A contact-less digital tachometer based on optical sensors
A tachometer is use to measure the rotations per minute (RPM) of a moving shaft or a motor. An example is a digital tachometer in a car that gives the RPM of the engine. This tutorial shows how to build a digital tachometer using a microcontroller and optical sensors.
The sensor unit consists of an IR LED that transmits infrared light and a photo diode that receives the light. This project uses a computer fan to demonstrate the technique. The fan is placed between the IR LED and the photo diode so that the blades of the moving fan interrupts the light received by the photo diode. Each interruption of the light gives a pulse that is counted by a PIC18F452 microcontroller. With a little math, the number of rotations per minute can be calculated. The result is displayed on a LCD screen.