Found in American documentation from the 1980s, this circuit has a diode feedback. At the exit we have an indicator LED. The LM3900 is a transconductance operational amplifier.
This circuit is from an Electronics World of 1964. The circuit can be made with modern...
This circuit is from an old American documentation from the 70s. Transistor 2N3904 can be replaced by...
We found this circuit in the March 1980 Radio Electronics magazine. The circuit used is the HA2400...