This optically coupled circuit was found in the February 1968 Radio Electronics magazine. The circuit uses an LDR in modulation. Transistors are types of the time.
This filter is suggested by Motorola's Linear Interface ICs of 1993. This same configuration...
This circuit was found in the March 1980 issue of Radio Electronics magazine. It is based on the...
This circuit not only indicates when the battery is on but also when it is low. The circuit is...