My friend, there are many kinds of speedometers, such as measuring rotational speed and linear speed. It is quite difficult to make these speedometers in spare time. This, we can see from its working principle:

First, the principle of radar speed measurement-measuring the speed of the car.

Radar velocity measurement is to calculate the frequency shift of the received reflected wave and get the moving speed of the measured object. Generally speaking, it is to set up a radar transmitter beside the road, emit a radar beam in the direction of the coming car on the road, and then receive the echo reflected by the car, and measure the speed of the car through echo analysis. At present, the police radar velocimeter is divided into two types: the fixed one is installed on bridges or intersections, and the mobile one is generally installed on patrol cars.

Second, a simple tachometer.

The instrument contains photoelectric converter, which converts the rotating speed into electric pulse signal. The pulse amplification shaping circuit amplifies and shapes this signal, and the digital display circuit displays the electric signal in the form of rotating speed.

For details, please refer to relevant materials. I won't elaborate here.