In the fist circuit the alarm triggers on when any sensor is CUT. In the second circuit the alarm triggers when any sensor is closed. Power suplly must source 2 V above the voltage of the relay, sincce this is the voltage fall in the SCR in the on state. The relay is any type with 50 to 100 mA coil. When the circuit is not triggered current drain is very low alloing the use of batteries as power supply.
Alarms Using SCRs
