This circuit that we found in a 1990 documentation uses an LED as a sensor. The operational amplifier is equivalent to the 741 and the trigger adjustment is made in the potentiometer.
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...