Michelin ECT Full Course Practice Exam 2026 - Free Practice Questions and Comprehensive Study Guide

A photocell detects light with a light-detecting element that converts photons into an electrical signal. That detecting device is a photodetector. Among the options, the broad term “photo detector/photo diode” best fits because it names the function—detecting light—rather than focusing on a specific implementation. A photodiode is a concrete type of photodetector, and a phototransistor is another detector option with built-in amplification; both perform the same sensing role, but the general descriptor is what matches the concept of detecting the light source. The LED indicator, by contrast, emits light and does not detect it.