Found in Maxim's documentation from the 90's, this circuit uses an integrated from the time providing a 12 V output from 10 to 12 V inputs.
This simple circuit controls the signal amplitude through a photoresistor optical coupler. The...
This circuit can be implemented with standard photodiodes consisting of a two input optical AND gate....
In the figure we have the basic configuration of an operational amplifier for audio signals. The...