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Chamberlain/Liftmaster garage door openers have a button which is connected to the opener with two wires. One wire is ground, the other 9600 baud serial UART data with 12v logic. This line also powers the button circuit.

I am working on a project to be able to open and close the door (and turn the light on/off) with an ESP8266. I'm able to read the serial data using a voltage divider per this diagram.

The question is how do I build a circuit to connect the TX line so that I can also also transmit data to the same wire?

Or is there another approach I should be taking?

enter image description here

Paul Wieland
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  • @jsotola Here is my [original question](https://electronics.stackexchange.com/questions/598717/reverse-engineering-garage-door-opener-signal?noredirect=1#comment1576073_598717) when I was trying to decipher the signal. I have since figured it out and can read the data being sent over the line. Let me know if there is anything else I can add to clarify... – Paul Wieland Mar 10 '22 at 00:36
  • @jsotola Doesn't work that way with Security+2.0 openers. You need to send an encrypted rolling code to the opener. – Paul Wieland Mar 10 '22 at 01:05
  • @jsotola If I wanted to do that I wouldn't have reverse engineered the rolling code algorithm ;) – Paul Wieland Mar 10 '22 at 01:10
  • just an observation ... the first screenshot in your previous question shows data from two different devices ... the first and second bursts have different voltage levels ... the second burst is probably from the main unit – jsotola Mar 10 '22 at 01:25
  • my guess is the the button module connects the power/logic line to ground, possibly through a dropping resistor ... you could experiment by placing a 10 k resistor across the wire pair to see how much the voltage gets pulled down – jsotola Mar 10 '22 at 01:30
  • yes, since then I have figured out that the button sends a command to the opener, the opener responds, and the button sends a final command (hence the different voltage levels). This is only for the door open command. The light on command is sent and no response is given. – Paul Wieland Mar 10 '22 at 01:31
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    Usually this kind of bus will have a npn bjt like a 2N2222 or BC548 to pull it low. This also inverts the signal so you’d need an inverter to put it right way up. The esp uart might be able to do that for you - read the tech manual. Have a look at the button device to see what circuitry it has. – Kartman Mar 10 '22 at 01:38
  • Thats correct - the button circuit pulls the data line low. A 2.2k resistor across the line drops the voltage to 160mv. – Paul Wieland Mar 10 '22 at 01:38

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