Telescope Digital Setting Circles Project



Digital Setting Circles allow a digital readout of the telescope position instead of using the regular mechanical setting circles. The interface board required for the DSC is fairly simple to build requiring only a handful of components.

The project comprises of:


Rotary Encoders and Hardware


Interface board


PIC Software


SkyMap Pro Driver


Rotary Encoders

The rotary encoders and hardware were purchased from Jim's Mobile Inc. See their model list to check out if your mount is listed. Don't worry too much if it isn't, neither was mine, but after e-mailing some pictures of the EQ5 mount I have they told me it was a Celestron CG5 mount. They must have been right as the parts fitted perfectly.

This picture shows the RA encoder installation.


And this is the Declination encoder installation.


This picture shows the complete mount.


Interface Board

The original DSC project interface board circuit was designed by David Ek, and my thanks go to him for allowing me to add my modified circuit to this page.

For details on the original circuit go to David's page at:


Interface Modifications

The modifications I added were for RS232 transmit and receive indicator LED's, and a PIC In Circuit Serial Programming (ICSP) connection point. I also moved the power LED to the output side of the 7805 regulator so indicating that the +5V is healthy and not the input voltage.

The ICSP plug means that I do not have to open the DSC box, remove the PIC from the board, put it in a programmer to re-program it and then replace it in the board and close the box. I simply plug the DSC box into the programmer directly and do it all from there.


Parts Required

The parts required for the DSC project are listed below:


Description Supplier
U1 7805 Voltage Regulator Maplin CH35Q
U2 PIC 16F84A + 18 Pin Socket RS 379-2897 + 402-787
U3 MAX232 RS232 Interface + 16 Pin Socket Maplin FD92A + 402-771
C1, C4-C7 10uF 25V Electrolytic Capacitor Maplin VH07H
C2, C3 0.1uF monolithic Capacitor RS 115-578
R1-R13 10K .25W Resistor Maplin G10K
D1 - D3 1N4001 Maplin QL73Q
LED1 - LED3 Rectangular Style LED RS 178-0937 (Red) 178-0959 (Green)
OSC1 4MHz TTL Crystal RS 316-677
J1 9W Female 'D' Type RS 239-5855
JP1 / JP5 2W PCB Header Maplin RK65V
JP2-4 5W PCB Header Maplin FY93B
Not on Diagram Enclosure Maplin LH14Q
Not on Diagram 5W Socket Housing Maplin BH66W
Not on Diagram 2W Socket Housing (Power+Reset) Maplin HB59P
Not on Diagram Solder Pins Maplin YW25C
Not on Diagram Power Connector + Plug Maplin FT97F + HH62S
Not on Diagram On/Off switch SPDT + Cover Maplin FH04E + JR79L
Not on Diagram RJ45 Connector RS 331-6437
Not on Diagram 16W IDC Cable Mounting Plug (ICSP) RS 121-0595
ICSP Cable 16W IDC Socket (ICSP) RS 409-868
ICSP Cable 16W DIP IDC Connector (ICSP) RS 100-8594
ICSP Cable 16W Ribbon Cable (ICSP) 0.05" Pitch         -

The items marked as Not on Diagram are parts that are fixed to the enclosure and therefore do not form part of the PCB.

The items marked ICSP Cable make up the In Circuit Serial Programming lead.


Circuit Diagram

The circuit diagram for the DSC project is shown below.

The board layout is shown here. It is obviously similar to the original but includes all the modifications I made.


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. Wire Links. Solder in the three wire links.

  2. Resistors. They have no polarity and can be inserted either way round. NOTE: R1 + D3 are joined in series and inserted into the board as a single component. See image below.

  3. 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.

  4. IC Sockets. You donít have to use sockets for either device, (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 ICís in their sockets yet. Pin 1 of the IC's on the PCB is designated using a square pad instead of the usual round ones.

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

  6. 0.1 mF 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 has an input, common and output connections. The input connection is to the left of the device when it is held such that you are able to read the device number. The second connection would then be the common and the last connection the output.

  9. Now install the connectors for the encoders, ICSP and serial connections.

  10. Install the reset switch if desired. I installed a two way connector although I have not installed a switch yet to connect to it.

  11. Install the power connector.

  12. Finally, install the ICís in their sockets, making certain they are installed in the proper orientation. Don't forget to program the PIC16F84 before testing.

The picture below shows the finished board with all components installed. The LED's are 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 enclosure I used was from Maplins (Part no LH14Q). The holes for the on/off switch, power input, RS232, programming lead and encoders were made first. Next I made the fixing holes for the PCB and then was able to accurately place the holes for the LED's and fit the board.

Once all of the holes were made I fitted the switch and power connector.


Programming Lead

The programming lead required a length or ribbon cable to be attached. Since I do not own the correct tool for the job I always find that a vice is adequate. Ensure that the cable is as straight as possible before using the vice.

Since the programming connector would have been crushed by the vice, a mating connector was plugged into it before using it.

The connections for the programming connector are shown in the table below.

Pin Name Programmer Pin Ribbon Connection JP4 Connection
MCLR 4 8 5
VSS 5 10 1
RB6 12 13 3
RB7 13 11 4
VDD 14 9 2


The Programming Lead and the DSC Programmer Connector


The programming lead is inserted into the programmer, (I have a PicStart Plus programmer), where the IC would normally be placed. Be careful to make sure that pin 1 on the programmer matches pin 1 on the connector.

The connector for the DSC can only be inserted one way around with the polarised connector that I used. If you do not use a polarised connector make sure that the plug is inserted the correct way around.

When reprogramming the PIC do so in the following order:

bulletSwitch off the DSC
bulletConnect the programmer
bulletProgram the device
bulletDisconnect the programmer
bulletSwitch on the DSC


Encoder Connections

The encoder cables that came with the encoder and hardware kit were terminated with an RJ45 connector. The internal DSC encoder cables were made from old PC cables, used to connect the front panel components (LED's reset switch etc), to the motherboard. The pitch of these connectors was the right size, 0.1 inch, and already had crimped connections one end. I found two that had four such connections in one plug, one for each encoder. If you do not have any of these connectors then equivalent part numbers are listed above.

The encoders RJ45 socket was an old network connection socket that I had and was already on a small PCB of its own. I cut this PCB to a suitable size and then soldered the cables I had to it, before fixing it in place with some silicon sealant. If you do not have a connector then the equivalent part number is listed above.

Below is a diagram of the encoder cable connections showing the RJ45 pin numbers for each encoder and the board connections.




The Encoder Cable RJ45 Connector And Internal Wiring


The Internal Connections All Plugged In


The Completed Project Showing The LED's On The Front


PIC Software

The PIC software for the 16F84A may be downloaded HERE. The file is in Intel HEX format.


SkyMap Pro Driver

I decided that I wanted to interface the DSC project with Sky Map Pro which would require me to write a driver as well.

For details on the driver go to the SkyMap Pro Driver for DSC Project page.

When I find some more time I will write an ASCOM driver for the DSC as well.


Check out the DSC project update with a new PCB that includes the Electronic Focuser.

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


Return to main telescope page


Copyright © 2004 - 2007 Phil Davis


Last updated 29th July 2014