In my apartment I have an old intercom to know when someone is at the entrance of the building and to unlock the door. I waned to connect it to an esp8266 to let me know remotely if someone ring and to open. The opening part is ok, I used a relay for that. But the tricky part is to be notified when someone ring.The intercom is running with 12v but when someone ring the current is going to 0v.

The intercom as only to cable. I tried to get signal by using this circuit. The idea is to detect current drop :

detect ring circuit

problem is: when nobody is ringing and 12v is present in the intercom wire, GPIO is going high and low constantly. Is there any reason for that ? Is there something wrong with this circuit ?

  • \$\begingroup\$ Are you sure it's 12V DC in that intercom circuit? Or just rectified AC but not well smoothed? \$\endgroup\$ – Finbarr Oct 14 at 13:48
  • \$\begingroup\$ Not sure and my voltmeter do not say more than 12v. But it’s what I was wondering. If it’s the case what can I do to smooth it to 12v ? \$\endgroup\$ – Dragouf Oct 14 at 13:49
  • \$\begingroup\$ Add a diode ahead of R2 (cathode at R2) , then a small cap and resistor (in parallel) to ground at Q1's base. Might try a 120k and 10nF to start. This would rectify any AC (and/or audio signals) on the line. \$\endgroup\$ – Nedd Oct 14 at 13:56
  • \$\begingroup\$ To the ground of the intercom you mean ? No risk to close the intercom circuit and get undesired behavior from it ? \$\endgroup\$ – Dragouf Oct 14 at 13:59
  • \$\begingroup\$ Stray noise current needs to be absorbed. apply 10k across Vbe. if it still toggles then you need a diode and cap on input \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 14 at 14:01

It sounds like your "12V" is AC, not DC. You could fix this in software by simply ignoring any high input that doesn't last for more than, say, 1 second.

  • \$\begingroup\$ Yes it’s what I did for the moment but I was not satisfied and a little bit scared of this behavior^^ \$\endgroup\$ – Dragouf Oct 14 at 14:11

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