Cad cell sensors respond to light; when the flame is present, the cad cell resistance changes in what direction?

• A. Increases
• B. Decreases
• C. Remains the same
• D. Fluctuates

Answer: (B)

Cad cell sensors are light-sensitive resistors. Their resistance drops as the light on them increases. When the flame is present, the flame emits light that the cad cell detects, causing its resistance to decrease and send a stronger signal to the control system. If the flame goes out and the cell receives less light, its resistance rises, which signals the safety control to shut off the gas.