This circuit was found in the March 1997 Electronics Now magazine. It uses a common analog indicator and the transistor is a general purpose one.
This circuit was originally created to monitor signals from a remote-control transmitter for the...
In this circuit the CA3140, an operational amplifier, is used as a comparator with the zener diode...
This circuit was found in old documentation. The frequency depends on R1 and C1. For the...