Digital tachometers are used to measure the rotational speed of objects and can be categorized into contact and non-contact according to the measurement method. The contact type digital tachometer is limited and not convenient enough to use, the safety is also lower, but the price is relatively low, in industrial control, we often use non-contact type digital tachometer. Digital tachometer is one of the important measuring instruments in daily life, and has a wide range of applications in steam, electronics, textile, paper and other aspects.