This resistance meter was found in an English documentation from 1989. The operational amplifier admits an equivalent and the transistor can be a BC558 or BC559.
The recommended distance between the emitter and the phototransistor is 10 mm for this...
The input and output frequencies of this circuit depend on the coils and capacitors of the tuned...
This circuit was obtained in a 1991 publication. What it does is to amplify a signal with unit...