Introduction: Star Tracker Using 28BYJ-48 Stepper Motor, Arduino and LCD Display to Vary Speed.
Hi, the winter nights are on me and my mind has turned to getting good images of the Milky way and other faint objects. To do that better results are obtained when you track the rotation of the earth and the simplest of these methods is by using a "Barn Door Mount" .
This design allows you to vary the speed with the keypad for accurate timing, and if the battery voltages drop you don't get the slowing down of the drive gears that other designs may suffer from.
I wanted the design to be able to change the speed of the motor, and direction so that it will work in the northern hemisphere and the south if required. "Why would I want to be able to change the speed " I hear you ask, well if you can't get the exact thread screw specified in the design you simply change the speed to compensate :)
I looked at a few designs but I saw good features in a couple so decided to use parts of both of these. May I offer my thanks to the authors of these designs for their inspiration and information.
http://garyseronik.com/a-tracking-platform-for-ast... I liked the fabrication of this design but not the electronics
http://kukuruku.co/hub/diy/diy-an-astro-tracker-in... I liked the electronics of this design so my suggestions an amalgamation of the great work presented by the other builders.
The benefit of the ideas I have combined is that its cheep and parts are easily available, so a short list though its covered in two links above.
28BYJ-48 stepper motor and driver board ( Ebay and others )
Arduino Uno ( Ebay and others )
LCD shield with keypad HobbyTronics. http://www.hobbytronics.co.uk/arduino-lcd-keypad-s...
Screwed rod, hinges, misc. nuts and bolts, 1/2" plywood, Ball Mount, etc covered in the other links above.
Old brass Meccano gears one 15 tooth one 60 tooth for a 4-1 ratio ( Ebay)
Batteries and holders to power it
A box to enclose it all.
Step 1: The Wooden Parts and Gears.
I started by building the wooden parts and figuring out the drive, you can see I printed out the shape of the motor full size from a data sheet I found on line.
Once I bent the screw rod to size on a 7 inch radius and cut it to 7 inches long for a fairly long track time.
The gears I got from ebay were next, the big gear needs a hole drilled in it just big enough to clear the screw rod. Its very important that when the nut to go on the screw rod is fitted is exactly in the centre, the method I used was to cut a 1 inch long piece of screwed rod and put the nut on one end, I then wrapped PTFE tape on the rod that would pass through the big gear to centralise it, then supported on short bit of tube I soldered them together using a small blow torch and flux, all joining faces were cleaned with a wire brush and sandpaper prior to soldering to ensure a good joint. Once its cooled simply remove the 1" piece of rod and the nut should be dead centre.
The small gear required the hole opened up to 5mm to suit the motor shaft and a grub screw fitted .
The author of one of the designs I used for my ideas has very kindly forwarded some laser cutting plans that has agreed to share should you wish to use them, I have no way of testing these as don't have the kit. The author is Alex Kuzmuk, his files are in zip form here he used them in his design Alex Kuzmuk's design
Step 2: The Electronics
These stepper motors are very inexpensive I bought two :) .
You require an Arduino Uno and an LCD shield ( as they are known )
The project I followed was written some time ago and the designer used a different LCD shield, but as Arduino parts normally just work with each other I did not expect it to fail :( , the good news is that I am currently following an online course for Arduino programming and I had covered enough ground to enable me to modify the software to work with the LCD units being sold by HobbyTronics
A picture is worth a thousand words so I hope the wiring image is easier to follow than me trying to explain.
When finally connecting it all up you require to feed the Stepper motor interface board with 5 to 12 volts ( check the polarity its on the board ) as well as feeding the Arduino, I believe its better to feed the motor that way as using the 5volt pin on the Arduino board could not really supply enough amperage to be reliable.
Step 3: Program the Arduino
I have had to alter the Arduino Sketch from the original to work with the LCD used here, Open the Arduino IDE and select New. Use the code attached here.If you require to change the direction of the motor look in the sketch about line 38 you will see this line of code.
double speeds = -271.6;
//changing this value from a negative to positive changes the direction of the stepper motor. This is important depending on the hemisphere you are in.
use 271.6 for the stepper to run clockwise or -271.6 to run anti clockwise.
If you are using different screw threads to the original designs ( 32 Tpi ) like me you will have to alter the speed of the motor, there is an excellent tool online that you can mess with to determine the number of gear turns you will require to get the tracker accurate
I'm going to use
.62 RPM / ((2Π / 1436) * 20.00 TPI) = 7.08 inches from hinge to drive screw.
so 37.2 teeth per minuet is my starting point, math is not my strong suit .
After a live run and very good results I have the timing set to -170, now I just have to wait for good sky :)
EDIT: Thanks and recognition to those other web sites that have proven helpful
Step 4: Some Modifications After the Build
I found after completing the build that there are some things that could improve the basic design.
The main one I think would have been to add 2 inches to the left of the lower board, cut a shallow V grove or some other method of your choice to locate a green laser pointer parallel to the hinges, green pointers stand out best but only use it for initial location of the pole star as they really show up in the atmosphere and could easily be a hazard to pilots and add to light pollution.
The rubber bands hold the two halves together and make the assembly easier to move about without the camera flapping about, it also serves to hold the gears in mesh for more reliable drive.
I found my camera hitting the board when I pointed it really high, so adding the additional mount below the ball mount solved that.