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.