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How does the wall button know, if there’s only two wires (+ and -), what’s a door open button press, and a light on press? Is it a voltage transmission difference?

  • With more than 1 button and only 2 wires it is simple to have a small resistance value that is different say 100 ohms on button #1 and 150 ohms on #2 for some systems this is how it is done. High end controls use addressable modules but this takes more wires as the device requires both communications and power , but even this can be done on 2 wires but is more expensive. – Ed Beal Sep 2 '18 at 16:31
  • Liftmaster/Chamberlain actually sends data over the 2 conductors. My wall station (LM888, an upgrade from the original) not only controls door and light, but also knows the door status and functions as an IOT device allowing internet control, which also shows door status. The magic is proprietary but I imagine RS485 which is slow but very good for process control where the same limited bits of data need bi-direction transfer. – Tyson Sep 2 '18 at 16:46
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You got me curious so I went to check my garage door opener. I have a Genie pro stealth. It shows 3.8vdc at the terminals where the switch(s) goes. The voltage at the terminals drops to 2.1vdc if I press the light switch and 0v if I press the open/close switch. So ... it uses a resistor plus the switch for the light and a just the switch for the open/close.

Technically you could say it is a voltage divider.

Sorry but I was too lazy to check for a signal with a scope. I know the sensors use a square wave with a dc offset (In my case). I built an alarm for the sensors a while back.

Hope that helps.

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