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 circuit presented is from a 2006 documentation. It can turn a set of lamps on or off when...
This second-order filter was found in a 1978 publication and can use common operational amplifiers like...
This simple tone control using two transistors was found in an Electronics Today International of the 90s....