Found in Popular Electronics from the 90s, this circuit uses a reed switch on the rails as a sensor. Instead of lamps, in a modern version, LEDs can be used.
Presented as an electronic siren, this circuit was found in Radio Electronics magazine from June...
This simple blocking oscillator using a transistor can be used in alarms or call systems. The...
This circuit was found in a 1992 documentation and has excellent sensitivity, using components that...