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Standard Autoguider port to relay interface

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#1 dgreyson

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Posted 18 March 2013 - 09:00 PM

Hi everyone. Is there a thread someone can point me to describing how to connect a standard ST-4 autoguider port like the one on an Orion easyguider to external relays?

I have a classic 12" f4.5 Cave astrola newtonian with 2 axis motors but the dec motor is 110vac reversable and the polar axis has a synchronus clock motor that uses an old criterion dyna tracker drive corrector.

SO I have Dec left, Dec right and Polar slow, polar fast.
I have 12 vdc available. I want to drive some isolation relays to switch the motions. 5 watts is max current for the dec so I may use reed relays as they are cheap.

But I dont know if the ST-4 guider port expects to output 5 volt logic level or if its dry contacts. Yes, I know, go look and see, but I'd hate to burn up an autoguider if there is a ready spec for this. Thanks if anyone knows. I have the port pinouts but thats about all I find.

#2 jrcrilly

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Posted 18 March 2013 - 09:12 PM

It doesn't source anything; just open collectors to ground.

#3 fmhill

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Posted 18 March 2013 - 09:27 PM

There is no good specification for the ST-4 interface, I believe Orion specs the SSAG as a 5 volt TTL compatible port however it is some type of open collector device, you need to add pull up resistors to 5 volts, probably an R value about 150 ohms per line...

The Shoestring Astronomy web site has a pretty good paper on the ST-4 interface...


#4 dgreyson

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Posted 18 March 2013 - 11:11 PM

open collector TTL, thanks guys!! I suspected as much, so I know what I need to drive my Cave now. I will show it when I have it breadboarded. Will definitely check out Shoestring. If yould like to see my scope there are photos in the classic forum, "Balance and fit test 12.5 F4.5 Cave Astrola " is the thread. It dosent accept digital control signals now, but that's about to change. I have a guidescope and hand controllers so I can still do the classic sort of hand guiding on exposures, but once youve done that, you know what is was like and can move on.

#5 jrcrilly

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Posted 18 March 2013 - 11:51 PM

open collector TTL,


Open collector, yes - but TTL, no. Most mounts present 12VDC to the open circuit so the internal switch is something that can handle 12V at the collector; usually CMOS or an opto-isolator.

#6 Geo.

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Posted 20 March 2013 - 06:02 PM

Your isolation can look some thing like this:

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#7 Geo.

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Posted 20 March 2013 - 06:44 PM

Without a look at the inside of your controller it's hard to say what you'd need for the mount/isolator interface. This is a Sidereal Electronics inverter/corrector. Joy stick, AC in, 12VDC in, Dec out (12vDC) and 120AC out. it uses a Texas Insts UA747CN dual general-purpose operational amplifier that drives complementary pairs of TI power transistors. Probably one transformer provides the power
for the op amp circuit when powered by AC. The other would be the inverter stepping up the 12vDC to 120vAC.
In either case the op amp changes the RA drive AC frequency in response to the joystick commands. Joysticks usually have a variable pot for each axis, hoever I was able to wire an Atari joystick up to a Nexstar hand controller to provide manual control for a fellow with MS just using four axis conductors and ground.

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#8 Geo.

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Posted 20 March 2013 - 07:19 PM

This is an inverter/controller I'm building that's based on Michael Covington's Alcor design. www.ai.uga.edu/mc/alcor.html

It assumes a low voltage DC Dec motor, controlling your 110vAC Dec would probably require a couple more circuits like these: http://downloads.deu..._PWM_Line_Vo...

I'd be tempted to look for a DC motor.

This is the Covington Circuit on Veroboard.

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#9 dgreyson

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Posted 20 March 2013 - 11:18 PM

Thank you for the schematic George.
The Declination motor on a cave astrola has twin opposing synchronus motors on the same shaft. Common neutral lead. 110vac to one coil slews clockwise, voltage to the other gives counter clockwise rotation. the dec hand controller is a box with two buttons that conduct line voltage to one or the other coils.

There is a 110vac synchronus clock drive motor on the polar axis. This is the factory standard configuration.

I also have the optional drive corrector, which has a transistor oscillator with fast and slow buttons in a seperate handset. it takes 12v and inverts it to variable frequency ac. The fast and slow buttons add resistance / capacitance for the one or the other speed to vary the oscillator fast or slow fron the normal 60 hertz tracking rate. So I definitely need two general purpose mechanical relays to switch the 110vac for dec and two small reed relays close to the buttons mounted inside the handset to switch the R/C network for the drive inverter.






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