This simple optical alarm circuit was found in the 1991 Electronics Experimenter Handbook. The circuit uses a lamp as the emitter but can use LEDs.
This circuit is from a Popular Electronics from the 90s. The integrated circuit admits equivalents...
This very interesting project was obtained in a 1980 publication. It consists of a device that is...
This circuit beeps when the sensor is triggered by the opening of the freezer door or other...