Normally alarm systems I’ve used have one of two different connections
The first is a normally open connection where power will not pass through till the relevant alarm is triggered.
The second is a normally closed connection where power is present till the alarm is triggered.
First thing to do is to check with the installers if attaching anything to these will void their support / warranty and check with your household insurance company to see if that would be void.
Next find out what voltage is present if any
on your side of the relay - below assume none
The safest way (as you will have zero effect on the alarm panel) is to use an opto-isolator chip set up like https://www.electronics-tutorials.ws/bl ... upler.html
at ‘An Optotransistor DC Switch’.
The relay contacts are the switch on the left and the Pi monitors the output on the right marked Vout.
Depending on the chip you use will determine the resistor value to stop the Pi frying.
If the relay is normally open, the pin will go high when the relay is triggered and low if normally closed.
Need Pi spray - these things are breeding in my house...