Tacho for Microprocessor Based Drives

For a microprocessor based drive system it is certainly necessary to trace the speed of the motor continuously. The circuitry given here gives an insight into a microprocessor based tachometer. Here only two ICs, a few LEDs and a phototransistor are used along with 8085-based microprocessor kit. The arrangement of phototransistor and LEDs is shown in the figure. To ensure the highest degree of optical efficiency the tubes, as shown in the figure, must be painted with white colour at their inner surfaces.IC1 (LM339) is configured as an inverting comparator.

When the shaft makes one revolution, the white spot will illuminate the phototransistor once, and IC1 will produce one pulse at its output. The variable resistor VR1 is adjusted such that IC1 will produce high output only when the photot- ransistor is optically aligned with white spot. Most of the microprocessor kits have interfaced 8253 timer. Here the counter 1 of 8253 is operated in mode ‘0’ to set a delay of ‘1’ sec. The clock pulses are obtained by external means by NAND gates ‘C’ and ‘D’ which produce 1kHz square wave output. The selection of counter depends on the counter which the manufacturer has provided for users. After one second delay the 8253 will interrupt the microprocessor through RST 6.5. The pulses obtained by IC1 are made to interrupt the microprocessor through RST 7.5 which is positive edge sensitive. The number of interrupts during one second is counted by initialising register ‘D’. The register will store the speed in rps (resolution per second) which is displayed in address field.For better result at lower speeds, the time delay may be increased to 3 or 4 seconds.

A lens can be used for avoiding disturbance by other light sources. It may be noted that assembly language listing calls for ‘UPDAD’ subroutine (or display data held in E register (double-digit packed BCD number) in the address field of 6-digit display. In case your kit has some other subroutine or provision for displaying the hex digits, you may use the same in place of ‘UPDAD’. The display can show up to 99 rps maximum.

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