Please see attached schematic.
I wanted to make a circuit that detects an intruder using a pressure mat (which I guess is really just a big switch you can stand on). The mat triggers a thyristor which powers the Arduino. When the Arduino has finished it’s tasks of sounding alarms etc… the last task it does is make a pin go from LOW to HIGH connected to a transistor which triggers a relay. The relay switches to expose the input of the thyristor to low current (ie below the holding level) and the Arduino switches off.
I made the circuit on breadboard and it worked when I just grounded the thyristor input using a reed relay but the current was too high and it blew the relay.
I would welcome some feedback on the new circuit using the SPDT relay but I’m not sure if there are better ways or I may blow further components etc… The zener diode is there to limit the voltage on the thyristor gate, which along with the resistor R2 is to limit power to the gate. I tried it on a simulator and it works in theory.