The cad cell is used in gas burner safety circuits to detect what?

• A. Temperature
• B. Pressure
• C. Light
• D. Sound

Answer: (C)

The idea is flame presence is verified by sensing flame light. A cadmium sulfide (cad cell) photoresistor changes its resistance when illuminated. In a gas burner safety circuit, the flame emits light that reaches the cad cell, lowering its resistance and sending a signal to keep the gas valve open. If the flame goes out, the light level falls, the cad cell’s resistance rises, and the safety control shuts off the gas to prevent unburned fuel from leaking. So this detector is about detecting light from the flame, not temperature, pressure, or sound.