Posted on Jul 3, 2012 9426
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.