London bus destination blind sign. Part 5.

August 16th, 2012

Last night I got the sign to the point where I could test it out. I made two clamps from sections of PVC pipe. These are cut a little shorter than the rollers and are split down their length. They are then put over the rollers. Because of the split I could lift up one edge and wedge it open with something (I used short lengths of plastic tube). Then I could put the edge of a long roll of paper under it and remove the wedges. The paper is then clamped in place. I am testing with a 5m length of cheap wrapping paper. This is much weaker than the bus blind so if it works OK with this the blind will certainly be fine.

IMG_2844_1 IMG_2846_1 Clamping the paper.

I fitted one end to a roller then manually rolled it up then fitted the other end. It is necessary to remove the sprocket on the drive motor to do this otherwise you can’t turn the rollers. The motor uses a worm and gear drive and in those the worm can turn the gear but the gear can’t turn the worm!

Attaching the ends this way means the blind isn’t modified or damaged in any way and also if the roller doesn’t stop before hitting the end of the blind it should just slide out of the clamp without being damaged.

I used a load of clip leads to temporarily wire up part of the drive circuitry. I am using two double pole relays and a double pole switch. Each motor terminal goes through a relay. The relays are wired so normally both motor terminals are connected to ground. When you energise one relay it switches that terminal to the positive voltage and the motor turns. If you turn that relay off again and the opposite one on it then switches the other terminal to positive and the motor turns the opposite direction. If you turn both on both terminals switch to positive so the motor won’t turn.

Using a double pole, centre off switch though I can arrange it so you can only turn one relay on at a time (or have both off). The switch is actually an old military DPDT but I only use half of it.

The final version will also have micro-switches as limit switches to cut off the power when the rolls reach the ends of the blind.

IMG_2848_1 IMG_2850_1 Switching.

Again here is a little film explaining and showing it working.


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