In an optical sensor system, when light from the transmitter reaches the receiver, which component conducts to energize the contacts?

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

Answer: (C)

Light reaching the receiver is converted into an electrical signal by a photodiode. A photodiode is a diode that produces current when it is illuminated, and when biased correctly this light-generated current flows through the circuit to energize the contacts. This direct light-to-current conversion makes it the typical choice for triggering the next stage in an optical sensor.

The other devices don’t fit as cleanly here: an LED is the light source, not the detector; a phototransistor can also detect light but relies on light-generated carriers to drive a transistor and often adds amplification and biasing complexity; a photoresistor changes resistance with light but doesn’t provide a straightforward, reliable current path to energize contacts.