Using an operational amplifier, this circuit from a 90s Popular Electronics, is characterized by its high sensitivity. R5 can be added to control the gain.
This circuit is from an 80's electronic component manual. It has the way to simulate an inductor...
This circuit was found in a 1974 Ferranti transistor manual. The frequency is given by the...
Indicated by a Popular Electronics of the 90s originally to measure the light determining the exposure...