Micro controller Based Tachometer

Learn interfacing of AT89C2051 with the display driver IC ULN2003. Application of 4-digit seven segment display. Use of the operational amplifier IC CA 3140 for amplification purposes. Use of the photo-transistor L14F1 as a sensor/detector. The circuit of the Micro-controller based Tachometer requires the user to point a light sensitive probe tip atop the spinning shaft towards the spinning blade, disk or chuck and read the rpm. The only requirement is to first place a contrasting color mask. A strip of white adhesive tape is ideal on the spinning object. It should be positioned in such a manner that the intensity of light reflected from the object’s surface should change upon rotation. Each time the tape spins past the probe, the momentary increase in reflected light is detected by the photo-transistor. The signal processor and micro-controller circuit counts the increase in the number of such light reflections sensed by it and thereby evaluates the rpm, which is displayed on the 4-digit, 7-segment display. The photo-transistor is kept inside a plastic tube, which has a convex lens fitted at one end. The photo-transistor is fixed on a piece of cardboard such that it faces the lens at a distance of about 8 cm. The signal detected by the photo-transistor is amplified by transistor 2N2222 (T5) and further amplified by operational amplifier CA3140 (IC3). The output from pin 6 of IC3 is fed to pin 12 of micro-controller AT89C2051. Note that pins 12 and 13 of micro-controller AT89C2051 are the inputs (+ and -) of its internal analogue comparator. Pin13 is adjusted to nearly half the supply voltage using a potential divider comprising resistor R7 and preset VR1 across the supply. The pulses picked up by the photo-transistor are sensed by the internal comparator of AT89C2051 and, through software, each pulse representing one rotation of the object is detected. By counting the number of such pulses, on an average per minute basis, the RPM is evaluated. It is displayed by a software routine to light up the LED segments of the 4-digit, 7-segment display.