Stepper Motor Controller with Autoguider

Autoguider November 2015

UPDATE (February 2016): I have recently built an Arduino-based stepper motor controller. For inquiries, kindly leave a comment below or send an email to

This stepper motor controller is an improvement of my previous tracker designs. The operation of the micro-controller is explained below:

1. Autoguider port – an RS232 port connects a controller to a computer’s parallel port for receiving guiding signals
2. Power supply – an internal auto-voltage regulator allows the controller to draw power from any 6 to 12 V power source
3. Autoguider RA+/RA- calibration – adjusts the pulses per second of RA plus and RA minus during guiding
4. Stepper Motor Port – an RS232 port connects the controller to the mount’s stepper motor
5. Audio feedback – tracking rate (pulses per second) can be analyzed through an audio port by connecting it to an oscilloscope (or through the computer’s sound card if a computer is used to emulate an oscilloscope)
6. Power on/off – turns on and off the device
7. Step rate indicator – provides a visual feedback of the current step rate
8. Step indicator – indicates which of the 4 coils of the stepper is energized
9. RA plus button – momentarily increases step rate to a predefined value, nudging the telescope to the west
10. RA plus indicator – lights up to indicate that the step rate is momentarily increased
11. RA minus button – momentarily decreases step rate to a predefined value, nudging the telescope to the east
12. RA minus indicator – lights up to indicate that the step rate is momentarily decreased
13. Mode – selects operation between Automatic and Manual mode
14. Manual override – when Manual mode is selected, step rate can be manually adjusted from 1 – 100 hertz (beats per second)
15. Speed selector – when Automatic mode is selected, step rate can be set from any of the 9 pre-defined step rates; for this controller, 1 is assigned to sidereal, 2 for solar, and 4 for lunar step rate, respectively
16. Sidereal calibration – adjusts tracking rate to accurately track stars, galaxies, and nebulas
17. Solar calibration – adjusts tracking rate to accurately track the Sun
18. Lunar calibration– adjusts the tracking rate to accurately track the moon

Related articles:
DIY Autoguider (Part 1: Introduction)
DIY Autoguider (Part 2: Setting-up the Guiding Software)
DIY Autoguider (Part 3: Wiring Diagrams)
DIY Autoguider (Part 4: Autoguiding and Polar Alignment)

For tutorials on how to get started with astrophotography, click here.
For DIY astronomy projects useful for astrophotography, click here.
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© Anthony Urbano (Manila, Philippines)


12 thoughts on “Stepper Motor Controller with Autoguider

  1. I wish I had your ability to create your own equipment….ever think of mass producing these things? An autoguide upgrade for the cg5(non motorized) already costs more than $500…too expensive for an average filipino income earner…

    Clear Skies!

    • Hi Weedbreeder,

      Thank you :) Building and modifying all sorts of stuff has become part of my hobby. Astro-equipment may be a bit costly, thus, I had to resort to improvisation to cut down costs (this particular upgrade costs just around $40 only, with all parts sourced locally).

      With your CG5, I believe it wouldn’t be much of a problem attaching a stepper to it. If you would be willing to devote some of your time to engage in equipment modification, perhaps I can help you design a tracker for your mount, and eventually adapt it to an autoguider setup.


  2. Hi Sir!

    I was checking out your website and I found the page for the clock drive for equatorial mounts. You have stated that the drive can be adapted to barndoor mounts. May I ask how?

    I currently have a manual-driven barndoor mount, which I use to take pictures of stars and some DSOs. I have been wanting to motorize the setup, but I don’t know how… hope that you can help me with this…

    Also, I’m interested in the afocal adapter for cameras… Can I use a DSLR with that afocal adapter?

    Any response to this email would be much appreciated. Thanks!


    • Hi Oliver,

      I hope you find my web site helpful for your barn-door mount project.

      Can you send me a picture of the barn-door tracker? If you are comfortable working with circuits (or perhaps someone you know), them I may just send you the schematics for a Variable Speed Stepper Motor Controller. For that setup to work, you will also need to look for a suitable gear system (perhaps the gears of a toy car, etc.) which will provide a means of transferring the rotating motion of the stepper motor to the shaft in your barn door tracker that needs to rotate. Some sort of adjustment knob will allow you to control the tracking rate/steps to find the exact rate which will eliminate the trails.

      And regarding the adapter, yes, the Universal Camera Adapter may be used to connect a DSLR to a telescope. This setup should be adequate enough for afocal imaging.


  3. Hello Eteny,

    Wow, you are quite talented! I would be very interested in the details of your stepper motor circuit with auto guiding capability. I recently built a barndoor tracker with 4rpm dc motor, 3v controller, and 1 to 4 gear ratio to get the 1rpm I need. It uses a 1/4-20 curved screw to maintain tracking. I’m still playing with it and am getting some modest pictures with a 200mm lens and Canon 1000d camera. I thought I would buy a professional mount after this but am curious if I can apply your circuit to my own barn door tracker first. Perhaps, then I could work up to a full mount like you did. The idea of building seems more fun then buying off the shelf :) I have electronics knowledge but I find soldering is still a challenge on those small prototype boards…ha..ha…


    • Hi Jim!

      The circuit only works with stepper motors :( and not compatible with a DC motor. However, if you can find a way to momentarily speed up and slow down the motor of your tracker by activating 2 separate relays (one for speeding up and one for slowing down), then it is very possible to devise a way for a computer to talk to your barn door tracker and regulate the speed with high level of precision. This will enable you to expose longer and use lenses with longer focal lengths. I believe a circuit that increases or decreases the voltage to a predefined level should do the trick. If you need help designing one, just let me know :) Here’s a brief intro to autoguiding.


      • Hi Eteny,

        I was considering changing to a stepper motor but am uncertain how one would have the computer output port adjust the 555 timer circuit frequency. Are you having the computer adjust the voltage on pin 5 of the 555 timer to increase and decrease the frequency – using it as a voltage controlled oscillator?


      • The computer port’s task is to switch on or off the 2 relays. The relays in turn provide another pathway for the electricity to pass through, thereby, with the help of 2 variable resistors, increase or decrease the resistance in order to control the frequency :) Variable resistors help achieve a predefined speed because without them, the relays will just either completely sever or short out the connections. Severing the connection (of the resistor) would result to a complete stop, and bypassing the connection (by shorting out the leads of the resistor) would result to maximum speed. We do not want a full stop or a maximum speed. What we want is a slight increase or decrease in the resistance which translates to a slight increase or decrease in the motor’s speed.

      • Hello Eteny,

        I was considering changing to a stepper motor. However, I am wondering how you control the 555 timer frequency up and down with the computer output port’s +5 volt signals. Do you adjust the voltage of pin 5 of the 555 timer with a circuit and use the 555 timer as a voltage controlled oscillator? I would love to hear how you approached this.


      • An external resistor usually controls the frequency of a 555 timer. By changing the resistance, it is possible to change the motor’s speed. 2 relays (which will be controlled later by the computer by sending 0V or +5V) can be connected in series or in parallel with the 555’s external resistor to either speed up or slow down the pulses. To have a better control of speed, trimmers are placed in series with the relays. It’s a rather simple solution, but it works:)

  4. Hi Eteny,

    I’m sorry for asking the same question twice above. I did’t know it posted the first time. Thank you for your responses. This looks like a really fun project. My biggest problem is that I don’t have a parallel port or card slot on my laptop. My serial USB adapter can not be assigned an LPT printer port number. The only commands I get through the cable (using a COM port) using Guidemaster are LX200 commands which don’t seem to toggle a voltage on any of the DB9 pins the right way. I’m not sure how to proceed…
    In any case thanks for posting so much interesting information :)


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