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.
This constant current charger from 1997 Linear Technology Applications uses common components at...
This interesting circuit was found in an Argentinean Radio Chassis Television magazine from March...
This different circuit uses a relay as an oscillator, whose frequency is determined by C1, and to...