This is a very cheap telesope tracker with the base components cost less than 10$. It is made for a EQ-5 mount but can probably be adapted to others. Part list:

• arduino nano or clone
• stepper motor type 28BYJ-48 incl driver pcb ULN2003
• 200mm closed "GT2" belt
• some sort of power supply (I use a single li-io cell, 3.7V)
• 3 pushbuttons of any size for fast forward, backward, lunar vs stellar speed
• printed parts
• a few screws

I actually drilled a hole into my telescope mount and cut a M4 thread into it to fix the base (see photos). You may want to re-design the base to do this less invasive. The 16 teeth pulley fits onto the stepper without any screw, the 64 teeth one can be fixed with a screw at the hour-axis drive (see photos). I also used an M4 screw there.

The included little arduino project is made for a 144 teeth main gear as featured on the EQ-5 mount. For other tooth counts, this can be patched.

Electrical connections: pins 9,10,11,12 from the arduino go to the stepper controller in1..in4 pins, 5V supply and gnd of both boards are just connected. The jumper that connects motor supply and board supply on the stepper controller board stays in place. Function can be checked without motor: The LEDs should act as arunning light. I use a single li-io cell (3.7V) and connect it directly to the 5V pins. The complete thing consumes about 180mA so that a 2Ah cell can easily last 10h. The thing should also run from a power bank via the USB connector. It will take a bit more current then, and you need to mount the arduino board so that the USB socket is reacheable. Pins 2,3,4 can be connected to some sort of pushbuttons that drag those pins to gnd when pressed (the arduino board features internal pullup-resistors). This toggles lunar vs stellar speed and does fast forward / fast backward. See the ino file for details. On the photos, this is the little 4-lead gray cable. Without any button the stepper just runs in stellar mode. It makes sense to connect the pushbuttons with a cable to avoid vibrations when operating them.

Programming:

• download arduino
• install the "timer1" lib via the sketch->lib menu
• open the tracker.ino file
• connect the arduino nano (windows may install some drivers)
• upload the program (you may need to check the hw manager to find the port)

If you mount the whole thing the other way round, or live south from the equator, you need to patch the code to reverse directions. Just replace if(d==1) with if(d!=1)

When connected to USB, the boards are supplied from there (and the stepper will also run). Therefore:

DO NOT connect both battery (especially if it is not 5V but goes to the 5V pin) and USB since this connects the battery voltage directly to the USB 5V pin!!!

UPDATE: I have just learned 2 things:

1. The stepper motor does not do one revolution for 4096 steps, but in fact for 64 * 32/9 *22/11 *26/9 *31/10 = 4075.7728 steps
2. The diameter of the pulley shall not be teeth2/pi mm but teeth2/pi-0.5 mm

I have therefore redesigned the pulleys and adopted the FW. As you can see, there are a few options on which output pins to actually use. I have also increased the number of teeth for the bigger pulley to 66 teeth to use up available space as much as possible.