This circuit is from old documentation, using a component that cannot be found easily today. The indicator is a galvanometer, and the input voltage is in the order of 1 V.
The phototransistor can be of any type and the adjustment potentiometer 4M7. This circuit was...
We found this circuit in a Popular Electronics from the 90s. Its purpose is to monitor a power...
This configuration can be made based on other operational amplifiers. For other frequencies, the...