This circuit for light modulated in frequency (FM) operating at 50 kHz was found in a 90’s Electronics Now. The circuit has an active filter. Modern equivalent transistors can be used.
This circuit uses unijunction transistors and drives a small speaker through an output...
This circuit, found in a publication from the 1980s, can have its output voltage determined by the...
The operational amplifier for this circuit, found in a 1979 publication, may be the 741....