This circuit is intended to detect when current flows through the opening of a car door. The circuit fires the horn for 90 seconds. The FET can be the BF245 and the NPN transistors can be the BD135.
This sophisticated circuit with two LF411 operational amplifiers appeared in the November 1984...
This circuit uses battery power, but it can be powered by an 80 to 200 V power supply, with no...
This interesting circuit, which can be used as an alarm, was found in a 1997 documentation. It...