This circuit was found in General Electric documentation from 1964. It makes use of a PUT, called the PNPN planar key at the time. The circuit has two supply voltages.
Using the four amplifiers of an LM324 this circuit was found in the June 1996 issue of Electronics Now...
This simple timer can be used in many applications such as egg timer. The LED glows up when S1 is...
In the figure we have a simple FM tuner using a tunnel diode. Can be adapted to other frequency ranges....