This circuit was found in the Electronics Experimenter Handbook from 1991. The sensor is a diode and the circuit has a relay according to the voltage used in the supply.
This circuit is from a Radio Electronics magazine from May 1989. The source must be symmetrical, and other...
The sophisticated signal tracker/follower with a JFET and integrated circuit 741 was found in a...
This circuit was found in Radio Electronics from the 90s. The transducer is a tweeter and the...