Adding bluetooth support to SynScan

[rkc]

I had a bit of a dilemma, choosing between using my Synscan handset, or a laptop with EQMOD, or SkySafari, or Stellarium, or Astrotortilla, or ... to control my mount (AZ-EQ6). Each seemed to need a different configuration of cables, possibly having to switch over after aligning.

Then it struck me that a serial link to the synscan covered most bases (since you can put the synscan into PC Direct mode to use it with EQMOD, or leave it in standard mode to use with Stellarium or SkySafari. But two wires into the SynScan felt messy.

THEN it struck me that a bluetooth to serial adaptor might work. Still a bit messy hanging off the SynScan. And how would I power it?

THEN it struck me that a small bluetooth to serial board like the HC-05's available for a fiver or so on eBay might fit inside the Synscan controller itself. And the controller has 12v power (of course, you can use the 12v power of the controller even if you use an external BT adaptor, since it's available on the RJ11 port).

So I opened up the SynScan controller and took a look, and sure enough, behind the screen there is a space just about large enough and deep enough for a BT adaptor and a small circuit board. Better yet, there is a 5v regulator meaning it's easy to find power for the BT board.

The inputs to the RJ11 'multi-purpose' port are at genuine RS232 levels, while the BT adaptor operates at TTL levels (actually most models operate at 3.3v), so a MAX3232 driver was also needed - I used one already mounted on a breakout board from SparkFun. A 3.3v LDO regulator one resistor, and one capacitor were the only other things I needed (I suspect it would _probably_ have worked just driving everything at 5v, and I suspect if I had looked hard enough I might have found a 3.3v supply on the SynScan board, but I had a regulator lying around and I didn't have any spare SynScan controllers or BT modules, so I figured better safe than sorry.

My first attempt at connecting everything up was a bit of a failure, as by the time I had connected everything to a stripboard, it was too fat to fit in the available space. But my second attempt wiring the two boards together directly, with a tiny circuit board for the regulator, fits just fine. I used a 2-pin header and plug so that I can disconnect the SynScan from the MAX3232 should I need to connect a standard serial port to the controller (for example to update the firmware).

Tried it all out last night, and it worked flawlessly. I run a network to serial proxy on a Raspberry Pi to allow control from SkySafari, or I can connect directly from Stellarium on my Mac, naming the bluetooth tty device as the port. THe hardest part of the project was actually getting bluetooth set up on the Raspberry Pi - this would not be needed if SkySafari on the iPad supported Bluetooth (as I believe the Android version does).

[rkc]

I uploaded some images showing the mod, in case anyone else is mad enough to try it

[chrisshillito]

Very neat Mod!

A while back I added instructions to the EQMOD project site on how to build a BT EQDIrect using the same kind of modules. One potential issue we've found is the inherent latency in comms transactions. Tests on the the EQDirect-BT showed transactions were taking around 50ms compared to 16ms using a directly wired cable. PC-Direct itself imparts its own overhead. For general mount controls these short delays aren't going to present any problem - but for pulse guiding such overheads could represent a significant proportion of the pulse duration - quite how, or if, this will affect guiding performance I've yet to establish. If you'd like to do some latency tests on the EQMOD/BT/PC-Direct combination the latest test release of EQMOD (available via the EQMOD group) includes a comms diagnostic within the toolbox application.

Please note that it isn't advisable to dip in and out of PC-Direct mode on the fly. It is a fundamental requirement of EQMOD that it must be must be in sole command of the mount at all times. Switching to any other method of control is likely to mess things up unless you have established a very controlled workflow that keeps all parties happy.

Chris.

[rkc]

I read the instructions for a BT EQdirect when designing this - in fact the original plan was to build one just like that...

I must admit I have not thought about latency a great deal - I will do some experimenting. I see myself using this mostly for visual though. In a setup with guide camera, imaging cameral, filter wheel, focuser control, etc etc one more cable is not going to make a lot of difference!

One thing I did find (slightly latency-related, I suppose) is that you need to keep the tty device for the bluetooth serial link open, rather than opening it on demand. In particular my network <-> serial proxy that I run on the Pi needed to be adapted to keep the serial port open all the time as SkySafari opens a new network connection for every command it sends (and it sends a position request once a second).

I have added a schematic to the gallery

[Mailer2010]

Hello rkc,

do you know if it's possible to build in a Bluetooth module into a Synscan Handset without the Sparkfun MAX-3232 module? Is it possible to connect it somehow directly to the MAX-323 (5V) module on the synscan board an doing some voltage regulation of VCC and TTL levels?

I currently have a plain HC-06 bluetooth board (only the green board without pins and integrated voltage regulator), one HC-06 with integrated voltage regulator (green bluetooth board on blue breakout board).

I also have several TTL level converters from 5v to 3.3v. But all solutions require a 3,3V source, the synscan handset however only has 12v and 5v supply points.

Greetings

flo