A speedometer is just a simple digital electronic transducer. Usually, it is used to measure how fast a shaft is moving. The number of rotations per minute (rpm) is important to know in order to understand any system that moves. For example, there is an ideal speed for drilling a hole of a certain size in a piece of metal. There is also an ideal speed for sanding, which depends on the material being finished. You might also want to measure how fast your fans spin. This microcontroller-based tachometer is easy to make and can measure the rpm of most shop tools and many household machines without a mechanical or electrical connection. Working of a microcontroller-based tachometer Fig. 1: Circuit of microcontroller-based tachometer Just point the light-sensitive probe tip on top of the spinning shaft toward the spinning blade, disk, or chuck and read the rpm. The only rule is that you must first put down a mask of a different color. On the thing that is spinning, a strip of white adhesive tape would work well. Set it up so that as the object turns, the amount of light that is reflected from its surface changes. When the tape spins past the probe, the phototransistor picks up the momentary rise in light that comes back from the tape. The 4-digit, 7-segment display shows the rpm, which is calculated by the signal processor and microcontroller circuit. It does this by counting the increase in the number of these light reflections that it senses. The phototransistor is kept in a plastic tube with a convex lens on one end. A common part used by watch repairmen and in cine film viewer toys is a convex lens with a

diameter of about 1 cm and a focal length of 8 to 10 cm. They can give it to you so you can set up the experiment. The phototransistor is attached to a piece of cardboard so that it is about 8 cm away from the lens. The phototransistor's leads are taken off and put into the circuit shown in Fig. 1. Figure 2 shows how a phototransistor should be set up. The signal that is picked up is amplified by the transistor 2N2222 (T5) and then amplified even more by the operational amplifier CA3140 (IC3). The operational amplifier's reference voltage point is found by a resistor divider network made up of R2 and R3. Pin 6 of IC3 is connected to pin 12 of AT89C2051, which is a microcontroller. Note that the inputs (+ and -) of the microcontroller AT89C2051's internal analog comparator are pins 12 and 13. A potential divider made up of resistor R7 and preset VR1 is used across the supply to set Pin 13 to almost half the supply voltage. Fig. 2: Suitable arrangement of phototransistor The AT89C2051's internal comparator picks up the pulses picked up by the phototransistor. Each pulse represents one rotation of the object, so each pulse is seen as a rotation. By counting how many of these pulses there are on average per minute, the RPM can be found. It is shown by a software program that tells the 4-digit, 7- segment display's LEDs to light up.

In Fig., you can see how the tachometer with a microcontroller works. The tachometer has an AT89C2051 microcontroller, a ULN2003 high-current Darlington transistor

array, a CA3140 operational amplifier, a common-anode 7-segment (4-digit multiplexed) display, and four anode-driving transistors. The AT89C2051 is an 8-bit microcontroller with 20 pins that is made by Atmel Corporation. It is part of Intel's 8051 family. Input pins 1 through 7 of the ULN2003 are connected to port-1 pins P1.7 through P1.2 and port-3 pin P3.7. The 10-kilohm resistor network RNW1 is used to pull up the pins on Port-1. They use internal inverters to power all seven segments of the display.