In optical sensing, the light-detecting element that energizes the contacts is the:

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

Answer: (C)

In optical sensing, the detector’s job is to turn light into an electrical signal that can drive the output. A photodiode does this directly: when light shines on it, it generates a current proportional to the light intensity. With the proper bias and circuitry, that current can switch or drive the output stage to energize the contacts. LEDs are the light source, not the detector. A photoresistor changes resistance with light but doesn’t provide a clean, directly switchable current for energizing contacts, and a phototransistor is another detector option with gain, but the device in this context is the photodiode’s fast, linear light-to-current response that activates the output.