Which component is used to detect light levels?

• A. Photocell
• B. Reed switch
• C. Proximity sensor
• D. Pressure switch

Answer: (A)

Detecting ambient light levels is done with a light sensor. A photocell, also called a light-dependent resistor, fits this role. Its resistance changes with how much light it receives—dropping in bright light and rising in darkness. That change lets a circuit infer the current light level and, in a simple setup, feed an analog input to a microcontroller so it can respond appropriately (for example, turning on lights when it gets dark). The behavior is intuitive: more light lowers the resistance, altering the divider voltage to reflect brightness. In contrast, a reed switch detects magnetic fields, a proximity sensor detects nearby objects or distance (often using IR or capacitive methods), and a pressure switch detects applied pressure. So the photocell is the component that directly detects light levels.