K3's AstroPhotography
"When I consider your heavens, the work of your fingers, the moon and the stars which You have set in place, what is Man that You are mindful of him?" -- Psalm 8:3,4

Modification of GEM1 for Computer Control

As my GEM1 equatorial mount has rather high periodic error, I decided to do something with it. The most walkable way for me it was to modify GEM1 controller unit and add homemade Control Interface Unit into the GEM1 controller box.

!WARNING! You can use the following information on your own risk. Author is not responsible for any damage of GEM1 controller or computer equipment. The following procedure was done successfully by author, but he cannot guarantee, that your modification will be successful.

Basically, Control Interface Unit (CIU) is an electronic switch equipped with computer interface for serial port. The electronic switches are connected to original mechanical switches. There was necessary to do two cuts of original PCB paths (see detail picture below). The electronic switches consists of CMOS switches and 2 relays (the DEC pushbuttons switch also motor drive current - about 150mA). The original GEM1 control unit still remains fully functional also without using computer. The CIU is realised on 50mm x 62.5mm stripboard.

The principle of operation
Serial cable with 4 wires (TxD, RTS, DTR and GND) is connected via 3 opto-insulators to 74HCT595 (8-Bit Serial-In, Parallel-Out Shift Register). Attention! 74HCT595 requires about 5V power supply voltage, while the GEM1 control unit is powered by 12V. That's why 78L05 IC is used for converting 12V to 5V. In 74HCT595 IC the serial data is converted to parallel and its outputs are connected to ULN2003 transistor array. 2 outputs of ULN2003 are connected to relay coils and the other are used for driving CMOS switches - 2x CD4053.

Click the photos to see detailed images.

GEM1 (Gazer Equatorial Mount)

GEM1 Dual Axis Controller

PCB of controller - side of connections

PCB of controller - side of parts

Control Interface Unit - prototype model

GEM1 controller with Control Interface Unit

Control Interface Unit on stripboard

Control Interface Unit placement in original GEM1 controller unit

Wiring CIU to GEM1 controller PCB

The final version of GEM1 computer interface

Detail of cut PCB paths

Schematic diagram

Programming the interface

The control byte sent via serial port is defined by the table:

Bit Meaning
0 RA+
1 RA-
2 RA Stop
4 DEC+
5 DEC-

The modified GEM1 controller enables telescope guiding, PEC and simple GOTO, now. The CIU is controlled by means of K3CCDTools.
In present time only guiding function is implemented in K3CCDTools.

Click the picture to see full sized screen capture.
At first no guiding was used and then autoguiding was switched on.

Very similar interface is used in PISCO project. You can also find description of programming interface with 74HCT595, there.

The page is still under construction. New informations are continually filled up.

See Tests and Analyses of Homemade GEM1 Guiding System

See Homemade GEM1 Autoguiding Setup

Back to Equatorial Mount Tracking Errors page

Back to AstroPhotography page

Computer generated images, real images, drawings and texts are property of the author and may not be reproduced or used without permission of author.


Last Update: 06.10.2002