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Revolution Meter

December 10, 1994

This is a simple rev-counter which can measure up to 100,000 rpm with method of strobe synchronization.

Its usage is easy. Put a marker on the shaft before measureing, light on the shaft by flashing LED and adjust flashing frequency with the potentiometer as keeping the marker still. The rotation will able to be read from scale of the dial. To avoid measurment error due to unintentional second or more harmonic lock, it should scan from higher value to lower value.

Measurement methodManual measurement in strobe synchronization method
Measurment RangeLow: 10,000rpm F.S.
High: 100,000rpm F.S.
Power supplyOne 9V battery
SizePocket sized
CostApprox. 3,000 JPY


Please show the circuit diagram. VR1 is a 10 turns potentiometer. Its output voltage is converted to frequency by the V-F converter. Thus the oscillation frequency is proportional to the dial position.

The pulses from the V-F converter triggers monostable and it drives strobe light (LED). The output pulse width also be controlled by the potentiometer, so that the strobe light is driven in constant duty ratio. The duty ratio is fixed to approx. 1:60.

LBT circut is a low battery detector. It stops the oscillator when supply voltage dips below 6.5 volts to avoid measurment error due to unintentional operation.

Build and Calibration

Because the circuit is simple and not fast operation, there is nothing to be considerd for assembling.

Next, the calibration is required after assembly, but it is a little difficult. Because measurement range is divided in two ranges, timing capacitors for each range must be adjusted with some capacitors.

First, set range to 100,000 rpm (SW2-OFF), set dial to maximum and adjust C5 to oscillation frequancy gets 1667 Hz. Next, set range to 10,000 rpm (SW2-ON), set dial to maximum and adjust C4 to oscillation frequancy gets 166.7 Hz. If you wish to avoid to adjust capacitors, fix to one range and adjust R3.

Accuracy of the Measurment

After all, this is built on a cheap analog circuit, the accuracy also corresponds to it. The accuracy depends on linearity of the potentiometer and the oscillator. Acturally, the linearity is 0.5 percent at 10,000 rpm range, 2 percent at 100,000 rpm range. At high frequency region of 100,000 rpm gange, discharge time (this is constant at all of a range) seems affects linearity.

To read the measured value, the value of multiplied maximum value at the measurment range by dial ratio is the rotation speed. e.g. when the dial value was [4] and 25 at 10,000 range, it becomes 4,250 rpm.

Technical Data

Circuit Diagram for Rev-Counter
The inside of the rev-counter.

Strobe Light
Flashing LED Light.