Can I use this MOSFET on my garage door opener?

April 15, 2024 7:27 AM

Can I use this relay -- which is apparently a dual MOSFET and not just a relay -- to wire into my old garage door opener, in order to simulate a button press? I am confused which wires to put into the four screw blocks, since the wall button itself only has two wires. Should I have bought a simpler module, with just POS, NEG, and SIGNAL wires?

I am comfortable with the soldering, ESPHome, and Home Assistant tasks. But my electronics theory is next to nonexistent. The GDO in question is an old Liftmaster (Chamberlain) unit with the purple Learn button, so it's pre- Security 2.0.

The little module has four ports for the screw blocks labelled "V+ in," "V+ out," "V- in," and "V- out."

I had figured that I would run a red and a white wire into the garage door opener's leftmost two blocks, as shown in this image.

I am stumped how to wire it up with four ports.

posted by wenestvedt to Home & Garden (7 answers total)

Best answer: I'm not positive but I think that isn't the best device for simulating a button press because the board also supplies the power to the controlled device. The key thing to know would be what is the voltage/signal going through the garage door button.

If the voltage across the button wires is between 5 and 36 volts DC you might still be able to use it. I would try connecting the + button wire to V+in, the - button wire to V+out, both V- terminals to ground, and the signal wires to your controller.

That said, I'm also just a hobbiest so take this all with a ton of salt.
posted by being_quiet at 7:50 AM on April 15 [1 favorite]

Best answer: That's not the right part for what you want to do. It's meant for controlling the power supplied to a load (light, motor, etc.), which means yes it does act like a switch, but not in the simple way you need.

If I'm reading the PCB traces correctly (no guarantee), it actually switches the ground/- side of the connection, and the V+ side connects straight through. And all it does is either connect or disconnect the Vout- to ground. Here's how I think it's connected:

Vin+ ------ Vout+

Vin-        Vout-
  |           |
Ground---/ ---+

Where --/ -- is meant to be the switch controlled by the "Signal" inputs.

So it might work if the existing button connects from 5V+ to ground, the garage door opener is acting on a voltage drop on the 5V+ side, and you connect the 5V+ wire from the garage door opener (the red terminal on the opener, I think?) to Vin- (you would ignore the Vin+ and Vout+ entirely). I guess it wouldn't hurt to try if you have the part already, but there are lots of things that could prevent it from working. I give it a 50% chance of working, 45% chance of doing nothing, and 5% chance of destroying your garage door opener in some way I didn't anticipate because I'm just a random person on the internet.

An actual relay, on the other hand, will do exactly the same thing as a button, and so it is just about guaranteed to work.
posted by whatnotever at 9:21 AM on April 15 [2 favorites]

Best answer: As above posters say I think it's not the right thing to use, you would be much better off with a relay board, which will have +5V, GND and INPUT connections on the Arduino side and a COMMON, N.C., and N.O. on the relay side.

By connecting the garage door to the COMMON and N.O. terminals it will connect them together when the relay is switched on - so the arduino just needs to turn it on for half a second or so as if someone is pushing a button.

You can find Arduino relay modules for less than 2 dollars each on Amazon, or there are plenty of esp8266 or esp32 boards with relays on board for ~10-20 USD.
posted by samj at 9:36 AM on April 15 [2 favorites]

Just a note on relay boards (or any boards, really): check what the switch does when you power cycle the board. It can close the contacts during reset/power on and act like a button press when you don't want one, and if you search the internet there are solutions to this, generally involving a moderate sized capacitor to make sure that the board needs to send a long on signal for the switch to close.

I was looking at doing this for a neighbour's gate controller, with a relay, and this was one problem that came up. That said, I got distracted and never finished the project.
posted by How much is that froggie in the window at 11:58 AM on April 15 [1 favorite]

I started heading down a similar research path last year, but eventually realized there were too many uncertainties. Instead I picked up a YoLink Finger... basically a wirelessly controlled motorized actuator that presses the old garage opener button on the wall. It's been working like a charm.
posted by departure lounge at 8:59 AM on April 16 [1 favorite]

Response by poster: Follow-Up: I bought a cheap board from AliExpress with a relay on it, and wired it to an ESP-32CAM microprocessor board. I installed ESPHome on that little guy, and then wired the relay into the garage door opener's contacts.

The Home Assistant software gives me a web front end to manually activate the relay, thereby opening or closing the door. It also lets me write a more complex script that activates the relay, waits a tenth of a second, and then toggles it off. And of course now I can automate things like alerts if the door is open after 10:00pm and much more.

(The original board I had, with two circuits each controlled by a dedicated MOSFET, might get used for some lighting project -- but you were correct that it wasn't what I wanted.)

Thanks again, everyone!!
posted by wenestvedt at 5:20 PM on May 1 [1 favorite]

If you ever want to upgrade to the functionality of the Chamberlain MyQ system, the next step would to be add an open/closed sensor (they appear to use a low power radio tilt switch to actually check if the door is open, but a wired magnetic reed switch would do just fine). They also have a beeper and light so you can't surprise someone by closing the door on them. You have plenty of outputs on your esp32 to add that in the future if it interests you.
posted by How much is that froggie in the window at 9:44 AM on May 4