Calibrated-tachometer

  
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Here is a simple tachometer circuit for use with a hand-held DVM or portable chart recorder. A novel feature is that the source frequency pulse/rev rate can be directly set on a ten-turn potentiometer to provide a convenient calibration of one V per 1000 rev/min. This is particularly useful when measuring a shaft or engine speed by sensing gear teeth.
Calibrated-tachometer - schematic

The circuit uses an LM2917 IC which is specifically designed for tachometer applications. The tenturn potentiometer, which provides the pulse/rev setting, is suitably configured in the output amplifier feedback path. The pulse/rev range is 100 to 1100, so the potentiometer dial mechanism should be set to start at 100 to provide direct calibration. The IC"s internal 7.5-V zener provides stable operation from a 9-V battery. The tachometer accepts an input signal between 50 mv and 20 V rms and has an upper speed limit of 6000 rev/min with the component values shown.





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