This is an interesting configuration of the LM317 voltage regulator found in an EDN from the 90s. The circuit uses an optical coupler and is intended to supply constant current to a sensor resistor.
The purpose of this circuit is to allow common analog multimeters to measure low resistances, on...
We found this circuit in a Radio Electronics issue from July 1969. The lamp remains on until C1...
This circuit was found in a 90s Popular Electronics. The sensor can be any phototransistor. ...