This circuit converts a high voltage into a frequency using an optical coupler. The transistor can be 2N2646 and the coupler admits equivalent. The circuit is from a 1974 publication.
Found in a Popular Electronics of the 90s this configuration can be seen elsewhere on our site....
Found in an Electronic Design of 1964 this circuit has a control for the frequency and another for the...
This circuit of the 80's makes use of components of the time. The circuit is from an American...