Remote, noncontact, measurement of the speed of rotating objects is the purpose of this simple circuit. Linearity and accuracy are extremely high and normally limited by the milliammeter used and the initial calibration. This circuit is configured to count the leading edge of light pulses and to ignore normal ambient light levels. It is designed for portable operation since the tachometer is not sensitive to supply voltage within the supply voltage tolerance.
Full scale at the maximum sensitivity of the calibration resistance is read at about 300 light pulses per second. A digital voltmeter.can be used, on the 100-mV full-scale range, in place of the milliammeter. Shunt its input with a 100-0 resistor in parallel with a 100-~F capacitor. This rc network replaces the filtering supplied by the analog meter.