Garage door safety circuits

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Mikey

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A neighbor had me over to fix his garage door -- it would go back and forth from full open to down-a-little and back to open. It looked like a safety circuit issue. His door has two such circuits -- a photocell sensor across the lower opening, and a pneumatic switch on the bottom of the door. According to the diagrams, each of these circuits is NO, then go to closed when they activate. I tried disconnecting them altogether, making the circuits permanently open, but the problem persisted. I eventually found a small wirenut had worked loose from one of the wires on the pneumatic switch, permanently opening the circuit. When I reconnected the wire, everything worked OK.

Why?

That was a commercial roll-up door. On my own door, I have a Sears system with a similar photocell sensor thing across the door opening, connected to the controller by a 2-wire cable. There appears to be no easy way to disable this system -- neither a NO nor a NC hookup will allow the door to operate. Somehow, the system knows whether the photocells are connected are not. Anybody know how this works?
 

Bob NH

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1. Check the manual. It will sometimes give hints based on symptoms.

2. Send and receive elements of the photocell may be misaligned or dirty.
 

Jadnashua

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The photocells draw current...it's fairly easy to monitor that. It could be that the voltage either has a one-shot, or just changes when the beam is broken.
 

Mikey

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The photocells draw current...it's fairly easy to monitor that. It could be that the voltage either has a one-shot, or just changes when the beam is broken.
That makes sense; my Sears (Chamberlain) unit has a bunch of solid-state logic on the board that would allow that. I can measure that and see if I might be able to simulate its presence with a suitable resistor in its place.

The neighbor's unit, however, is all relay logic -- no steenking ICs or anything fancy. Looks like it was built by the Russians, and will continue to operate during EMP warfare. The photocell sensor is optional, and the door operates OK without it. The pneumatic switch is standard, is a pure mechanical NO microswitch triggered by a diaphragm, and generates an effective one-shot closure because of air leakage. But the door won't operate without it connected. Right now I'm mystified, but the door works. Next time it breaks I'll dig deeper.
 

Billy_Bob

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These are intentionally designed to not be bypassed. The circuits are looking for a specific "resistance" or sometimes a specific "capacitance".

So neither open or closed will work, it is between these and a specific resistance usually. Might try a 1k ohm resistor or a 500 ohm resistor which you can get at an electronic store. These are common values to other similar circuits (non-garage door opener).

Also you can try searching for the specifc brand of opener on google.com and along with the brand name include the words photocell and resistor or photocell and capacitor.
 

Mikey

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I've got resistance & capacitance decade boxes kicking around somewhere, but my suspicion is that resistance is enough -- I doubt they'd spend the money to measure anything more complicated, although polarization could enter into the works as well. It's a good rainy day project, but the way things are going down here, it'll be a long time before we get a rainy day :(.
 
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