Digital tachometer using 8051

Posted on Feb 5, 2014

A three digit contact less digital tachometer using 8051 microcontroller which can be used for measuring the revolutions/second of a rotating wheel, disc, shaft or anything like that is introduced in this project. The tachometer can measure up to a maximum of 255 rev/sec at an accuracy of 1 rev/sec. What you just need to do is to align the sensor

Digital tachometer using 8051
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close to the reflective strip (aluminium foil, white paper or some thing like that) glued on the rotating surface and the meter shows the rev/sec on the display. The circuit diagram of the digital tachometer is shown below. The first section of the circuit is the optical pickup based on photo transistor Q4 and red LED D4. Every time the reflective stripe on the rotating object passes in front of the sensor assembly, the reflected light falls on the photo transistor which makes it conduct more and as a result its collector voltage drops towards zero. When viewed through an oscilloscope the collector waveform of the photo transistor Q4 (2N5777) would look like this: Next part is the signal conditioning unit based on the opamp LM324 (IC1). Only one opamp inside the quad LM324 is used here and it is wired as a comparator with reference voltage set at 3. 5V (using resistors R16 and R17). The job of this comparator unit is to convert the spiky collector wave form into a neat square pulse train so that it can be applied to the microcontroller. Every time the collector voltage of the photo transistor goes below 3. 5V, the output of the comparator goes to negative saturation and every time the collector voltage of the photo transistor goes above 3. 5V, the comparator output goes to positive saturation resulting in a waveform like this: From the above two graphs you can see that the negative going edge of the waveform indicates the...

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