Based on the LF256 operational amplifier, this circuit was found in the September 1989 issue of Radio Electronics magazine. The indicator is analog, and the power supply must be symmetrical.
In this circuit of a 1968 publication the unijunction may be 2N2646 and the transistor BC548. The...
This circuit was found in a 1969 CQ magazine. It is intended for coupling telephones to radio...
This circuit, which can be used as a shield, was found in Radio Electronics from the 90s. The...