We found this circuit in the July 1992 issue of Radio Electronics magazine. An optical system produces the light beam that, when interrupted, triggers the alarm. The sensitivity is adjusted by R4.
This adjustable source was found in a 1979 documentation, but all components are still common...
This temperature control was found in General Electric documentation from the 90's. The circuit uses...
This SCR circuit was found in an American 1980s documentation. The circuit is still current,...