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 stage was found in a 1964 Electronic Design. The transistors are of the time and the shape of...
The traditional circuit was taken from a French magazine Radio Plans that no longer exists but can...
This circuit, which is present in other points of this website, was found in a 90s Hands On...