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.
This circuit was obtained in a magazine Hands-On Electronics of June 1987. It is a circuit that...
The purpose of this circuit found in an old Elektor is to light a lower power lamp when the bigger...
Found in a 90s Popular Electronics, this circuit uses two PLL tone recognizers to trigger by touch an...