A contact-less digital tachometer based on optical sensors

Posted on Feb 6, 2014

The circuit for the Digital Tachometer/RPM Counter consists of only a few devices. Wire them up according to the following circuit diagram. The PIC I`m using is on a demonstration board, which means the clock, power and ground pins are already wired up so I didn`t bother to include that on the schematic. This is the digital tachometer portion

A contact-less digital tachometer based on optical sensors
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of circuit that will send +5v pulses to the RC2/CCP1 pin of the pic. The CCP module, specifically the capture module #1 on the PIC is located at this pin. This is the standard 16x2 character LCD and the interface consists of 8 bits of data, with 3 control signals. Power, Ground and Contrast are the last signals to wire up. The PIC sends commands to this 16x2 LCD over the control and data lines and the LCD does what it is told. The HD44780 system that this LCD uses is standarized and the datasheet is widely available. First, let me explain the overall theory of how the circuit and microcontroller will work to achieve our goal of building an rpm counter. The 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 occured. Now, if we keep track of how often that change occurs using a timer, we can estimate the instantaneous RPMs, making a digital tachomter! The actual output signal from the photo-interruptor portion of the circuit will look similar to what you see above. The length of the +5 pulses are determined by how long the emitter and detector are interrupted. If we know the period of time between...

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