Star tracker

I had some spare acrylic from another project, so I decided to get this project underway. The basic idea is very similar to Alex Kuzmuk's design.
https://kukuruku.co/post/diy-an-astro-tracker-in-two-nights/

I desired a simple and reliable star tracker, that was also small enough to carry, particularly for hiking or travelling.

The use of the Arduino provides a lot of flexibility in gear and thread choice. Mine uses an M6 thread to drive the barn door.

You may need to adapt it to your tripod/camera mount. There's a 6.5mm (1/4) hole for the camera mount. I used a 3d printed base for a ball joint from an old monopod, bolted in with a 6mm bolt. The tripod mount is designed to fit my Vanguard tripod with foot dimensions of 10mm high, top 32.3mm square, base 42mm square.

There's a couple of potential improvements that could be made. The hinge could be redesigned to incorporate bearings (see update 19/06/17). The gear system could benefit from bearings as well. In a 'perfect' system, the main gear would rotate around a bearing, and the guide rod would be constrained by the gear. In this model (and most other barn door star trackers) the threaded rod constrains the main gear, and is itself constrained by the hole in the acrylic. As the hole in the perspex has to have some leeway for the threaded rod, this could result in vibration or oscillation as the rod and gear move around the hole.

This design includes a picatinny rail for mounting a sighting scope.

Electronics consists of an Arduino Nano (just the board - no riser pins), an illuminated switch, and a 5V BYJ48 stepper motor. Speed control is defined in the code, so just flick the switch to start tracking.
https://youtu.be/23A9o62kssQ

I'm not going to draw a sketch for the electronics. I'll just list the wiring below. There's not much clearance in the case for wiring, so be neat. :D
Pin 12 -> switch -> ground
Pin 3 -> stepper IN1
Pin 4 -> stepper IN2
pin 5 -> stepper IN3
pin 6 -> stepper IN4
5V -> stepper positive
Ground -> stepper negative
5V -> switch LED (If you want an illuminated switch)
ground -> switch LED (If you want an illuminated switch)

The whole thing is powered over USB from a battery pack. In my case I'm using a 4AA Goal Zero power pack.

Fasteners used were:
10 x M5 countersunk. I cut mine down to the right length, but if you have options, then lengths should be around 6x12mm (picatinny rail and hinge) and 4x 25mm (tripod mount).
2x ~3mm screws, ~15mm long (to screw the control box to the tripod mount).
6x #4x12mm screws (for Nano enclosure and control box lid).
2x 3mm bolts, ~ 15-20mm long (for motor).
1x M6 bolt (for camera mount)
1x M6 threaded rod.

In terms of weight capability, the main factor is probably the acrylic panels and plastic rather than the gearing. I don't see any problems with mounting things up to 1.5KG on mine. If mounting heavier stuff, just be aware that the tripod and camera mount are plastic. In terms of gears, with a 5/1 gear ratio to the stepper and another ~3/1 in the barndoor, I suspect that there is no camera heavy enough to could stop the gears from turning.

Warning: Yes, you can rotate your camera around to tip it over the hinge. If you're concerned then figure out your balance point and cut the threaded rod a bit shorter than that.

Update: 19/6/2017
I have now developed a bearing hinge that uses a couple of 608 bearings (Roller skate bearings). The files for the bearing hinge are "Bearing Hinge Interior" and "Bearing Hinge Exterior". The bearings are clamped in place with a couple of M8 35mm bolts, with captive nuts in the Interior portion. There's 1mm clearance between the two sections, so make sure you don't overtighten the bolts. This design means that all of the bearings are captive, rather than just relying on a push-fit, and can centre the hinge components so that there is no rubbing anywhere between plastic.