My project idea is to attach "something" to my Intercom and when it receives an electric signal (someone rings the door bell), contact a local web service in a home server and that server sends a push notification to phones.

The part of the project I currently control is the one of the server and the push notification (I'm developer), the part I'm lost is the hardware part.

It seems the ESP8266 looks like a good option for me. But, I need to know how to make it react to the ring signal, and how to make it being powered with the Intercom power source, can you give me some advice?

This is the intercom.

Maybe the SparkFun ESP8266 can be a better option?

Based on Chris Stratton comments, Will it be a better way to have a ESP8266 with is own power supply and track the acustic signal of the Intercom rather than use the internal electric of the Intercom?

If yes, what I need to make my device track the acustic signal?, and, is there any battery source that informs about power life duration? Or the life of the battery is long enough to not worry about it?

Continuing with my initial idea I found this Current Sensor Module ACS712. I understand that this could "bypass" the cable that comes from the portal to the Intercom and that causes the intercom to ring so that when this cable receives power (someone called), could I detect that change?

It seems to me a more precise and simple option than the acoustic detection.

  • Power supply: - Audio: 12 Vac - Video: 18 Vdc no much else in the document, I guess he has to open it up and see what it has. Commented Jul 20, 2017 at 12:03
  • Wiring: - Audio door entry system: 4 common wires + N (1 call wire per home). - Video door entry system: 7 common wires + 1 video coaxial+N (1 call wire per home). Commented Jul 20, 2017 at 12:04
  • In a few hours I can open it and put a pic if you want.
    – Mulflar
    Commented Jul 20, 2017 at 12:20
  • 1
    the link is fine @Mulflar but, it would be better to have the power rating to understand the device better. But hopefully, we have a universal standard for that. Commented Jul 20, 2017 at 12:22
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    You'll likely do best finding a way to acoustically or electrically detect the ring output rather than trying to snoop on the cable. But beware of galvanic connections to the intercom system unless you fully understand its power supply. Your system will probably need its own line supply - batteries won't last, and it's not clear that you can get usable power from the intercom without excessive reverse engineering. Commented Jul 20, 2017 at 14:46

2 Answers 2


I found the below article telling, how you can use a LM567 circuit to detect a tone via microphone input and where output turns LOW when detection happens.

That solution needs some soldering, and some resistors and capasitors to supply the microcircuit.

In the article there is a mention that you'll have to tune the detector by playing a loop of some short part of the detectable tone. A LED light is used for tuning showing when the value is going done.

Reading the tone can be done with ESP8266 or whatever microcontoller that can read high and low values. Note that ESP8266 uses 3.3V voltage and LM567 5V.

You are better of if you have separate powering for each circuits mentioned.

[1] http://www.scary-terry.com/more_stuff/tonedet/tonedet.htm

[2] http://www.electrodragon.com/product/special-sound-identifier-module/

(same circuit without soldering needed)

  • Doesn't the LM567 require a power supply inside the intercom? Commented Jul 22, 2017 at 5:56
  • My suggestion is to use own power supply for every device.
    – mico
    Commented Jul 22, 2017 at 17:20

I am not quite sure about whether there will be an interruption in the Intercom signals if you use any device (maybe an IC) that sucks power from the Intercom's supply.

If it doesn't, things are quite simple. As Mr. Stratton mentioned in his comments, this is a free waiver of many associated complexities!

You could use an Arduino or any other microcontroller circuit and connect one of it's input terminal to the supply of the InterCom.
Interfacing the circuit to the Intercom's circuitry should be taken care of.

Each circuit should be powered separately since their power requirements may be different or it may cause interference.

Program the microcontroller to ping the server when it detects signals on that terminal.

The ESP8266 module can be used for Wi-Fi connectivity.

I mentioned Arduino because of the ease in programming. It's generally used for prototyping purposes only! I am making things easy to understand. Please comment if you need any clarification or find anything wrong.

  • So thinking again in ESP8266 (or maybe SparkFun ESP8266) my question again is how to power it from Intercom, and how to detect that someone ringed
    – Mulflar
    Commented Jul 20, 2017 at 12:59
  • it should be powered separately. You need to have a 5V power supply module for both the Arduino and ESP8266. Its better this way. Commented Jul 20, 2017 at 13:27
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    @Prashanth Benny Note that ESP runs from 3.3 V and not from 5 V as original Arduinos. Commented Jul 20, 2017 at 14:08

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