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Cheap and easy AC controller

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#1 idp

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Posted 31 January 2013 - 01:59 PM

Hi to all,

I have almost finished the 10" f/7 I have been working on for a while. It saw the first light a few nights ago, not going to win prizes for elegance or craftmanship but I am very happy with it.

The RA tracking uses an AC motor and I am fine with it since I plan to leave the telescope home all the time; however, I would like to have a simple controller, like stop/fast so that I can center objects in the field at high magnification, overcome irregularities and delays in tracking etc. It was a must on my old Starfinder mount, and this one is much older.

I really don't know anything about electronics, so I would be grateful if someone could explain me how to do it. I really just need stop/fast (or slow/fast); adjustable speed as a bonus, but only if easy to implement.

Thanks in advance folks.

Ivano

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#2 DAVIDG

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Posted 31 January 2013 - 02:46 PM

A simple method is purchase a DC/AC inverter off of Ebay. Almost all these units use 555 or 556 IC's to control the frequency. The frequency time base is made up of a resistor and capacitor. You add two push button switches that add or remove resistance to this circuit. This causes the unit to increase or decrease of the output frequency and change the speed of the AC motor.

- Dave

#3 Geo.

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Posted 31 January 2013 - 03:30 PM

www.ai.uga.edu/mc/alcor.html
http://code.google.c...or/wiki/design2

This is about assembly and burning firmware to a microprocessor. The electronics design and software code has been done for you. I've done a layout on Veroboard which simplifies assembly. Features are simple to build hand control and ST-4 autoguiding can be implimented.

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#4 idp

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Posted 31 January 2013 - 05:42 PM

A simple method is purchase a DC/AC inverter off of Ebay. Almost all these units use 555 or 556 IC's to control the frequency. The frequency time base is made up of a resistor and capacitor. You add two push button switches that add or remove resistance to this circuit. This cause the unit to increase or decrease of the output frequency and change the speed of the AC motor.

- Dave


Thanks Dave. So, if I well understand I'd have to give up using AC current and get batteries? Also, I saw lots of inverters on ebay and vastly different prices, what kind of product should I get?

#5 idp

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Posted 31 January 2013 - 05:47 PM

www.ai.uga.edu/mc/alcor.html
http://code.google.c...or/wiki/design2

This is about assembly and burning firmware to a microprocessor. The electronics design and software code has been done for you.


Hi Geo,

I took a look at your webpages before posting here, but I thought it's overkill for what I need and what I can do. I don't even understand many of the symbols in the picture. As I said, just stop/fast, variable speed nice to have but not really needed. No thought of implementing autoguide.
Thanks a lot though.

#6 DAVIDG

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Posted 31 January 2013 - 06:20 PM

A simple method is purchase a DC/AC inverter off of Ebay. Almost all these units use 555 or 556 IC's to control the frequency. The frequency time base is made up of a resistor and capacitor. You add two push button switches that add or remove resistance to this circuit. This causes the unit to increase or decrease of the output frequency and change the speed of the AC motor.

- Dave


Thanks Dave. So, if I well understand I'd have to give up using AC current and get batteries? Also, I saw lots of inverters on ebay and vastly different prices, what kind of product should I get?


All the commerical telescope controllers made for AC motors are running off of DC. The one that can use both AC and batteries have a built in AC to 12 volt DC power supply. When you run off 12 volt batteries your bypassing the internal power supply.
As for which DC/AC inverter to purchase, you just need to find one that is using 555 or 556 IC. I used one from Radio Shack that was given to me because it wasn't working. I replaced the bad FET's and then modified it. If you know someone that has an inverter and you can pull the case off and take a look inside, that might be your best bet.

- Dave

#7 Ed Jones

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Posted 31 January 2013 - 06:39 PM

Ivano,
The last one I modified was this one, only $17. I didn't take any pictures, however, and the resistors are the micro ones but accessible. I took a pair of nippers and crushed the micro resistor and then soldered wires to the ends that were left.

#8 mattflastro

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Posted 31 January 2013 - 09:17 PM

Ivano,
The last one I modified was this one, only $17. I didn't take any pictures, however, and the resistors are the micro ones but accessible. I took a pair of nippers and crushed the micro resistor and then soldered wires to the ends that were left.

To be honest, that's not all you had to do to make it work . You also had to solder another resistor in place of the one you crushed , plus 2 more plus wire the 2 buttons . Which brings the total to removing one resistor , soldering 3 resistors in its place and figuring out the values so that when no button is pressed, the equivalent resistance of those 3 resistors is equal to the resistor you broke off. When one button s pressed, the resistance is lower than the original, and when the other button is pressed, the resistance is higher than the original. That's how you have the slow/fast and nominal rates. Of course then is the question of WHICH resistor to remove in the beginning . For that you need to be able to figure out the pcb layout and read the 555 datasheet.

#9 Ed Jones

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Posted 31 January 2013 - 10:05 PM

I assumed all that was obvious. As I recall the capacitor was all alone and I just used a DVM to measure the frequency while I added a resistor to nearby resistors using probes looking for an increase in frequency to find the RC resistor. If I had one on hand I could shoot a picture.

#10 John Carruthers

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Posted 01 February 2013 - 04:56 AM

AWR do a VFO for ac drives;
http://www.awrtech.c...eledriv.htm#VFO
a lot of scopes were (and still are) driven this way.
You would need one built for 60Hz rather than UK 50Hz.

#11 Geo.

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Posted 01 February 2013 - 09:53 AM

A good 555 tutorial: http://www.kpsec.fre...om/555timer.htm






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