An electronic tachometer operates by receiving voltage pulses from which source?

• A. An ignition signal
• B. A DC power supply
• C. A rotating encoder
• D. A temperature sensor

Answer: (A)

An electronic tachometer measures speed by counting how many voltage pulses occur in a given time. Those pulses come from the engine’s ignition system, which fires a spark and creates a voltage spike each time a cylinder fires. The rate of these ignition pulses is directly proportional to engine RPM, so counting them and converting to revolutions per minute gives the correct reading. A DC power supply merely powers the instrument, a rotating encoder would require a shaft with an encoder (not typical for engine ignition signals), and a temperature sensor outputs a slowly varying signal, not a rapid pulse train.