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Telescope Digital Setting
Circles + Focuser Mark II PCB

Introduction

Having added the focuser to the original DSC project board, I decided to design a new Printed Circuit Board (PCB) to remove the need for all of the internal wiring, which had now got a bit out of hand.

Original PCB

As can be seen from the original photo of the finished DSC project the wiring does leave a bit to be desired.

The focuser board below would also be added to the design as opposed to the custom fit (bodge) of the original which can be seen above just below RS232 connector.

I decided that the new design would have all PCB mounted sockets so that no wiring was required at all.

Another modification to the original was the addition of a higher powered voltage regulator and heat-sink, as I had found that the original 7805 1A regulator got hot during use after I had added the focuser mod.

This was because I had been only inputting 7.5V into the regulator before the mod, but the focuser required 12V for the stepper motor, and used the input voltage of the regulator as the supply, so it had to be increased to 12V.

I used an old regulator, of which I had several knocking about, which is rated at 5A. This had been already been retro-fitted to the original design and proved to be quite effective.

Design Tools

To design the PCB I used Easy-PC for Windows V6 from Number One Systems.

To check the outputted Gerber design files I used GC-Prevue from GraphiCode.

The printed circuit board was manufactured by PCBTrain.

PCB Layouts

The two sides of the new PCB are shown below.

The bottom layer of the new DSC project board.

The top layer of the new DSC project board.

Parts Required

The parts required for the new DSC project are listed below:

Circuit Diagram

Click on the diagram below for a larger image of the circuit of the new DSC project.

Circuit Board Construction

The order in which I install the parts on a PCB is always the same. Simply start with the component that has the lowest profile on the board and work upwards. This usually means starting with any wire links or resistors. Here is the order that I used on the project.

1. Resistors. They have no polarity and can be inserted either way round.

2. Diodes. Make certain you install them with the proper polarity according to the circuit diagram. One end of the diode has a band around it. Align the component so the band on the component matches the band part of the diode on the circuit diagram.

3. SIL resistor and IC Sockets. You don’t have to use sockets for the IC's, (although I would recommend it), since the PIC can be programmed without the need for it to be removed. Ensure that the sockets are the correct way around. Do not install the IC’s in their sockets yet.

4. Oscillator. Make sure that the device is installed the correct way around according to the PCB layout.

5. 0.1 uF capacitors. They do not have a polarity so either lead can go in either hole.

6. 470nF and 220nF capacitors. They do not have a polarity so either lead can go in either hole.

7. Electrolytic capacitors. These devices do have polarity so ensure correct installation. Normally the negative side of an electrolytic device is marked on the body.

8. Voltage Regulator. This device can only be fitted one way around. I also fitted a heat-sink as the original became fairly warm.

9. Install the two and three way PCB headers. Fit a link to PL3 and PL4 as shown in the photo below.

10. Install the RJ45 encoder connector.

11. Install the 16W IDC programmer connector.

12. Install the RS232 connector.

13. Install the 5 pin DIN focuser connector.

14. Install the power connector.

Final Assembly

The picture below shows the finished board with all components installed. The LED's will be installed on the underside of the board since the board is fixed on the front of the box and they need to show through.

The PL3 and PL4 links are to control the stepper motor controller IC mode of operation. The linking shown below emulates the original focuser project.

Final PCB Assembly - Much neater I think

Enclosure

The enclosure I used was from Maplins (Part no LH14Q). As can be seen from the picture above the board was designed to be as large as possible to fit the box. First I made the fixing holes for the board and fitted M3x10mm screws to the lid with an M3 nut.

The board was then placed on the screws and fixed using M3 washers and nuts.

Next, using a cut off from one of the resistor legs, I marked the inside top of the lid by pushing the leg down the LED's component holes in the board. The board was then removed from the lid and holes drilled, between each pair of marked holes, for the LED's to show through.

The LED's were then soldered to the underside of the PCB and the board fitted to the lid for the final time (I hope).

LED's showing through top of box

The holes for the RJ45 connector, Programming connector, RS232 connector, Focuser connector and power input connector were made next.

RJ45 Connector Hole

Programming, RS232 and Focuser Connector Holes

Power Plug Connector Hole

The finished box with all the cables connected

Testing

To start with I ensured that the IC's were removed. Next, with an ohmmeter, I checked for shorts between ground and the +12V and +5V power rails of the board. No shorts fortunately.

I then connected the power and measured the +12V and +5V rails. Both were perfect.

I programmed a PIC device with a small program that simply flashes the two Rx and Tx LED's alternately. I programmed the device using the programmer and NOT the programming lead. Once the PIC had been placed in its holder I powered up and to my delight the LED's flashed. The power LED also worked.

Now I knew that the PIC was running correctly I tested the programming interface. The PCB was attached to the programmer and the current program verified. Verification was OK so communication between the PCB and the programmer must have been achieved.

I then compiled the real DSC software and programmed the device. Again it verified correctly.

I installed the MAX232 RS232 IC, and hooked up the board to the laptop to test the DSC command set. Again all was well.

Since the lead from the focuser on the telescope and the original DSC was a keyboard extender, one end had a male connector and the other a female connector. As both the focuser and the DSC now have female connectors I had to make a small gender changer lead with a male connector at each end.

Gender Change Lead

I plugged the small lead into the DSC and the other end to the extender and finally the extender to the focuser. I tried the focuser command set and, once again, all worked fine.

All that was left to test was the encoders connection to the DSC.

The tripod and mount were assembled and the encoder connections made. I started SkyMap Pro and ran the setup procedure for my scope. Moving the scope in any direction moved the crosshair on the screen correctly.

The new DSC is now fully tested and waiting for clear skies.

Spot The Difference

PIC Software

The PIC software for the 16F84A is the same as for the original project and may be downloaded HERE. The file is in Intel HEX format.

New DSC Mark III now designed and about to be built.

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