by Don Patton
modern motorcycle image tachometer by Oleg Mitiukhin from Fotolia.com
A tachometer is a meter that indicates the rotational speed of a car engine. Traditional mechanical tachometers had an analog needle to indicate engine RPM. Some modern electronic instruments also use an analog stylus instead of a digital display.
The operation of a mechanical tachometer is similar to that of a mechanical speedometer. A flexible cable with a rotating shaft connects a moving part of the engine or transmission to the meter. Inside the instrument, the rotary shaft controls the position of a needle to indicate motor speed.
The heart of a mechanical tachometer is an eddy current sensor that contains a moving magnet driven by the rotating input shaft. The rotating magnet on the sensor imparts a force on the indicator needle proportional to the engine speed, while a spring counteracts the force from the sensor.
An electronic tachometer uses a magnetic pickup placed near a rotating part of the motor to produce electrical pulses at a frequency proportional to the speed of the motor. The meter circuitry converts the pulse rate to display the engine RPM using an analog needle or a digital readout.