pics/page18.html) then use this vacuum to collect the dust!

Nice brush. Boom! Boom!

With the brushes removed (and cleaned up with electrical cleaner) you can remove the stator. There is a metal spring clip that holds it in place. If you press this down the stator should then slide out.

Spring clip holding stator down.

The electronic controller is attached to the stator and will come put with it. You can see the top of a TO220 type device sticking out of the top of the plastic housing. We get to that in a minute. The inside of the stator and housing were both covered in the sanding dust so I cleaned these up as well.

Stator removed. Note the electronics are still attached.

Next you can carefully pull out the rotor. This has bearings on each end and the lower bearing is a press fit into the housing. You need to carefully pull this out. The rotor will come out in one piece. Be careful not to lose the little flat spring washer though.

Rotor removed.

The observant of you will probably have noticed one of the problems with the motor. The commutator on the end of the rotor, that ring of copper strips the brushes rub against, are filthy and scored. To fix this I carefully mounted the rotor in my mini-lathe. You only need to grip it very lightly in the three jaw chuck. I made sure it was running true and turned it on. I then used some fine wet and dry sandpaper folded into a long strip to carefully sand down the commutator.

Rotor mounted in lathe.

I didn’t try to get the commutator perfectly smooth as I didn’t want to sand too much away. It still has a few small scores around it but it doesn’t need to be perfect. The deep scoring is actually where the edges of the brushes are in contact with the commutator so the brush is in contact with smooth copper on most of it’s face.

Commutator after sanding.

Next I cleaned up the aluminium blower which was quite clogged with dust. A bit of electrical cleaner and a poke around the fins with a long cable tie did the trick.

After doing all this and cleaning everything to remove all the dust I reassembled the motor. Since I had given it a good clean with electrical cleaner I left the motor on top of my dark coloured garage roof to make sure it was fully dry before trying to run it again. I wanted to make sure all the cleaner had evaporated out of the motor and windings.

Unfortunately after putting it back in the vacuum cleaner and reassembling everything (with a little Loctite around the rotor bearing where it pressed into the housing) the motor was still dead! I had to take it apart again. This time I removed the motor, opened that up and removed the motor electronics. Again thanks to nice design this module just unclips since it is held in place with spade connectors.

Motor electronics.

The electronics on the motor are incredible simple. Basically it’s just a TRIAC and what I think is a thermal cutout device.

TRIAC and thermal cutout thingy?

About now the problem was pretty obvious. This TRIAC was burned out! A close inspection and a little prodding showed that TRIAC was burned out. Two of the legs were not even connected to the body anymore.

Well there’s your problem!

I am not sure why this happened. I am guessing a combination of a badly connecting and arcing commutator and a motor clogged with sanding dust ended up cooking things. The TRIAC itself is a T2550h 600T which is a 25 amp TRIAC. These are available in NZ but not from the easy places like Jaycar or Dick Head Smith (who don’t really do electronics anymore despite their name). You can probably get them from the bigger suppliers like Farnell or RS but they would cost a bomb and you might not be able to buy just one. So I looked on eBay and found someone in the UK sells them for just a couple of quid. I ordered one of them.

This is the data sheet for this particular part: http://www.datasheetcatalog.org/datasheet/stmicroelectronics/6697.pdf

Once that arrived a week or so later it was a simple matter to unsolder the dead part and solder in the new TRIAC. I reassbmbled everything again (after this many time apart you get good at this bit) and finally everything was working again!

All that was actually done several months ago and the vacuum cleaner is still working happily now. I know these things aren’t supposed to be customer serviceable but it is nice to see that they are engineered in a way that means a customer with the right skills can successfully get in there and fix things.

My house and separate garage.

One thing that always worries me about being in the garage is making sure no one goes into the house (and vice-versa). Locking the doors each time I go from one to the other quickly becomes annoying so instead I decided to make a little door alarm. Basically, when I am in the garage, an alarm sounds if someone opens the front door of the house and, when I am in the house, an alarm sounds when someone opens the door to the garage.

To build this I used two small boxes, some magnetic reed switches, some switches and LEDs (with current limiting resistors), a six volt power supply and four wires of the data cable I had run between the house and garage when I had it built.

Circuit diagram.

The circuit is really simple. Everything on the left is in the house, everything on the right in the garage. Basically each little box contains a toggle switch, an LED to indicate the alarm is on, a buzzer and a magnetic reed switch attached to the door.

The alarm works by sounding a buzzer when the opposite door is open. So if you are in the house and have the alarm switched on the buzzer in the house sounds if someone opens the garage door. If you are in the garage and the alarm is on the buzzer in there sounds if the house door is opened.  The toggle switch on each box turns off it’s buzzer (say you are in the house and you know the garage door is open and don’t want it buzzing). The LED just shows the alarm is on and will buzz when the opposite door is open. The reed switches are wired so that the switch is open when the magnet is in place. The reed switch itself is attached to the door frame and the magnets are on the door. When the door opens the switch closes and the alarm sounds.

The power supply is an old 6 volt wall plug I had lying about. To completely turn off the the alarm I simply pull out the power supply lead.

Garage alarm box showing wiring and reed switch.

Garage alarm box (the power was off so the LED isn’t glowing).

House alarm box next to normal home alarm.

Now I can work in either the garage or house and leave the doors unlocked (but closed) and know that if anyone opens a door I will hear it where ever I am.

http://www.asciimation.co.nz/bb/2009/10/18/a-rocking-horse-for-sophie

With time moving on I had to really move to get the rocking horse done. After protecting the foam with the tin foil I applied layers of fibreglass and polyester resin. I built up a nice, thick skin of glass all over the horse.

Layers of fibreglass to make a shell.

After the fibreglass shell had hardened I gave it  a quick sanding just to remove any sharp bits then set about adding in Horseome Paul’s super secret feature – a hidden hiding place where Sophie can keep things. The cavity was made form a piece of PVC piping. A hole was cut in the horse and the pipe gently inserted.

Gentle insertion of secret hiding place.

Once the pipe was fully inserted the excess was trimmed off with a cutting disc on a Dremel.

Cutting off the excess.

Once the cavity was done I then covered the body in several layers of 5mm thick upholstery foam. This was glued on using Ados F2 spray contact adhesive. I put extra layers on the head for some extra padding in case of face plants by the young rider!

Covered in foam padding.

Next I had to figure out how to cover him in fur. A visit to Spotlight (a material and craft super shop – kind of like Bunnings for girls) provided me with some great fake fur and some pink frilly stuff for the mane and tail.  I roped in a friend to help with the sewing (at the cost of making her dinner). I wrapped the horse in plastic from an old plastic bag then wrapped that in layers of brown packing tape. The idea is to make up a tape shell that you can then cut off with scissors and cut to form flat pattern pieces that will exactly match the three dimensional shape you are trying to cover. I was going to buy a sewing machine and learn to do all this myself but with only a few weeks before Christmas I ran out of time.

Wrapped in plastic and tape.

So, with time running short my friend Dani came over to help with the pattern making and sewing. One look at Horseome Paul though and we decided it was going to be trickier than we though. Dani makes quilts which are flat. A three dimensional horse was a much trickier problem. We decided I should try to find someone with a bit more experience to help. Still made her dinner though

As it happened there is an alterations place near to where I work (errr, used to work as I was made redundant just before Christmas). I popped in there and talked to a woman who didn’t speak much English but after must gesturing and explaining she realised what I wanted to do and said she could help. I took the horse and material along to her and she sewed on the covering for me in a very short time and for an extremely reasonable price! She also made a bridle and sewed on buttons for eyes and even gave him a little charm necklace. He has a zip running down his back so the fur can be removed if needed.

Horsey head!

In the mean time I completed the wooded base. I originally painted it pink but in the end repainted it white and the pink become overwhelming! I attached some cardboard stickers I got from Spotlight to give some detail and to put Sophie’s name on the base. They were supposed to be self adhesive but I found I needed glue to get them to stick. Finally the body was simply bolted down to the base and held with large fender washers and Nylock nuts.

So here is the final product!

Horseome Paul is complete!

I ran out of time to make a saddle but I think he doesn’t really need one anyway. Christmas day is today so we shall see what young Sophie things in a few hours or so! Hopefully it doesn’t scare her!

Oh, that secret hiding place. Well, you can get to it by simply unzipping the zip under the tail a little. I am not sure my sister is going to approve…

And finally, 6 months later, an actual picture of Sophie on her horse. Yes, it is still bigger than she is!

Sophie aged 2 years 5 months (or so). Horse aged 6 months.

I decided to make the base rocker from wood and build a horse approximating shape on top of that for Sophie to sit on. The horse is made from polystyrene with pool noodle legs which is then covered in fibreglass to form a tough but light shell. The shell will be covered in foam then given a horse skin coat (that makes me think of ‘It rubs the lotion on its skin…’).

First I needed some polystyrene blocks to carve up into the basic shape. A sensible person would get some solid blocks from some sort of specialist polystyrene shop or something. I decided instead to glue together a few sheets of underfloor insulation foam since that was easy for me to get from Bunnings (where I seem to end up several times a month for some reason). I glued the sheets together using Ados F2 spray contact adhesive. The legs are made from sections of pool noodle.

Foam blocks and pool noodle legs.

Once I had the foam blocks I needed a way to cut them so I threw together a hot wire cutter. This is simply a wooden frame with a handle, kind of like a hacksaw frame. Stretched across the opening of the frame is a piece of nichrome wire. I use small screw eyes to attach it and at one end have a spring to keep the wire under tension. If you try to attach the wire to the screws eyes directly when it heats up it will expand and go slightly slack. The spring ensures it is always taut. A wire runs from either end of the nichrome and attaches to my car battery charger. At 12 volts the wire draws about 1 and 3/4 amps of current.

Hot wire cutter.

Next I drew something vaguely horse shaped on some cardboard then cut that out and transferred the shape onto the blocks.

Template and shape draw on foam.

Once I had a basic outline I used the hot wire cutter to remove anything that didn’t belong on a horse. Then after the basic shape was cut out I used a long knife to cut holes in the base of the body to push the legs into. I then used a 40 grit sanding disc on an angle grinder to sand the basic shape smooth. You need a gentle touch doing this as the disc will eat into the foam rapidly.

Basic shape cut and sanded.

Since the horse was taking shape I decided he needed a name. The horse is named in honour of an awesome ex-workmate of mine, Awesome Paul (so called because he is so awesome). The horse is of therefore known as Horseome Paul. Not many people can say they have a rocking horse named after them. Not sure many people would want to…

Once the shape was done I cut and pushed wooden dowels down the hole in the middle of the pool noodle legs. These extend right into the body to make the legs nice and strong. I drilled the ends of the dowels and inserted in some long 8mm bolts with the heads removed to turn them into threaded studs. These studs will attach Horseome Paul to the base. They are held in with extra strength Araldite epoxy.

8mm bolts glued into wooded dowels in the legs.

Next I started work on the base. First I drew a rocker shape onto some 6mm thick MDF to make a template. I used some scrap cardboard and a steel rule curved against a large steel block to form half of a smooth curve in the rocker. Once that curve was drawn on the cardboard I cut that out and transferred the first half of the curve to the MDF. I then turned the template over to draw the same curve on the other side of the rocker. The MDF template serves two purposes. First it gave me something to trace around onto two pieces of wood to give me a line to cut out with the jigsaw to form the rockers. Then I was able to screw it to the roughly cut wood and use the MDF as a guide on a ball race router bit to make the rocker edges square and identical. When tracing around the template I use a wide Sharpie marker to give a nice thick line. I then cut around the outside of the line meaning the rough rocker is about 1mm over-sized all around.

Template for the rockers.

The routing takes some explaining. What I do, and this is probably dodgy as hell, is bolt my router to a flat piece of MDF to serve as a base. I then screwed that to my temporary work bench I was using (a bit of chipboard sitting on a panelbeaters frame). This holds the router nice and firmly upside down with the bit facing upwards (Note to self :  I really must make a proper router table someday). I then screw the MDF rocker template firmly to the roughly cut rocker making sure it is lined up so the 1mm extra is even all around. Then I run rocker against the router bit (making sure as you push the shape around that the tool cuts INTO the wood) with the bearing on the tip of the bit running against the edge of the MDF template. Because the bearing is following the edge of the template the wood will be cut to the exact shape of it too.

Routing around the edge of the template.

Once routed out the two rockers are almost identical. Just to be absolutely certain I screwed the two pieces together then sanded the edges together to make them smooth.

Random orbit sanding the two rockers together.

With the two rockers made I then cut two small supports that are screwed to the inside of the rockers. The tops of these supports were cut with a curve on the top face to match the curve on the rockers. Two wooden dowels attach each end of the rockers together. These are screwed in place then small wooden plugs were glued over the screw holes. Wooden slats were then screwed across the rockers, the outer two being slightly thicker than the middle ones so the legs can bolt down onto them.

Base completed.

While Horseome Paul is off his rocker(!) a temporary MDF board drilled to match the holes in the rocker base keep the legs fixed in the correct position so the bolts will line up once he is fibreglassed. Without this the legs might shift position during the glassing process then not fit through the holes in the base.

Temporary leg support.

One final detail was making a handle to go right through the head. The handle is just a piece of dowel. It does need to be removable though so later on I can fit the horse suit over the body. I found a piece of PVC pipe the dowel slides nicely into. I pushed the pipe through the head and made sure it was flush on each side. When the body is done the dowel will slide into this and be glued into place (or perhaps wrapped in tape and just held by a friction fit).

Handle and PVC pipe being inserted. Went right through! Didn’t hit the brain!

Finally before fibreglassing I needed to cover the polystyrene foam to protect it from being eaten by the polyester resin I was going to use. To do this I glued tin foil all over the body to form a barrier to the resin. By the way the polystyrene ears  are just there to remind me that I am making a horse and not a sheep/dog/cow. The proper ears will be sewn on at the end along with a mane, tail, bridle and saddle.

Ooohhhh, shiny…

Finally fibreglassing could begin. To be continued soon.

Part two is now available here:

http://www.asciimation.co.nz/bb/2009/12/25/finishing-sophies-rocking-horse-before-christmas-just

The ladybird I was going to copy.

My idea was to make the bookends with ladybirds like the one shown above but with LEDs in the wings to make them light up at night. Originally I wanted these to be solar powered so that they would charge during the day and glow at night in the same was as solar garden lights. I did experiment with that a little but found there isn’t enough light indoors to reliably make this work. For now I am using a small power supply instead but I might revisit the solar idea later.

I started by making the base of the bookends. These are made from MDF and quarter round moldings. These were pretty quick and easy to knock up. I then painted them with a lot of coats of gloss white paint. On the base I glued some sheet rubber to stop the bookend from sliding about.

The wooden bases made from MDF.

After I made the bases I started on the ladybirds themselves. These were made from more MDF cut out on a band saw into the basic shape. The wings are made from white acrylic sheet cut to the appropriate shape. These were sanded to make them diffuse the light from the LEDs I was going to use to make them light up. Each wing is held to the wooden base by two screws. There are two holes in the bodies that allow the LEDs to be inserted and shine through the wings.

The ladybirds and their acrylic wings.

Once the bodies were made (and painted pink) I glued small pieces of felt onto the face. Two of these cover the screws that hold the wings on. On the back I inserted the  LEDs and their resistors. These are held in place with hot glue. In the center of the body I epoxied a threaded brass stud which is used to secure the back. The backs contain a power socket and are made from the lids of small plastic screw jars.

The back of the ladybird showing the LEDs and threaded stud.

The ladybirds are attached to the bases with two short lengths of brass rod covered in clear plastic tubing which spaces the body out from the base. These were epoxied in place.

The power socket in the backs are attached via short wires then the back is put in place over the  brass stud and held on with a nut and a dome nut. Using two nuts allows me tighten them together to lock them in place so wht backs won’t come undone.

    The back in place.

With the backs on it was a simple matter to make up some power cords. I use a small 5 volt  wall plug power supply. The LEDs are only drawing 20mA each so the power supply doesn’t need to be very big at all and the whole thing uses very little power. The LEDs make the wings glow very nicely. It is a bit tricky to capture in a photograph though!

And these are the finished bookends! Along with some books every little girl should read. Sophie is a little young now but she’ll grow into them!

The skin is completely dried and is very tough and all it’s internals seem to have dried to nothing. I want to somehow remove all the flesh and just leave the bones (don’t ask why, it’s just one of those things you have to do).

What though is the best way to remove all the old tissue? Since my poor pet newts died I have nothing to feed the white worms they lived off to. I was wondering if I put the worms in a container, soaked the rat in water then put it in with the worms would they do the work of eating away all the dead flesh?

I know there are special beetles used for just such a purpose but goodness knows if they are available in NZ at all. Either that or some sort of chemical treatment?

 

 

The finished SV Chronometer.

Time. We are all obsessed by it and it catches up with us all in the end. Especially women who don’t moisturise their elbows properly! I have my own little obsession with time. I often have a stop watch in my pocket, just so I can time things, I wear a 24 hour analogue watch (they are cheap on eBay!) and I get very, very annoyed that all the clocks in my house all show different times! You can set them all to the same time but eventually they all drift.

For example the exact time now is 17:34. The clock/weather station beside my PC shows 17:32. My wristwatch shows 17:35. My mobile phone shows 17:32. The living room clock shows 17:37. The microwave shows 17:35. The video shows –:–. And my Spanked Woman Rotating Legs clock shows 8:55. I suspect her batteries have gone flat! Speaking of time you might need a bit to read all of this. It is a somewhat rambling discussion of how this thing works!

The drifting problem is something I just have to live with. I have no control over the actual timepieces so they are all going to drift over time and I know I have to keep resetting them. They question is when I do reset them what do I reset them to? What is the correct time? This little project is to solve that problem.

I decided to build a stand-alone clock that I can use as a master time source to set all my other timepieces to. All it has to do is show me the correct time but I wanted it to be interesting to look at and interesting to use. A plastic box with an LCD display on it would be effective but not nearly as much fun! I decided to build some kind of mantel clock and I wanted to base my design on an old art deco Hammond clock. The time is displayed on a screen inside the box which is only visible when the user peers though the eye pieces and cranks a handle on the side of the casing. The name of the device, SV Chronometer, comes from the fact that it is using GPS satellites to receive the time. In GPS language the satellites are referred to as Space Vehicles.

There are various ways other to get accurate time. Radio clocks are one method. Network Time Protocol (NTP) is another. Accurate time signals, called PIPS,  are often broadcast over radio stations. And there are speaking clocks available as well. The Measurement Standards Laboratory has a list of the various time standards available here in New Zealand.

To be honest my PC would probably be the easiest way to get an accurate time signal. Microsoft provide simple(!) instructions for setting up Windows XP to use NTP. As my computer is always on and always connected to the Internet I should always have an accurate time source. Instructions on the easy way to do this in NZ are available from MSL. Another easy way to get accurate time in Auckland at least is the bus stops. I suspect these might be using GPS time also as they are always in sync with my watch!

I should point out here that when I say accurate I am meaning only to within a second or so. I am using the time to set physical clocks some of which can only be set to within a minute or so accuracy anyway. If all my clocks are accurate to within a minute I am happy.

Once upon a time I worked for Navman,  a company that makes GPS navigation devices. Having played with GPS there and knowing a bit about how it all works (for a REALLY good explanation have a look at the Trimble GPS site) I knew you can get a very accurate time signal from a simple GPS module. I happened to have such a module from my time at Navman. Actually as it turns out that one was a lame duck but I managed to find an exact  replacement cheaply on TradeMe.

  GPS Module.

This particular GPS module is pretty simple. It has an onboard antenna as well as a connector to attach an external antenna. It runs off 5 volts and it has a very simple serial interface. I think most GPS modules available these days work in a similar way. The serial interface on this module outputs NMEA 0183 sentences. Basically the module is spitting out ASCII text that is giving you the GPS information. There are different sentences containing various pieces of information. Things like position, heading, speed, satellite information and, of course, time. Not all receivers give out all of the different sentences but time is one of the most basic so I think all GPS modules should have this available. Some pieces of information are available in multiple messages. Here is a complete list of the NMEA messages available.

The first thing I did was connect up my hand-held Garmin eTrek GPS unit to my PC using it’s serial cable. I was easily able to get that working and see the raw data in HyperTerminal. Next I tried connecting my Navman module up to the serial port on my PC to see what sentences it was giving me. I used a MAX232 chip between the module and the serial port to do the signal level conversion. I was rewarded with a nice stream of NMEA sentences.It provides the GPRMC, GPGGA, GPGSA and GPGSV messages.

One thing about the Navman module is that it runs at a very high serial speed, 57600bps. This was to make things interesting later on. From the NMEA messages I was getting I could see I could pull out the data I wanted. I can use the GPRMC message to get th time and the GPGSA message to get the fix accuracy.

This is an example of the actual GPS raw data the module outputs:

$GPRMC,051735.997,V,36000.0000,N,72000.0000,E,0.000000,,041108,,*34
$GPGGA,051736.997,0000.0000,N,00000.0000,E,0,00,50.0,0.0,M,26.2,M,0.0,0000*41
$GPGSA,A,1,,,,,,,,,,,,,50.0,50.0,50.0*05
$GPGSV,3,1,09,16,71,075,,20,52,293,00,23,48,221,,31,30,122,*70
$GPGSV,3,2,09,01,21,068,,13,20,230,,25,17,271,,06,14,037,*7E
$GPGSV,3,3,09,03,12,025,*47

Once I knew the information was there the question was how to get at it inside a stand-alone clock. I decided I could use a PIC16F628 microprocessor to receive the NMEA messages from the GPS module then parse them to get the bits of data I wanted. The PIC is well suited for such a job as it has a built in serial port, it’s own memory and it can easily be programmed (using assembler) to drive an LCD module which is what I decided to use to display the actual time.

Circuit and GPS module during testing.

Basically how it works is the GPS module sends out a group of NMEA messages every second. The PIC receives the serial data and writes it all into an internal buffer. I maintain an index to my position in the buffer. NMEA sentences can vary in length and the PIC only has limited memory but I worked out that the information I need is always in the first 13 bytes of the message. I made my buffer long enough to fit just the bytes I need. As the bytes of the sentence are received from the serial port they are put into the buffer until the buffer is full. If I receieve a ‘$’ character I restart at the beginning of the buffer as I know this is the start of a NMEA sentence. When the buffer is full everything else is then dropped.

As the bytes come in the PIC looks out for a ‘*’ character in the sentence. This happens regardless of whether the characters were written to the buffer or not. This character indicates  the check sum at the end of the NMEA message. I don’t use the checksum but I do use this character to indicate  I can examine what I have in the buffer from the message I have just received.

By examining the first few characters in the buffer I can determine what sentence I received. If it is one I am interested in I then copy the relevant bytes from the buffer into storage variables. . In my case I simply want the time, which is always UTC time, and the fix information, which tells me how accurate my satellite fix is. 

Because the Navman GPS module is running at a fast serial rate I found I didn’t have much time (measured in clock cycles) for parsing the messages. Luckily there was just enough time to extract out the time and fix data. It might have been nice to get the date also but that would cause other problem later on when I need to work out what time zone I am in.

Even though the serial data itself is transmitted very quickly the burst of data only happens every second. This gives me lots of time to actually manipulate and display the data between bursts once I have it.  To know when I can start displaying data I use one of the PICs internal timers. Timer 1 is set up so that it is reset each time a byte is received from the serial port. As data is received the timer is being continually reset. Once the serial data stops however the timer is left to run and eventually trigger an interrupt and I set about displaying the data. This will happen once a second after the data burst from the module.

Originally I was going to have the clock display as much information as I could. GPS could give me the location, heading, speed over ground, etc but really none of this information is very useful on a mantel clock that doesn’t move! In the end I decided to display only a minimum of information. The clock will show me the current UTC time, the time zone corrected time and the fix information. Time is displayed on a  2 line LCD display. The fix information is shown on the analogue meter on the front of the clock. When you don’t have a GPS fix the module will still output time but it is its own internal time. This is probably accurate enough for my purposes during the periods when the clock doesn’t have a real GPS fix.

Assembled on back plate.

Displaying the time on the display makes use of some simple LCD routines I wrote for the PIC. The UTC time is displayed almost exactly as it was received from the GPS module only formatted in hh:mm:ss 24 hour format. The local time is displayed by adding on a  time zone correction to the UTC time.

That was actually an interesting problem. My clock won’t work for all time zones because I had to simplify how I handle the time zones in the PIC and also due to my almost non-existent user interface. I don’t actually store a time zone as such. All I do is store a value from 0 t0 23 in the PIC to represent the hour correction that needs to be added to the UTC time. This is enough to cover most of the time zones in the world as most are at hour offsets from UTC. There are a few odd time zones in the world with half or even quarter hour differences. I figured the chances of me moving to such a place is slim!

To change the time zone I have a simple push button on the back of the clock case. Each push of the button increments the internal value. When it goes above 23 I reset to 0 again. This value is stored in EEPROM on the PIC so even if the power goes off it remembers your time zone when it restarts. To display the local time I simple add on this value to UTC. This is where not displaying a date makes things very easy. Being able to correctly show a local date worked out from a UTC date with a time zone correction is actually quite tricky. You need to take into account if the time zone is a positive or a negative offset then you need to adjust the day appropriately. When you adjust the day you need to account for the start and end of a month and adjust that appropriately. And once you do that you need to account for leap years. It all starts getting complicated and needing look up tables and so on which is a little more than my poor PIC can handle. Also I generally know what day it is anyway! You also have to account for daylight savings time which is another complication if you wish to do it automatically. On my clock you simply change it manually. Hell, it’s only 2 button pushes once a year and 22 button pushes six months later! Or is it 23? I am always off by 1!

The display (showing ‘Estimate’ instead of the UTC time).

The other main parts of the circuit don’t involve the PIC at all. The first is simply the MAX232 chip which I left in circuit between the GPS module and the PIC. On the back of the clock case is a RS232 port so I can connect the clock to any PC and see the raw NMEA messages if I want to. The second is more to do with the aesthetics of the clock which take some explaining.

The outer housing of the clock is obviously inspired (actually shamelessly copied) from the art deco Hammond clock. It is made from MDF which I then paint with thinned down polyurethane as a sealer then I spray ti with gloss black enamel paint. I didn’t want a simple clock face though. That would be far too boring. Instead what I have is a box you have to peer into in order to see the display. The front plate is painted steel and the eyepieces were made from a set of old, broken binoculars I removed the lenses from. To make the display look distant from the user and give the insides some depth there is a periscope system made up of two mirrors inside the casing . The display isn’t actually behind the eye pieces but is instead at the bottom of the casing and you are viewing it via the mirrors.

The inside showing mirror periscope.

To further obfuscate things (and this is where the interactive part comes in) the display isn’t actually visible if you just peer into the box. To actually read the time you need to crank a handle that is fixed to  the side of the housing. I was inspired by old fashioned “what the butler saw”  machines (actually called Mutoscope machines). Originally I played around with a mechanical shutter system where the crank flipped a little metal plate up and out of the way of the display so you could see it. The plate  was ‘bounced” by a cam so that you couldn’t simply turn the handle to one position to lift the shutter out of the way. You had to be actually rotating the crank for the screen to be visible.

The flywheel and switch.

The mechanical system worked but was finicky and also it made a horrible rattling noise! It occurred to me I could achieve the same effect electronically. Since the box is almost light tight and dark inside you have to use an LCD screen with a back light in order to see the display. To simulate the flickering effect I simply turn the back light on and off!  It worked out to be an elegant solution.

The crank handle is held to the case on a bearing carrier (something from something I junked so long ago I can’t remember what it was). I made the handle from a piece of steel suitably cut and drilled welded to an 8mm shaft. The shaft is held in place by a hex nut on the inside of the case. Attached to the inside of the case wall is a micro-switch with a lever on it. The lever runs against the hex nut and as the nut turns the switch is turned on and off six times per revolution. It also makes a great mechanical clicking noise which adds greatly to the whole cranking experience. To make the crank feel balanced I made a heavy lead fly wheel by melting down collected wheel balancing weights (I must admit I have a habit of collecting them off the road) and casting a rough disc. I machined that on the lathe to make a nice, heavy flywheel. One side has the lead hollowed out to help balance the handle and give a nice, smooth turning motion. Machining lead is not really a nice thing to be doing so I made sure I was wearing gloves, face mask and goggles (which you do for any machining of course). By keeping the speed right down I avoided chips flying all around the workshop and when I was done I was able to collect all the stray lead pieces and re-melt them into one lump. The lead is very easy to machine being so soft. The flywheel was then painted black to seal it.

Machining the lead flywheel.

The LCD back light isn’t connected directly to the micro switch. If I did that then you could stop the handle in one of 6 positions and read the display which ruins the interactivity of the whole project. Instead the switch is connected to a small circuit. The circuit uses every electronics hackers favourite friend the 555 timer! The timer is used to send a fixed length pulse via a transistor switch to the back light to turn it on briefly. A largish capacitor is connected across the back light LED to smooth out the pulse so instead of the display harshly flickering on and off it tends to rapidly fade in and out. The 555 is wired up slightly differently to what would be a normal monostable setup though to account for the fact that the trigger pulse can be longer than the timed pulse. Normally the trigger has to be shorter. This addition means stopping the crank in a position where the switch is pressed still only results in one quick pulse rather than the back light staying on.

The meter on the front of the box is used to display the fix information as previously mentioned. The meter will show no fix, a 2D fix or a 3D fix by flicking the meter needle on a 0 – 400 scale. 0 – 100 for no fix, 0 – 200 for a 2D fix and 0 – 300 for a 3D fix. I use the PICs built in PWM module to generate a 25%, 50% or 75% PWM signal that drives the meter to the correct position. If you have no fix the LCD will also display ‘ESTIMATE’ for the UTC time. Since the clock is updated every second between data bursts the meter needle flicks every second also. 

One quirk of the meter is when there is no fix the meter needle doesn’t move properly back to the zero position. This is due to it being 1, old and 2. dropped. Several times! The back of the meter housing was shattered so I had to repair it with a small piece of plastic. I have developed a habit of coming up and tapping the glass on the front of the meter when there is no fix to unstuck it and get it moving properly again. I find this just adds to the character of the clock!

A further addition is a simple LED inside the box which flashes in time to the meter on the front of the box. I used an ultra bright orange LED and this creates a nice, neon like glow that at night is visible flashing deep inside the box when you are lined up just right with the eye pieces.

Back plate.

On the back of the casing is a simple panel with connections for power, the external GPS antenna and the serial port as well as the time zone push button. The power supply has a bridge rectifier and a regulator so you can plug any ac/dc power supply up to about 15 volts or so in to power the clock. You can use any cheap power supply such as an old phone charger or wall power supply. The serial port is wired up to use a straight through cable. The external antenna connection means I can use an antenna on the roof to get a nice 360 degree view of the sky although I find it actually works pretty well inside on the window sill even though the view is limited. An external antenna is required since the actual module is mounted inside the box and it has no view of the sky.

Closing the back showing the internal layout.

And here is a link to a little YouTube video of the SV Chronometer in action. It is very hard to film it on my digital camera in such a way to show exactly how it works but hopefully you get the idea from this.