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I'm trying to connect a relay/switch to a Garage door(Lift Master). I know that there are two wires to trigger the garage door. I'm not sure how it works exactly. I'm trying to control the garage door remotely (greater than 1 mile).

How do you trigger the door to open with the wires only?

How do you trigger the door to close with the wires only?

Is there any risks to shorting the two wires together?

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My opener manual has instructions on what wires to short to open/close it (they provide this as a way to test whether its the remote or the logic board that is broken). Have you checked the manual? –  Steven May 17 '12 at 17:55
    
Do you know the model number? –  Tester101 May 17 '12 at 17:58
    
haven't looked through it yet, I have probed the back of the garage door to check the voltages. I am just not sure if it is shorting the voltage to ground to trigger the garage door. Are there a lot of different ways to trigger a garage door? I don't have the model off the top of my head, but I'll add it tonight –  Ashitakalax May 17 '12 at 17:59
    
Careful applying current (making contact with the multimeter could do this if one of the terminals is energized) - it might be a dry contact in which case applying current might damage the controller. –  Steven May 17 '12 at 18:27
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up vote 3 down vote accepted

Do you have a button switch on the wall to open - close? There are usually two wires going to that. I don't think it will hurt to short those wires momentarily to test it. If it did, Lift Master would get a lot of returns because I'm certain many people have shorted the wired pair upon installation. My Craftsman has two wires going to a multiple switch panel. I can open - close the door, turn on-off the opener light, and lock the remote controls. These functions are controlled by adding a resistance to the wired pair. each control has a different resistance. The open-close is just a short. the unit remembers what direction it went the time before. IE: if you short once to open the door, then short again will close the door.

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+1 When I was a kid, I took some speaker wire and a random push button switch to make my own door opener (I lived over the garage and didn't feel like walking down to the door to let people in). The button just closed the circuit when pressed. –  BMitch May 17 '12 at 21:51
    
SteveR, I do have the button switch. cool I'll try shorting it. thanks –  Ashitakalax May 17 '12 at 22:12
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I believe the light and lock buttons introduce a diode (with opposite polarity for each function) rather than using different resistances. –  TomG May 18 '12 at 0:44
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@TomG- I believe mine is current sensing, by using different resistances. I remember monitoring the voltage on the pair and it was 18 VDC static. With an accessory button pushed it would drop a volt or two, polarity stayed the same. I did not take apart the control panel so I can't say for sure. If it were diode steering as you suggest, I would think the pair would be polarity sensitive showing only the FV of the diode. If I get time I will take the switch panel apart to see for sure:) –  SteveR May 18 '12 at 13:03
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