Here are some different types of tachometers:

* Mechanical tachometers: These use a rotating shaft connected to the device whose speed is being measured. The shaft drives a mechanism that displays the speed.

* Digital tachometers: These use sensors to detect the rotation speed and display it digitally.

* Optical tachometers: These use light beams to measure the rotation speed.

* Stroboscopic tachometers: These use a flashing light to create a "stroboscopic effect" that allows the user to visually determine the speed.

The type of tachometer used depends on the specific application and the required accuracy.