A Low Cost RA Stepper for Small Telescopes

[c3po]

Thanks Alan. My unit is finally fully functional and ready for its first use in anger. Just bought a T-ring for my dslr and sky glow filter... Just need some clear skies and a night off work (I've been on nights for the past few weeks :-( ) to try it out.

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[nagarajan]

Hi,

  I'm going to the market to purchase all the components. I have also sources two motors from a old dot matrix printer. My only worry  is the worm gear. hope I'll get it some how. wish me good luck.

[nagarajan]

Hi, 

   Finally purchased all components. Finished the gear box with gears taken from old printers only item missing is the worm gear which ill get it soon as i have made some contacts for that.  i couldn't get smd capacitors and resistors in low quantity so i purchased conventional ones. being new to electronics i have some hiccups. so let me list it out kindly help me.

     I don't know how to convert eagle layout into pcb layout V1.10 so that i can take a printout and and make etching. I tried removing layers but still not getting it right. all the pins in the microelectronic seems to be merged in that. moreover i am using eagle free version. If anyone have the pcb layout file kindly help me so that i can take a printout( makes my life easy ). If its RA v1.20 PCB please share the eagle file also for reference.

[AstroTux]

Hi Nagarajan,

sounds like you're moving along quickly with this.

Did you buy the ATmega8A in the 32-pin TQFP package ( for surface mount ), or the more usual through-hole mount 28-pin DIP package? If you're new to making PCBs you may find surface mount components more difficult at first as there is a trick to soldering them correctly. However, there are some excellent articles on the Internet you can look up to help you with this. You may want to make everything with the usual through hole components instead as it makes the task a lot simpler. I use surface mount as I have been an electronics engineer for over 35 years, and so am used to such things.

If you want an up to date layout using surface mount, here it is:

RAControlSchematics1-21.zip

I have included a version of the board that will handle the new code and a focuser motor if you want to include one.

You will also find inside this file a script called 'layout.scr' that I use to help with identifying and printing the board layout in Eagle. Just put this in your Eagle/Scripts directory and you will be able to run it using the 'scr' icon at the top of the layout screen. One of the options is Display - here you will find that you can set it to display the top or bottom layers for printing. Also included is the ability to display all, or just the placement of components. This helps a lot when you have to solder the jumper wires that connect parts of the board to others.

NOTE: you should only print a layer once you have used the Ratsnest command in the layout screen, as this fills in the polygons that create the ground plane of the board.

You will also find a file called 'Metrics for QFPs.dru'. This is the set of design rules that I use to check a board before making it, but you will probably want to come up with your own at some stage. To use this just place it in Eagle/Design Rules, then when you want to check the board with DRC ( Design Rule Check ) load it and the board will be checked using this set of rules.

By the way, all of this work was done with the Eagle freeware edition, so you should be able to modify whatever you wish in terms of components or layout. Don't be frightened to play with Eagle, but do spend some time and learn it. It can be a very powerful tool when used properly.

Hopefully this should get you pointed in the right direction, but if you have any problems just ask. However, I am away on one of my regular climbing holidays to the NW Highlands of Scotland tomorrow, and the Internet connections there cannot always be relied upon, so don't be disappointed if I don't reply for two weeks or so.

hope this helps,

Alan

[nagarajan]

Dear Mr.Alan,

                     thanks for the help. I purchased all  IC's as SMD components. I know thy are difficult that's why i bought three each so that even if i goof-up while soldering or programming i have a backup.  I am using this project to learn bits and pieces in electronics may be i will learn programming also in near future. More than just coping the design i am learning each components functions and the design philosophy as detail as possible. i am find this task very difficult but determined. Time is also a factor as i spend 12 hrs at work. i will study the new layout and ask for your help if required( after two weeks) so enjoy your trekking.

Regards

Nagarajan K

[nagarajan]

Hi,

    It’s been a quite a journey so far. I’m currently in the process of making PCB through tonner transfer method. So far I have learned how not to make it may be 7 or 8 times. Pretty soon the copper is going to vanish from my rubbing. I tried Kodak photo paper lot of colorful shiny paper, A4 sheets it all ended horribly wrong. Tonner doesn’t stick uniformly or the tracks from the microcontroller smudge or the paper refuse to go away from the tracks. I’m putting too much pressure or too little. Anyhow I’m determined to see till the end even if I have to destroy couple of copper clad and few bottles of nail polish remover. Let me see who wins at the end me or the paper.   

    I have a small doubt can I use 21pF or 22PF capacitor instead of 20pF for C3. 20pF is not available here.

10Mhz crystals I’m getting are all 50ppm tolerance  will it affect the accuracy?

The journey continues.

Nagarajan

[nagarajan]

Dear Mr.Alan,

             Take your own time to reply i dont want to spoil your vacation. and do share your treaking experiance. i was a  a outdoor person couple of years back. been to lot to treaking in forests, tiger santuries and jungle water falls. love to do it again hopefully.

[AstroTux]

Hi Nagarajan,

I'm currently indoors as the weather here is atrocious. Going out on the hills is dangerous due to the high avalanche risk caused by wet snow and rain, and it hasn't frozen at all so everything is loose. Not good!!! :sad:

Making your own PCBs is not a straightforward process and requires a lot of trial and error. What works for some doesn't work for others and many variables are at work. I sometimes think it is more of an art than a science :grin: .

I haven't used the toner transfer method myself as I prefer to use photo-resist spray ( Positiv 20+ http://www.rapidonline.com/Mechanical-Fastenings-Fixings/Kontakt-chemie-Positiv-20-Photo-Resist-Spray-100ml-87-0705/?source=googleps&utm_source=googleps&gclid=CP2FoKzY6bwCFfPItAodb2UAyg ) which is probably available somewhere in India. I spray this on a cleaned board ( cleaned with Vim to remove all dirt and grease ) and let it dry in a dark place for a couple of days. The artwork is printed on a laser printer transparency and placed over the board as a mask which is then exposed in a UV light box for about 1 min 20 sec. The board is then developed using a commercially available developer which leaves a perfect etch mask, and the board is then etched in ferric chloride.

If you cannot get the photo-resist spray you could always try photo-resist prepared copper clad board instead, but will have to take the time and learn how long to expose the artwork for. If you look around on the Internet there is a lot of information that will help you when using photo-resist for PCB making.

BTW, I cannot get on with the photo-resist film that is commonly available, but you may find that you can.

The capacitors can be any value near to 20pf as they just allow the crystal to vibrate. They have a small impact on the accuracy, but not much. The crystal can be 50ppm if you wish, but I have covered the effects of this in my notes where accuracy is discussed. If you can use ebay, you will find lots of 20ppm crystals available from Chinese suppliers.

hope this helps,

Alan

[nagarajan]

Dear Mr.Alan,

                    

Hope the weather got clear by this. I sourced 20 PPm crystals from Banglore through my friend. Only thing not to my satisfaction is the 20 pf capacitor I settled for 22pF. Your method of PCB fabrication seems to be too expensive for me. All chemicals are available here but not in small quantity. Photoresist dry film seems to be promising that too is available only in bulk. So planning to take my pcb to a local screen printer and get it done but that’s a last option.  Before I decide I want to do a bit of research before I make it. Hope it come good. Your help is invaluable to me. Thank you.

Nagarajan

[nagarajan]

Dear Alan,

          Finally made the Pcb using Tonner transfer method. All other process seems to be expensive. After a long trial and error I finally made three board.  First one using a normal paper with reasonable good result.  Second and third using photo paper. I just went to local studio and asked for it and the gentleman gave couple of small sheets free of cost. Using photo paper I faced problem of track smudge near the microcontroller. With great difficulty I finally succeed. I used the first board to practice soldering the SMD components. Though I have normal types I thought smd’s are cool. Anyhow I don’t know to solder so I thought of starting with SMD’s. It’s better to aim high and fail and learn. First try I learned the result of overheating and how to burn a good pcb. On the second board I learned how not to solder a smd microcontroller. My attempt resulted in shorting all the pins and when tried to rectify that I severed the tracks.

     By the third board I got the legacy data of failure and avoided them. I soldered all the components properly. Checked the continuity, everything is good. Some more soldering is pending like LCD, Jumpers and connectors. I think it will be a simple task.

     I am currently using 22pF capacitors. When I brought the crystal the dealer told me its 20ppm but I’m not sure. So I contacted my friend from in US to get crystal, 20pF capacitor and USB avr programmer from eBay and they are on the way. EBay India is total Rubbish and everything is expensive compared to US. Once I get that I’ll replace them.

       My gear box is still waiting for the worm gear. The One I got was a total misfit. The one you are using seems to be from an old HP deskjet 600 series printer so I have starting chasing those now.  I have a doubt why do we need a cnc connector. What difference it makes compared to a direct metal shaft connector?  Just curious to know. I have planned to make two board one for me and one as a gift to whoever deserves it, someone like me who is struggling to have one of these but not able to. I might be disturbing you when I start programing, kindly bear with me.  

 

I want upload the photos but its not working.

 

Thanks

Nagarajan

[AstroTux]

I have a doubt why do we need a cnc connector. What difference it makes compared to a direct metal shaft connector?  Just curious to know.

The CNC connector is for attaching the output shaft of the gearbox to the RA slow motion connector of the mount.

Without it the gearbox cannot easily be connected to the mount. You could try and directly connect the RA connector to the gearbox, but then you would not be able to remove the gearbox for any adjustments. Also, any vibration from the motor will be transmitted straight into the mount affecting your views through the eyepiece at high magnifications.

Hope this helps,

Alan

[Ags]

Great project - I am looking at adding a stepper motor to my barn door tracker, although I plan to use a raspberry pi and hopefully Java...

[nagarajan]

^{Dear Mr.Alan,}

                ^{I have a query regarding the open circuitry on pin number 19(ADC6) in microcontroller and pin number 5(R/W) in LCD is a switch?. I completed the assembly and awaiting for the USB programmer I have ordered. Meanwhile I powered the board and found no reaction on the LCD. I was expecting backlight of the LCD will be glowing. Please let me know the ideal state of the board before programing.} 

 

^Thanks

 

^Nagarajan

[nagarajan]

me dumb i figured out the LCD will come alive only through the microcontroller. my doubt is only on the 19(ADC6) in microcontroller
 

Nagarajan

[AstroTux]

my doubt is only on the 19(ADC6) in microcontroller

Nagarajan,

don't worry about pin 19 ( ADC6 ), it is an extra piece of circuitry that I put into the original design so that I could also implement battery monitoring using the A to D converter module. ADC6 can only be used as an analogue input pin to the A to D converter and is only available with the TQFP package, the DIL ATMega8 does not feature this pin. To use it will require two resistors configured as a voltage divider on the power input stage, but as I have not got around to implementing the code it is currently redundant.

hope this helps,

Alan

[nagarajan]

Dear Alan,

       Thanks for the info. i was little puzzled about that. Now i'm ready to go to the programming stage. 

Regards

Nagarajan K

[udaiveer]

Wow..

Reading before commenting

[udaiveer]

Wow..

Reading before commenting

[happy-kat]

Fascinating thread but quite likely way over my capabilities but have bookmarked it.

[cuivenion]

Fascinating thread but quite likely way over my capabilities but have bookmarked it.

Same here. I have got some pretty good stepper motors salvaged from some old disco lights, so it'd be interesting to to see if they'd be any use.

[Langy]

This is something that I would love to have a go at, but my main issue is the gearing and putting it together.

I have no problem with the microcontroller and programming as I play around with the Arduino controllers a lot and can program in various languages.

If I can get hold of all the parts then I'd certainly give it a go.

Paul

[paul_astro]

Has anyone ported the the code onto an Arduino?   Pins and timing should be minor changes.

Arduino is now the cheaper than the Amtel development boards, and there are ready made shields for stepper control. 

regards
Paul

[Jack22]

Astrotux:

 

Came across your super post last week and would like to know if you are still willing to share ideas with me.  You have probably moved on to newer and greater projects and that i understand.  I am interested in the tracking / focusing device you describe here, but need advice on availability of parts etc.

 

 

[JamesF]

Doesn't look like he's logged in for more than two years, so I'm not sure you'll get a response from him.

James