DIGITAL TACHOMETER

DIGITAL TACHOMETERNelson Murwariya (100104077)Rohit Raj (100104108)Sumit Rajput (100104130)

INTRODUCTIONThe word comes from Greek (tachos) i.e. speed and metron meaning measure .Atachometer(revolution-counter,tach,rev-counter,RPM gauge) is an instrument measuring the rotation speed of a shaftor disk, as in a motor or other machine.The device usually displays therevolutions per minute(RPM) on a calibrated analogue dial, but digital displays are increasingly common.

HISTORYThe first mechanical tachometers were based on measuring thecentrifugal force, similar to the operation of acentrifugal governor. The inventor is assumed to be the German engineer Dietrich Uhlhorn; he used it for measuring the speed of machines in 1817.Since 1840, it has been used to measure the speed of locomotives.

WORKING PRINCIPLEPulses are fed to the tachometer at the frequency to be measured. A scale factor is applied to produce readings of desired types (linear speed, flow rates etc.).

Tachometer works on two basic principles:- a) Principle of the fixed time base tachometer b) Principle of the reciprocal tachometer

CLASSIFICATION Tachometers can be classified on the basis of data type Analog tachometer. Digital tachometer.

COMPARISON BETWEEN ANALOG AND DIGITAL TACHOMETERSANALOG TACHOMETERHas a needle and dial type interface.No provision for storage of readings.Cannot compute average, deviation etc.DIGITAL TACHOMETERHas LCD or LED readout.Memory is provided for storage.Can perform statistical functions like averaging etcCONSTRUCTION 1) PIC 18F452 Microchip's PIC microcontroller will be used to display text to the output LCD and also for counting the pulses from the IR rpm counter circuit.

2)PIC Programmer The PICkit 2 PIC Programmer was used to put the final software code on to the pic microcontroller.

3)Trimpot The trimpot is used as a variable resistor for both the LCD screen and for the IR rpm counter circuit. The LCD screen needs to have a certain resistance in order for the contrast to be right, then the text will show up perfectly. The digital tachometer circuit uses the trimpot to bias the IR Detector properly, sending pulses whenever there is a change between the Detector and Emitter.

4) IR Emitter/IR Detector The IR Emitter device sends an infrared signal just like any normal LED lights up a room. The difference is you won't see any light shinning from the IR Emitter because our eyes cannot see it. The IR Detector will detect if any infrared light is being shined at it. If IR light shines at the detector, it allows current to pass through it to ground.

5)16x2 LCD The 16x2 LCD gives us a nice platform for displaying text that is easily readable and very accurate. This LCD uses the HD44780 standard .

Other useful equipments are 100 Resistor , Breadboard , Wires , Desktop Computer Fan.

OPERATIONThe IR circuit will output pulses whenever it is interrupted (this type of IR circuit is also known as a 'photo-interruptor' circuit).The PIC microcontroller will stand by waiting to see the rising edge of one of these pulses.Anytime a rising edge is detected the PIC will interrupt the current software and run a special subroutine to take note that the change on the signal occurred.Now, if we keep track of how often that change occurs using a timer, we can estimate the instantaneous RPMs, making a digital tachometer. Output Signal From IR Circuit

FOR CALCULATING THE RPM The Fosc is the current frequency of your system. For a 4 MHz oscillator. Since the PIC takes 4 clock cycles to execute 1 instruction, we have to multiply it by 4. On the bottom, the reason we multiply the period between photo-interrupts by 7 is because the fan used has 7 blades. Period*7 = 1 full rotation of the fan.