The cad cell on an oil burner detects what?

• A. Pressure
• B. Light
• C. Noise
• D. Flame

Answer: (D)

The cad cell is a flame detector. It works as a photoelectric sensor that responds to the light emitted by the flame. When a flame is present, the light lowers the cadmium sulfide cell’s resistance and sends a signal to the burner control to keep the fuel flowing. If the flame goes out, the signal changes and the control shuts off the fuel to prevent unburned oil. It’s not measuring pressure or noise, and it’s not reacting to general ambient light—it’s specifically detecting the flame’s light.