A tachometer is a device that is used to measure the speed of rotation of any device. Using the analog voltage reading property of an arduino, this can be easily implemented using the appropriate parts. Tachometers have a variety of applications, including measuring the speed of dc motors in order to ensure they are running according to specifications. In order to make a tachometer, we will have to convert the speed of rotation into a readable form. The only form the arduino can read is in terms of electrical voltage. It is well known that if voltage is given to a motor the motor wheel will turn, the speed of which is determined by the amount of voltage given. However the converse is also true, meaning that if we were to rotate the motor by ourselves, we can obtain a voltage across the two ends of the motor terminals. 

Here is a trial of building a tachometer using AT89S52 Microcontroller. This is a digital tachometer which works using infrared rays. The tachometer designed here can measure rotational speed up to 4200 rpm which is sufficient for small scale use. If the speed goes beyond 4200 rpm, it gives a message on the display screen as “OUT OF RANGE”.Take out the wheel of a damaged optical mouse and make a hole at the center of the wheel. Put the steel rod at the hole and glue it tightly so that the rod and the wheel rotate simultaneously. Make the plywood frame as shown in figure and screw the screw bearings on both sides so that the rod with the wheel can be mounted. After mounting the rod (with the wheel) on the screw bearings, try rotating the wheel with your finger. While rotating, the rod should have a minimum friction with the screw bearings.