TACHOMETER ABSTRACT Tachometer is used to measure the speed of a running motor. The heart of this project is 89c51 microcontroller. this basicslly consisits of 3 modules- microcontroller unit, display unit , interface unit . In the interface unit, a combination of reed switch and a magnet is used to measure the sped of a running motor.the magnet is fixed on the shaft of the running motor. and every time a rotation is made, the reed switch comes in contact with the magnet and a pulse is sent to the microcontroller. 89c51 microcontroller has 2 counters\timers. they can be programmed to act as a counter or as a timer. here one counter is programmed as a counter and the other as a timer. the reed switch is connected to the counter pin of microcontroller and the frequency is measured . counter\timer value gives the frequecy. it is converted to speed and displayed on the 7 segment led display. the output of the microcontroller is bcd . therefore cd4543 decoder is used to convert bcd to 7 segment display. if the speed exceeds 1500 rpm, a red light flashes an d which is a n extra feature. with a slight modification in the interface ckt, this can be used as a heart beat monitor,pedometer, frequency meter.