How does a garage door motor wall control on a two wire circuit determine between door control and light control?

I am trying to recreate the garage door wall switch (making a custom assembly that has two buttons to toggle the door and toggle the light). The first thing that I am attempting to understand is the voltage being passed from the motor, to the wall switch and then back to the motor.

While looking at different articles and other places on the internet I discovered that the COM port on the motor passed power down to the wall controller and when the door button was pressed closed the circuit for a brief amount of time to make the motor move the door. To control the light, I thought that the same voltage was passed down through the same wire but instead a lower voltage was passed back up (making the switch more of a voltage divider). There is only one wire going back into the motor meaning it has to be something of a varying voltage to control the door and light.

When using a Fluke Multimeter I was getting a jumping reading from the wire connected to the COM port on the back of the wall control around 0.750 VAC. (I didn't like that it wasn't giving me a constant reading, originally I thought the meter was broke but put it into a standard outlet and got a solid 120 volts). When pressing the door control the voltage on the wire that went back up to the motor read 0.750 VAC. I figured that jumping the circuit would toggle the door, which it did. This made me think that the door opening was a simple instance of closing the circuit for about half a second but made me unsure of what the true voltage was.

The second challenge was how to tell the motor to turn the light on/off through that same wire. A few articles I read stated that the wall control acted as a voltage divider but not by how much or if the motor needed said amount of voltage for how long. When measuring with the light switch pressed down I got readings jumping around 0.350 VAC, but once again nothing concrete.

Here are my questions from all of this, and I am mainly asking those who have tried something like this before or know the common circuitry of these products.

How many volts are sent through the COM port on the motor to the wall controller?

How many volts are returned to the motor from the controller to open the door?

How many volts are returned to the motor from the controller to toggle the light?

I have a Genie Pro Max system, but from what I can tell and the fact that manufactures sell universal door switches I think most systems operate around the same circuitry.

Thank you in advance for your help.

• I've not seen this documented anywhere but it should be a simple matter to use a voltmeter on the wires between the opener control board and the control switch and measure what's going on when the different functions are selected. I will add that the ones I've worked with only have a single function that activates the motor. It's a simple open/close switch. They must be doing something different with the light activation. Oct 16 '20 at 14:57
• Perhaps you should try measuring the current in the wire with open/close vs. light on/off. It's possible that they use a resistor for the light and the current is below the threshold needed to trigger the motor. Oct 16 '20 at 15:00
• check the resistance between the wire terminals when each switch is actuated. One standard method is putting a known voltage across the circuit and then measuring the current using a shunt resistor in series Oct 16 '20 at 15:53
• You may have better luck if you provide details about the exact opener make/model, and switch. A voltmeter may not be sufficient to comprehend the protocol. A modern opener could easily use some variation of a serial data connection, with the "jumping" you notice corresponding to a voltmeter's inability to accurate describe the modulating voltage that such an interface would use. You might need an oscilloscope to correctly characterize the interface. Even if not, the oscilloscope would tell you exactly what's going on electrically. Oct 16 '20 at 17:00
• Hope this helps: diy.stackexchange.com/questions/146259/…
– Rodo
Oct 16 '20 at 18:13

2 Answers

I haven't played with a Genie unit, but I spent some time with an oscilloscope and a Chamberlain/Lift Master unit several years ago. I found it has two modes of operation. One is the simple contact closure -- just short the two wires together and the motor will operate. The other mode is digital. The two wires carry power to the wall control, but they also carry a serial data communication protocol between the motor and the control. The only way to control the advanced features, such as turning the light on and off, is to use one of the wall controls with the digital interface.

As I recall, I also found that the two modes can't be used together. For example, you can't put the fancy wall control in one place in the garage and put a simple doorbell type switch in another place. When the simple switch shorts the wires the door operator goes into "dumb switch" mode and the fancy wall control ceases working (until the door operator is power cycled).

I couldn't say for sure whether your Genie works the same as that Chamberlain did, but I would not be surprised if it does. In any case, you wouldn't get far with discovering this nor reverse-engineering it armed with only a volt meter, unfortunately.

Methinks you are tilting at windmills here.

Modern garage door openers are tiny computers with a set series of programmed responses, not dumb robots with simple voltage controls. If you want an overhead light controlled by a wall switch, you'll get much better results installing an overhead light and a wall switch for it than trying to hack your garage door opener into turning on its light without opening or closing the garage door.

Among other reasons, if you do manage to hack your opener and it then goes on to malfunction and hurt/kill someone, (as they still do from time to time, which is one reason that they have become so highly computerized) the manufacturer will be off the hook and you will be on the hook for responsibility because you have modified the opener.