Telescope equatorial mount motor controller

This is an Arduino-based controller for RA and DEC motors. I've installed this on both a Great Polaris mount and an old Meade Starfinder. This project includes all the printed parts for the Starfinder installation. Included in the project:

• RA motor mount. The RA motor replaces the 1/10 RPM motor in the original Starfinder. The motor attaches to the mount with four M3 screws, and the mount attaches to the gear assembly with two M4 screws and nuts. The motor drives the worm gear directly with a printed part that mates to the worm gear shaft.

• DEC motor mount. The DEC function requires the declination arm option offered by Meade. The motor drives the declination arm screw via a GT2 timing belt.

• Motor housing. This replaces the original plastic housing for the RA gears. It includes mounting locations for the controller PCA and all the connectors and switch.

• Control pad. Includes a housing for the keypad PCA, RJ45 connector, front panel, and keys.

The project includes two printed circuit assemblies, a controller PCA based on the Arduino NANO and two DRV8825 stepper motor controllers, and the keypad. You can order printed circuit boards for either of these through JLCPCB at the links below.

The current draw for this controller and motors is 980mA.

Bills of materials:

Controller PCA:

• Printed circuit board. https://oshwlab.com/charlesodonnell/gp-mount-motor-control_copy_copy

• Arduino NANO. https://a.co/d/faer9xY

• DRV8825 stepper motor controller (2 req'd). https://a.co/d/9mnK7W8

• 47 uF 25V electrolytic capacitor https://a.co/d/gXJ10xh

• 2-pin terminal block (2 req'd). https://a.co/d/e4GhavR

• 4-pin terminal block (2 req'd). https://a.co/d/e4GhavR

• 8-pin terminal block. https://a.co/d/dpvWNWE

• Sockets (optional). https://a.co/d/iob9XRY

Keypad PCA:

• Printed circuit board. https://oshwlab.com/charlesodonnell/motor-control-keypad

• Pushbutton switch, MC32829 (6 req'd). https://a.co/d/hdghlzt

• 8-pin terminal block. https://a.co/d/dpvWNWE

RA motor:

• NEMA-17 2.0A stepper motor. https://a.co/d/3TkMI4R

DEC motor:

• NEMA-17 1.5A stepper motor. https://a.co/d/1abx970

Misc. parts:

• GT2 timing belt, 2mm pitch, 162mm long. https://a.co/d/eOuzwsJ

• RJ45 connector (2 req'd). https://a.co/d/aUa2zUw

• 12V power socket 5.5x2.1 mm. https://a.co/d/3yB7RVw

• 5-pin aviation connector, male + female. https://a.co/d/16HfTAQ

• Miniature switch, SPDT. https://a.co/d/gX1wspX

• 8-conductor coiled cable, RJ45 connectors. https://a.co/d/cQN9WJt

• M3 screws & nuts

• M4 screws & nuts

• No. 4 x 3/8" sheet metal screws

Printed parts:

• RA motor interface. Mounts to the shaft of the RA stepper motor with M3 hardware. Mates to the shaft of the RA worm gear.

• RA motor mount. Mounts to the RA gear assembly with M4 hardware. RA stepper motor mounts to this part with M3 screws.

• Control housing. Replaces the original gear assembly housing. Provides mounting locations for the controller PCA, power jack, DEC motor, Solar/Sidereal switch, and control panel cable

• RA motor cover. Slides into the control housing over the RA motor.

• Zip-tie anchors. (4 req'd). Epoxy to the control housing to tie down cables with zip ties.

• DEC motor mount. Mounts to the declination arm assembly. DEC stepper mounts to this part with M3 screws.

• GT2 pulley 48T. Mounts to the shaft of the DEC stepper motor with M3 hardware.

• GT2 pulley 32T 1/4x28UFT thread. Replaces the adjustment knob of the declination arm screw. Secures with M3 hardware.

• Focuser control housing. Provides mounting location for the control panel keyboard.

• Motor control panel. Cover with legends for the control panel.

• Focuser control key (6 req'd). Pushbuttons for the controller.

Assembly notes:

• I recommend sockets for the NANO and DRV8825s, but you may be comfortable soldering them directly to the PCB.

• The DRV8825s must have their current limits set. I set the limit to 2A for the RA motor and 1.5A for the DEC motor. You can find the procedure here: https://www.makerguides.com/drv8825-stepper-motor-driver-arduino-tutorial/

• The 47uF 25V electrolytic is not optional. This is to protect the drivers.

• You must compile and load the sketch to the Arduino. The sketch is available at the link in the BOM above. When compiling the sketch for the Starfinder, comment out the line "#define GP"

• The slide switch selects either sidereal (star) rate or solar rate. The motor runs at sidereal rate if the switch is open and the solar rate if closed.

• The slide switch is mounted to the control housing on two plastic posts and heat-staked in place with a soldering iron. The result appears to be solid.

• I recommend printing the zip-tie anchors and gluing them in place with 5-minute epoxy. There are four outlines in the housing for the placement of the anchors.

• The 8-pin terminal block on the keypad PCA is soldered to the underside of the board, opposite the pushbutton switches. The connections to the terminal block are shown in the schematic at the link in the BOM. Use a zip tie to secure the cable to the PCA through the two holes provided.

• PCAs are mounted with No. 4x3/8" sheet metal screws. The control keyboard panel is also mounted to the case with No. 4x3/8" sheet metal screws.

• When printing the panel with legends, edit the gcode file to insert an M600 command (filament change) at Layer 10 (assuming 0.2mm vertical resolution). Start the print with white PLA. When the print pauses, change to black PLA to print the legends.

• Pay attention to the connections to the 8-pin terminal block on the controller and keypad PCAs. It's easy to get these reversed, or to stake the wires to the wrong legend on the RJ45 connectors.

• Also pay attention to the connections to the 5-pin aircraft connectors for the DEC motor. It's easy to get these reversed, too.

• I highly recommend heat-shrink tubing for all soldered connections to connectors and the switch. I use 3/32" tubing, shrinkable to 3/64".

Most parts are printed in PLA, although I printed the keypad keys in translucent PETG. PLA parts can be printed at 0.2mm resolution. I don't recommend printing the RA motor interface, DEC motor mount, and GT2 pulleys in PLA. I printed mine using Siraya Tech Blu resin (https://www.amazon.com/Siraya-Tech-Lava-Black-Engineering-Grade/dp/B0D95KDJCK/?th=1). PLA will not hold up in this application for those parts.

Category: Hobby