Q: How do those automatic switches for faucets and toilets work?
A: The trick is infrared light, says Perry Sandstrom, an instrumentation engineer with Ice Cube, a giant neutrino telescope at the South Pole headquartered at UW-Madison. These automatic plumbing fixtures have infrared LEDs (light-emitting diodes) and detectors like those used on TV remote controls.
The sensor detects when infrared light is reflected from the user. A small microchip sets rules to reduce the chance of a spurious activation when a person simply walks past the sink or toilet. When necessary, the switch sends an electric current that activates the faucet or toilet's valve.
Because most of these devices are battery powered, they need to save energy. First, the LED is pulsed "on" for only a very short burst about once a second, Sandstrom says, "so it does not waste battery reserves while waiting for someone to show up. And the water valves are set to require electric current only when they move from open to close, or vice versa. They use no energy when they remain in the open or closed position."
Infrared technology is mass produced for so many purposes that both the LEDs and receivers are inexpensive, Sandstrom notes. Because the power is very low, the beam of invisible light is not hazardous.