Found in an RF Design from the 90's this circuit is intended for crystals of less than 1 MHz. Transistors are types of general use today.
Using an operational amplifier, the figure shows how to convert the waveform to low frequency...
This basic circuit for a common phototransistor was found in an Electronics Now from the 90s. The...
This photometer is based on a Wheatstone bridge that must be balanced by the linear potentiometer P1. The...