A star tracker, also known as barn door tracker, is a device that lets you make long exposure photography of the sky without star trails. To achieve this, it compensates the rotation of the earth by spinning at the same speed but in the other direction. The movement can be generated by hand or by using a low RPM motor coupled with some gears to match the earth rotational speed.
I built this start tracker using birch plywood and a 4 RPM DC motor. An 18 teeth gear is fixed on the motor’s axis while a 72 teeth gear is placed on the curved threaded rod. This 1:4 ratio gives a good approximation of the desired speed.
The black box plugged to the platform contains a 9V battery as well as a circuit based on the LM317 voltage regulator. The motor’s speed will slightly vary when the battery drains out. A variable resistor enables a precise and constant voltage at the poles of the motor even when the battery loses power.
You can find more details on Gary Seronik‘s website. A cheaper manual and easy to build version can also be found here.