PCB design questions for telescope drive corrector handbox

Community PCB Design
1

Hey folks,

My name is Steve Rosenow and I was referred here by a member of a car forum.

Before I begin with my question, I have a little background on my project.

I own a Meade LX-3/2080 Schmidt-Cassegrain telescope - with a servo motor/worm gear-driven drive base that dates back to the mid-1980s. These telescopes upon their introduction in 1985 were considered the “cream of the crop” and were the first large-scale deep-sky astrophotography platform available to the mass consumer market. As part of the telescope package, Meade offered users - as an optional accessory - a manually-driven motor drive corrector which allowed the user to completely control the tracking rate ensuring the ability of exposures longer than five minutes. These drive corrector hand controllers, known as the Meade #36 Drive Corrector, were produced in a limited quantity until the end of the LX-3 production run in 1990. Over 10,000 LX-3 telescopes were produced in 8 and 10-inch optics, with less than around a thousand hand controllers. Today, these hand controllers are either failing due to aging electronics, or are becoming extremely hard to source.

Since I am an astrophotographer who actually enjoys these older units, I would like the ability to keep control of my telescope’s tracking rate to ensure I can do exposures longer than five minutes (to eliminate horizontal star trailing due to drive error and to bring out the fainter deep sky objects).

To that end, I have sourced the original Meade wiring/component schematic from an astronomy website, plus a photo illustrating the original hand controller.

[<LINK_TEXT text=“http://farm9.staticflickr.com/8042/8010 … e179_b.jpg”>http://farm9.staticflickr.com/8042/8010422173_ef906fe179_b.jpg</LINK_TEXT>

[Meade LX3/2080 handbox by [Loowit Imaging - Steve Rosenow, Photographer, on Flickr

This is a photo of what the finished product looked like. The rough size is about two inches wide or so, a little over five and a half inches long with a depth of an inch and a half.

[<LINK_TEXT text=“http://farm9.staticflickr.com/8451/8042 … 4ac3_o.jpg”>http://farm9.staticflickr.com/8451/8042698852_99df614ac3_o.jpg</LINK_TEXT>

[LX3Paddle by [Loowit Imaging - Steve Rosenow, Photographer, on Flickr

In light of that, the N/S/E/W buttons are SPDT momentary pushbuttons in a normally-closed circuit. The slight problem in faithfully recreating this controller is that the hardware Meade used (namely, the buttons) is long since out of stock. No supplier I have checked with supplies these buttons. As a result, I am having to use SPDT momentary buttons from Radio Shack (link here: http://www.radioshack.com/product/index … Id=2062541). These switches are vastly different compared to the original switch buttons Meade used.

As for the “QUARTZ/MANUAL” slider switch, it is a DPDT slider (although I have been told that a SPDT slider or toggle will work). The power cord is a six-strand coil cable terminating at a large 6-pin DIN plug.

Using the schematics, I have produced a basic idea of what I hope to achieve, which is a rectangular hand box controller very similar to the Meade original, with a toggle and the adjustment dial for solar/lunar/sidereal tracking speed. From the outside, the layout will be identical.

The main question, is that I am for the most part stuck. How would I go about creating the PCB for this?

Many thanks in advance for any assistance I can get. I have been wracking my brain for a few weeks on it.](About Steven Rosenow | Flickr)](LX3Paddle | Steven Rosenow | Flickr)](LX3Paddle | Steven Rosenow | Flickr)](About Steven Rosenow | Flickr)](Meade LX3/2080 handbox | Steven Rosenow | Flickr)](Meade LX3/2080 handbox | Steven Rosenow | Flickr)

2

Since you are only making one or two and the electronics are simple, I would be tempted to use wirewrap technology and skip the PCB. If you really want a PCD, then start learning Eagle or some other layout tool. Eagle has lots of tutorials on the web.

3

Why are you stuck, exactly? Is it just that you want to make a PCB, but don’t know how?

If so, I agree with fll-freak: don’t bother with a PC board for a one-off. This is a simple circuit and can be built using perfboard like [this and point-to-point wiring techniques quite easily. I don’t do wire wrap any more, but I do use [wrapping wire for my perfboard wiring.

As far as the switches are concerned, give us some more information and we can probably find you switches either identical, or very close to what Meade used.

If you do want to make a bunch of these, then like was suggested, you’ll either need to learn CAD or pay someone to do a PCB layout/prototype for you.](Wire Wrap Wire - Yellow (30 AWG) - PRT-08029 - SparkFun Electronics)](ProtoBoard - Square 2" - PRT-08811 - SparkFun Electronics)

4

I’m mainly stuck on the part of the schematic where the potentiomenter as well as the 6.2k resistor and 10k trimmer pot come in. I need to know where to wire them in and how.

5

If you simply use perfboard or “doughnut per hole” type board, then your trim pot, get connected in with lengths of wire. Your fixed resistors would get put onto the perfboard and connected into the circuit with either wirewrap or some point to point wiring. Open up any old stereo or VCR and you will see how the switches and pots where not often integrated into the circuit board but rather tacked on with a wire harness.

6

Yeah, I’m aware of that. The 1k potentiometer on the diagram as far as the Meade handbox controller goes, was affixed to the top cover of the handbox and wires led to the circuit board (as far as I can see in photos of an actual unit).

As for the switches, I am fine with using the Radio Shack switches, they seem to work fine.

I just need to know where I wire in the potentiomenter and all the resistors into that circuit, and what wiring path I should take.

Eventually I plan on paying for a PCB.

7

We can’t tell you where to place the parts. You need to buy some perfboard and start some trial layouts to minimize the crossovers and interference with other parts. As for resistors and the like, get them through hole and just bend the leads to fit into perfboard holes. One the backside you use wirewrap or other fine wire to make the connections. I find that a wirewrap tool works wll not just on the special wirewrap posts, but also on many through hole components. If not, I strip a bit of the wire wrap, form a loop with tweezers, place it over the lead and solder it in place. I did a 144 pin part once this way.

8

Okay, I think I cracked the code!

Can I get an evaluation of this, based off the Meade schematic, to tell me if this will work? It is a rough draft sketch in Paint.

Any corrections needed?

[<LINK_TEXT text=“http://farm9.staticflickr.com/8032/8046 … 3275_c.jpg”>http://farm9.staticflickr.com/8032/8046460157_e65efb3275_c.jpg</LINK_TEXT>

[Meade LX-3 handbox wiring/PCB schematic - rough final draft by [Loowit Imaging - Steve Rosenow, Photographer, on Flickr](About Steven Rosenow | Flickr)](Meade LX-3 handbox wiring/PCB schematic - rough final draf… | Flickr)](Meade LX-3 handbox wiring/PCB schematic - rough final draf… | Flickr)

9

Steve, did you build this yet?

Jim