DIY: Focus Control at the Laptop

[iceman]

Hi all

Dennis Greeve (Dennis G) has kindly written an article describing how he build a motorised focuser so he can have Focus Control at the Laptop.

You can read the article by going to the IceInSpace Projects & Articles (http://www.iceinspace.com.au/?projects) page, or directly by clicking on the link below:

Focus Control at the Laptop (http://www.iceinspace.com.au/index.php?id=63,450,0,0,1,0)

Thanks to Dennis for writing the article. If you'd like to submit an article or review to IceInSpace, please Contact Me (http://www.iceinspace.com.au/?contact).

[iceman]

Article uploaded.

[AndrewJ]

Just for fun, i also recently made a remote focusser,
but for my PST.
I made up a rough new base that allows the fitting of
1) a "gunshield" at the front ( to reduce burning of the top of my noggin )
2) a motor bracket at the back, to run the focuser.
Makes remote focusing in daylight a much easier process
Rough focusing is done by swinging the motor out of the way
and using the knob as per usual.
Once close, the motor is engaged and a remote HBx is used to
control the motor to get best focus whilst sitting at the lappie.

Andrew

[rogerco]

Would it be possible to get an exact link to the motor control circuit that was used in this project, just looking up the guy's name resulted in a big site with many circuits and nothing obvious could be found.

Jaycar in Sydney, Australia has the motors by the way.

Thanks
Roger

[MrB]

You were searching for a motor control circuit huh?
Took a bit of sleuthing to find it, but the circuit he used is Here (http://www.discovercircuits.com/DJ-Circuits/4013oneshots.htm).
The one-shot circuit at top.

Note that it won't directly control a motor, you will need a transistor to handle the motor's current draw. You'll also need a DPDT switch to reverse current to the motor for direction changes.

[MrB]

I drew up a couple of schematics that should help.
First one for the motor driver with direction control.
Second one for a view of the switch wiring(rear view)
Third is the pinout for the Transistor#
Any DPDT* switch will do, be it slider or toggle or rotary.

Note that the base resistor(1k5) in the motor driver circuit is for a 5v supply.
If you run the logic on the same 12v supply as the motor, use a 3k3 resistor.

#The transistor marking(part code) is facing away from you in this view - Metal tab is facing you.
*(Double Pole - Double Throw)

Edit, a pinout for the transistor will help :D

[MrB]

Lets get rid of the direction switch altogether and replace it with a second pushbutton!
Would be easier to control: Push the up button to move the focuser in, down button to pull it out....

Wouldn't be hard, remove the current switch and the motor drive circuitry, add a second one-shot circuit and a H-bridge circuit for the motor drive. Hmmm.
There's a H-bridge circuit Here (http://www.discovercircuits.com/DJ-Circuits/hbridge2.htm) but it would need some logic 'glue'.

[AndrewJ]

Why not just get rid of the buttons as well ;)
My motor control units just use a single pot where off is in the centre
Rotate the pot either way from centre and the motor goes that way at a constantly increasing speed.
Very simple to use one handed

Andrew

[rogerco]

Thanks so much for that effort, I kniw where I am going with it now. The hardware is coming along and I am looking forward to it. Good to have something to dom it yet another cloudy nught uo here in Sydney.

Thanks

Roger

[MrB]

Haha, I rambled-on a bit yesterday, no sleep and lots of coffee :D

Hi AndrewJ, I think the original idea for this particular controller was to have incremental control, kinda like a stepper without a stepper ;), will continue with that theme. Cheers tho.
A two button unit would be one handed too I think, I imagine using a small handheld box with just two buttons, rest the thumb between the two buttons(or even a single self-centring SPDT toggle switch) and just push up or down. Certainly easier to use than a single button with a switch to change direction - no disrespect to Dennis' original tho - I just imagine I would keep forgeting to flick the direction changer and go the wrong way.

Roger, if you're liking the two button/no switch idea, forget the H-bridge in my previous post, too much mucking around, too many components.($$)
I've found the perfect solution tho; a cheap, available, single-chip H-bridge with only one external component needed. Even simpler than the original motor driver!! Will post a schematic later today.

Wish I had a 'scope with a real focuser(has a helical), would make one of these for myself.

[AndrewJ]

Gday MrB

I understood the mechanism used in the initial post, but the general topic was about having remote focusing ability at a laptop.
A single shot incremental unit limits you to lots of small moves if you want to say walk through focus. Also, loading effects due to pointing angle, ( i have a Meade SCT with Microfocusser ) supported load etc will give different results if its only a simple DC motor, and may not always move the same amount for a given impulse.
Having infinitely variable speed control means you can do what you want.
I use mine at high speed to walk back and forwards through focus, then drop down in speed to final tweak.

Note, The above is written based on the fact we have no absolute positional feedback, hence we are still using the Lappie screen view as our means of determining best focus position.

Andrew

[MrB]

Oops! sorry. Nearly forgot.

The chip is a L293D and has a maximum input voltage of 7v, so a 5v regulator is included in the schematic to power the chip.
The two One-Shot circuits and their buttons will also have to be run on the 5v supply so that their outputs don't exceed the limits of the L293D's inputs.
Note that this does not limit the voltage to the motor to 5v, the schematic shows 12v for the Motor(on Vss), but up to 36v can be safely applied to this pin.

Heatsinking is probably not going to be needed with these highly geared motors and their intermittent use. The chips have thermal protection, so if the controller does cut-out occasionally, then you may need to provide some heatsinking. If so, the GND pins (4,5,12,13) are heatsinking pins, leaving more copper track(if using vero-board or designing your own pcb) attached to the pins will help, or solder some copper sheet to them.

InputA and InputB are from the individual One-shot circuits.
Accidentally pressing both buttons at once is safe.
The L293D actually has the ability to drive two motors, so another motor or two solenoids/relays/heaters etc could be added, but heatsinking will likely be needed.

The L293D chip is available from the following Australian sources: (Direct Links)
Altronics (http://www.altronics.com.au/index.asp?area=item&id=Z2900)
Futurlec (http://www.futurlec.com.au/test13.jsp?category=ICOTHERS&category_title=Others&main_menu=IC&sub_menu=ICOTHERS)
Microbyte (http://www.microbyte.com.au/page/shop/browse/a/category/e/components_intergratedcircuits)
Microzed (http://www.microzed.com.au/chips.html)
Wiltronics (http://www.wiltronics.com.au/catalogue/shop.php?cid=377)
of which Futurlec appears to be the cheapest.

[rogerco]

Getting away from the electronics does anyone know of a shop in Sydney that sells small pulleys and belts, I think the notched timing belt type would be best, but I can't find a shop. I want to look at them first, I have found a web site but they have a terrible catelog and are rather pricy so I want to avoid costly mistake by actually handling before purchase. :help:

Roger

[MrB]

You could try contacting a local Photocopier repairer or something.
While not in Sydney, www.smallparts.com.au (http://www.smallparts.com.au/) is well worth checking! :D
Smallparts Belts (http://www.smallparts.com.au/cgi-bin/store/product.pl?alpha=B=post) & Pulleys (http://www.smallparts.com.au/cgi-bin/store/product.pl?alpha=P=post)

As a last resort, RS and Farnell have belts and pulleys of different sizes, though their prices aren't far short of robbery.

RS Mechanical Index (http://www.rsaustralia.com/cgi-bin/bv/rswww/searchBrowseAction.do?N=4294842878&forwardingPage=browse&name=SiteStandard&obs=bObs&callingPage=/jsp/browse/browse.jsp&callingPage=/jsp/homePage/homePage.jsp&BV_SessionID=@@@@1729663984.1202712 260@@@@&BV_EngineID=ccdeadedfidgjijcefeceel dgkidhgn.0&cacheID=aunetscape&Nr=avl:au)
RS Belts (http://www.rsaustralia.com/cgi-bin/bv/rswww/searchBrowseAction.do?N=4294842875&forwardingPage=browse&name=SiteStandard&callingPage=/jsp/browse/browse.jsp&BV_SessionID=@@@@1729663984.1202712 260@@@@&BV_EngineID=ccdeadedfidgjijcefeceel dgkidhgn.0&cacheID=aunetscape&Nr=avl:au)
RS Pulleys (http://www.rsaustralia.com/cgi-bin/bv/rswww/searchBrowseAction.do?N=4294842847&forwardingPage=browse&name=SiteStandard&callingPage=/jsp/browse/browse.jsp&BV_SessionID=@@@@1729663984.1202712 260@@@@&BV_EngineID=ccdeadedfidgjijcefeceel dgkidhgn.0&cacheID=aunetscape&Nr=avl:au)

Farnell Mechanical Index (http://au.farnell.com/jsp/search/browse.jsp?N=1001744)

[rogerco]

Thanks,

I hadn't thought of RS, their prices are the same as the other site I found but at least they have an understandable catalog.

Regards Roger

[MrB]

Oops, sorry, those RS links worked fine yesterday, now they don't.

Be sure to compare prices with Smallparts, some things appear to be 1/2 the price at RS, some 1/2 at Smallparts. Far bigger range of choices at SP.

[rogerco]

No problem I found my way around. Eventually got the two pullies and a belt from Smallparts. The main problem was getting shaft diameters to fit the shaft on the focuser, 2.5mm anyway I ended up with a 3:1 reduction combo which combined with the 10:1 on the focuser and the 36 turns per minute motor should give very slow, perhaps too slow focusing. We will see.

Regards :work:

[MrB]

I'd offer to bore-out the pulleywheels' bores(if undersize) to any size for you on my lathe, but I'll be OS for the next month, after that however I'm glad to help if needed.