Light-Path Sensor and Actuator
by Dennis L Feucht

This project will appeal to anyone who needs to turn an electrical device on and off without touching a mechanical switch. It uses a light path that is broken by reaching hands. Applications include actuation of water valves for washing hands, food preparation, or as a safety verification mechanism that a connector is plugged (or not plugged) into a given socket.

This circuit was originally designed for use on airbag instrumentation. The light path spanned across the diameter of a front-panel connector and detected whether the plug of a pulse output cable was indeed plugged into the socket. Later, my rural neighbor in northwest Pennsylvania -- a tool and die machinist by occupation -- had a booth at the county fair each year where (among other things) he sold cotton candy. His cotton candy machine, which wisped out the pink fluff onto a paper handle, needed to be turned on and off without touching the switch with candy-coated hands. I used the pulse generator circuit, which was abandoned in the design for the simpler solution of two additional shorted pins on the plug detected on the socket side within the instrument. The circuit diagram is given below, scanned directly from the project notebook.

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