I want to hook my home security system's alarm into my Raspi, so whenever the alarm is triggered, my Raspi sends me a text message.

The gentleman who installed the system said whenever the siren goes off, current is running through pins 3 and 4. According to the attached picture (wiring diagram) this looks correct.

My Question :

Based on the wiring diagram, would It be ok to attach wires from Pins 3 & 4, directly to the Raspi 2 pins? Notice It says 12.5vdc to 10.9vdc, and something that looks to be a value of 2.2k. What do I need to do to safely read/get the voltages into the Raspi?

I appreciate any help In advance!

Internal Alarm Panel


Apart from the voltage, there are myriad problems interfacing with an external system. You could probably make it work with suitable protective circuitry, but it is likely to be simpler/safer to use a relay. Other possibilities are opto-isolators if you want a non mechanical system.

  • Thanks for the advice. I think I have a relay that came with my Arduino Kit. Would any relay be suitable, or could you get more specific with the relay option? Thanks :D
    – Guy
    Sep 25 '15 at 0:48
  • It all depends on the operating voltages of the relay switch - I think @milliways is suggesting to use the relay on the 10-12V signal coming from the alarm, and have it switch the appropriate 3V3 signal to the rPi (connect 3V3 to the Relay's IN terminal and a GPIO pin to the OUT terminal, with a suitable resistor in between). The relay you have with your Arduino might not work with 10-12V input signals, as the Arduino's output pins switch at 5V. You need to check this.
    – Phil B.
    Sep 25 '15 at 1:56
  • Note also that it's a (supervised) bell circuit, meaning the overall resistance of the circuit can't change (both at alarm voltage and the lower supervision voltage when the bell is at rest), the purpose of supervision of bell circuit is to discover when the wire from this panel to the actual bell has been opened(cut) or shorted, which would prevent it from working. You really do need to find out how to properly wire a relay here, not for 12v output as drawn below the bell circuit but for contact closure... That's what you can wire to your pi.
    – Tyson
    Sep 25 '15 at 4:33
  • So I got a 12V Relay. Just before I go to wire everything up, can someone confirm If I'm about to do this right, lol -- i60.tinypic.com/oaax51.png -- The Relay Normally Open (Pink) to GPIO0 (PI Pin 3), the Red to Alarm Positive, the Orange to Alarm Negative, the Relay Common (Blue) to the PI Pin 1 (3V3), and the Green on the Relay (Normally Closed) goes to nothing. Is this all correct?
    – Guy
    Sep 27 '15 at 23:26

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