This is a device that produces a voltage proportional to the speed of a rotating shaft. Common examples include the rev' counter found in many vehicles. This measures the rotation of the engine crank shaft. The speedometer in vehicles measures the wheel rotation speed. Wind speed gauges use a similar system.
Here is a convenient way to detect the rotation of a shaft.
Shaft rotation can also be measured with a mirror attached to the shaft. Another alternative is to attach a magnet to the shaft and place an induction coil near the magnet. As the magnet passes the coil, a voltage pulse will be induced.
As the notched wheel rotates, the light beam is interrupted. This appears as a small square (ish) wave output from the photodiode. When the shaft rotates faster, more pulses are produced but the pulses are also shorter. This means that the average output remains constant. To make a useful measuring device, the variable length pulses from the photodiode need to be converted into fixed length pulses. A monostable circuit achieves this. The monostable needs to be triggered by short pulses.
Here is a system diagram.
- The photodetector generates low voltage, variable length pulses.
- The amplifier increases the the amplitude (size) of the pulses to be compatible with the trigger input of the monostable (5 volt pulses for TTL chips). The limiter ensures the pulses all have the same voltage.
- The monostable converts the short variable length trigger pulses into longer fixed length output pulses.
- The integrator smooths the pulses to an average DC level. A passive low pass filter can be used for this.
- The volt meter measures the DC level. This voltage is proportional to the shaft rotation speed.
This is a linear system. This means that the tachometer output voltage is directly proportional to the speed. A graph plot gives a straight line as shown below. This system has been neatly calibrated to give one volt representing ten km/h.
