In an optical sensing pair, which element in the receiver conducts when illuminated to energize the contacts?

• A. LED
• B. Transmitter
• C. Photodiode
• D. Phototransistor

Answer: (C)

In an optical sensing pair, the receiver’s job is to convert light into an electrical response that can drive the circuit. The element that conducts when illuminated is the photodiode. When light from the transmitter strikes the photodiode, photons generate electron-hole pairs, producing a photocurrent. With appropriate bias this current flows through the circuit and can energize the contacts, effectively turning on the output. The LED is the light source, not the receiver, and while a phototransistor also conducts under light, the photodiode’s direct conductive response to illumination is the mechanism described here for energizing the contacts.