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Homebrew GPS module for Celestron Nexstar Telescope mounts

My el-cheapo Celestron telescope, with a computerized Go-To mount, didn't include a GPS module, which meant I had to re-enter date, time, and location every time I turned the thing on. By building a simple clone of the Celestron GPS Accessory (for a LOT less money) this problem was eliminated.

The know-how for all of this comes from this site/thread:
https://www.cloudynights.com/topic/567421-homebrew-celestron-compatible-gps/

That thread discusses several versions of the design as it evolved, and IMHO the best version was that by user Lord Beowulf, hosted on GitHub:
https://github.com/LordBeowulf/nexstar_gps

I have taken the Arduino code from there, observed it in action with a digital logic analyzer, and then fixed a few things. Basically I tidied things up, removed some triplicated code, changed the serial protocol to 19200_8N2 to match what I see on the logic analyzer, and also added logic to wait for RTS to go high before dropping it again and transmitting a response.

My version of the arduino code is here: nexstar_gps_mlord.ino (right-click and SAVE).

Here is a simplified hook-up diagram, based on Lord Beowulf's schematic. The only change there is I added two random diodes (from my junk box) in series with the GREEN +12V power supply from the AUX bus, to drop the voltage there by a couple of volts, just to give the built-in power converter on the Pro Mini board a bit more margin. Not necessary, but an easy precaution if you have a couple of diodes laying around that can be salvaged for this job.


IMPORTANT NOTE: The 6P6C plug/cable I used has the wire colour order reversed from standard wiring for 6P6C. This adds unneeded confusion, but emphasises the requirement to verify end-to-end that the signals end up on the correct pin numbers, regardless of actual colours. Pre-made cables often get the colours reversed (as here). If the signals end up on the wrong pins, the telescope mount could be irreversibly damaged.

If you want to try building this, then here (Oct/2020) is an Amazon.com link to a pair of 5V Arduino Pro Mini boards along with a USB-Serial convertor for programming them: WayinTop 2pcs Pro Mini ATMEGA328P 5V 16MHz Development Board + FT232RL .. (no affiliation with the seller). Similar/better deals may be found at AliExpress, Sparkfun, AdaFruit, eBay, RobotShop, etc..

And here is an Amazon listing for the BN-220 GPS module that is required for this circuit: BN-220 GPS Module (again, no seller affiliation here). As with the Arduino boards, similar/better deals may be found at AliExpress, Sparkfun, AdaFruit, e Bay, RobotShop, etc..


A Logic Analyzer trace showing the finished protocol in action is here:

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November/2020: Updated sketch for slightly better handling of RTS signal.

October/2020: Published sketch (program) for Arduino Pro Mini (clone) board.