This circuit was found in documentation for radio amateurs of the 1980s. This is the so-called roger-beep, which is the beep that is transmitted when the talk-listen function is changed.
This circuit is from a Linear Technology manual from the 90's. The integrated circuit is from the...
The original design of this circuit is from a 1978 publication. The circuit can be implemented with a...
The PNP transistor can be a BC548 and the NPN any common phototransistor. The circuit is from 1968...