Entrance gate eletric lock: existing intercom + arduino

Dear all,

at the moment I have a cheap “video intercom system” that serves as doorbell and opens the entrance gate.

This intercom has two cables that go to the eletric lock of the gate. When I press a button on the intercom, I can read a 12vAC voltage on these cables for a second (or less) and the eletric gate opens.
The intercom is powered by an external power source. No 12vac is in input to the intercom.

In the picture attached:

  • on the left, the power source
  • on the right:
    • on the top, a 4 wires connector. These wires go to the external unit (where the doorbell and the camera are)
    • on the bottom, a 2 wires connector. Red and black, these are the wires where I read the 12vac only when the “open door” button is pressed, and just for a few seconds.

The project I would like to realize is: while leaving the existing as is (i.e.: the intercom should keep working as it is now), I would like to be able to add the possibility to open the gate with an Arduino.

Actually, I think I will use a nodemcu, that is a board based on esp8266 12e and compatible with arduino.

The key point to start is: how can I “output” 12vac from an arduino? Should I use a relay? I am not able to find a 12vac power source… can you help me?
What if I use a 12vdc do power up my arduino and then find some converter to get some 12vac?

The second important point is to be able to “switch” from the intercom and the arduino in safe mode. I mean, I would like to avoid that the eletric gate receive the input from the both of them at the same time. But I think I could, again, use two relays to block/allow the input from the intercom. E.g.: the circuit always opens the “channel” between the intercom and the electric gate but when the arduino get the command, switch to its channel to the intercom.

Do you think it is feasable? Do you have any suggestions/idea?

The key point to start is: how can I "output" 12vac from an arduino?

You can't. You can CONTROL an external 12VAC source, using a relay or transistor.


You are looking at this from the wrong end. Forget the intercom system, look at the gate controls. I don't know anything about your specific gate system but I sometimes connect telephone systems to electric gates and door releases and they are all the same, they all have an input to which you connect either a relay contact or a normally open push button. You short the 2 terminals of the input together and that operates the gate or door release. Look inside the control box of the gate system and you should be able to find them. If you can't then search on the internet for the part number of the control box or phone the manufacturer and ask for a connection diagram.

Good luck.

I suspect you hear the gate latch "buzz" during those few seconds when you also read 12vac. This is a simple unit and just outputs the power to drive the gate solenoid release latch on those two wires. Yes you can drive it from the ESP8266 but have a lot of Power issues to deal with. You need 5v or 3.3V for the NodeMCU. You need 12VAC (DC might work depending on the solenoid used in the latch). You were on target in thinking you could use a relay. I would go with DPDT and put the pole on latch, NC contacts to the existing output panel, and just 12vac on the NO contacts. Then when you drive the single relay closed for the few seconds required to release the latch, it will just disconnect the panel from the NC contacts and provide 12VAC directly from the NO contacts. You will need a transistor circuit to power the relay. Plenty of circuits to follow out on the internet for driving the relay with the ESP8266. Since this device is also WIFI you could build a simple web page to open the door, and also provide a simple way to do it from anywhere over the internet. Sounds like a fun project.