An electronic circuit that monitors readiness and transmits a signal when there is a problem is a :

• A. Microwave signal
• B. Mechanical signal
• C. VHF signal
• D. Trouble signal

Answer: (D)

A fault-detection signaling function. In fire protection and life-safety systems, the built-in monitor checks that the system is ready and, when it detects a problem, it sends a signal to alert maintenance or the control panel. This specific signaling of a fault condition is what’s called a trouble signal. Microwave and VHF refer to radio frequencies and don’t denote fault reporting, while a mechanical signal implies a physical, non-electronic indication rather than an electronic fault alert.