This circuit triggers a buzzer or other warning device when the light falling on the sensor is cut off. We found it in the Encyclopedia of Electronic Circuits Volume 5 of 1995.
In the original version of this 1987 circuit, the loads are lamps, but it can be adapted to drive a...
This transmitting circuit for an infrared intercom is from Texas Instruments. The operational...
This circuit emits a sound at regular intervals to prevent you from falling asleep, for example. It...