This circuit is from an American manual from 1975. The transistors are types of the time, but equivalents can be used, as well as the relay that can be according to the supply voltage.
This alarm with NO and NC sensors was found in the January 1975 Radio Electronics magazine. The...
Found in a 90s Popular Electronics, this circuit checks if a remote-control transmitter is...
This small experimental transmitter can send its signals to distances up to 100 meters. L is...