I am trying to upgrade our existing bell with my arduino to detect whether the bell is used and to mute the bell.

The circuit for the bell runs with 9V AC. In order to minimize the wiring I want to serially connect something to the bell such that it is possible with my arduino to turn the bell on and off and I can detect whether the bell was used.

I tried to use two relays: one as a switch which can be used by the arduino and one as a switch which will be closed when the bell is used. They were parallel connected. But since I am a beginner nothing worked as intended :)

Is it possible to do this with two relays? Or is there a better way to make it work?


Thanks for the answer. The bell isn't earthed. I tried this:

Idea with two relays

What is wrong with it? :)


1 Answer 1


Here's your detection circuit:


simulate this circuit – Schematic created using CircuitLab

The output will pulse 4.7V at the same frequency as the source when the bell is being triggered, which is appropriate for a 5V Arduino. Use a different zener diode if your Arduino is running at a different voltage. Note that R1 should be a 1/2W device or higher since 200+mW of power will be dissipated through it.



simulate this circuit

This new schematic connects to an input with the internal pull-up enabled. It will pulse low at half the frequency of the source.

  • 1
    \$\begingroup\$ What if the bell circuit is earthed? Better to use an opto. \$\endgroup\$
    – Andy aka
    Dec 20, 2014 at 23:22
  • \$\begingroup\$ Agreed. I'll add that in. \$\endgroup\$ Dec 21, 2014 at 16:33
  • \$\begingroup\$ It is easier for me to add something in series in the circuit of the bell. Since I am renting the flat I want to minimize the wiring :) I added some schematic of my idea. Is it possible to do it somehow my way? \$\endgroup\$
    – Marc Bury
    Dec 21, 2014 at 17:24
  • \$\begingroup\$ The problem with yours is that the control connection short-circuits the detection relay. \$\endgroup\$ Dec 21, 2014 at 17:27
  • \$\begingroup\$ Sorry I don't understand why. I thought there are two resistors (1 ohm and 700 ohm from the relay) which are parallel connected. \$\endgroup\$
    – Marc Bury
    Dec 21, 2014 at 21:24

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