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 emits an audible signal when the mains voltage ceases. The circuit is from a 1973...
We found this circuit in a 1966 RCA manual. The integrated circuit is unusual, and the Triac must...
Short duration pulses or "clicks" can be generated with the circuit seen in the figure. The...