I've spent some time to take the concept of an XBee GPS off the breadboard and have built a 2nd generation prototype of an XBee GPS.




First, I wasn't very impressed by the Copernicus GPS I used on the breadboard first generation. In my office, which is on a northwest corner of my house, even with an external GPS antenna, I could rarely get it to lock on.

So, I decided to try a new GPS and went with the Sirf III that Spark Fun is selling. After some initial experiments with the Sirf III, I was very impressed. Even inside my office, with no external GPS antenna, it could get a fix.


Next, I wanted to try to conserve power as much as possible and reduce space with a smaller Lipo battery. So, I went to a 3.7V, 2000 mAh flat lipo from Spark Fun and a low-drop-out 3.3V regulator from Microchips. This regulator has only a 300mv max dropout, so there was a lot more headroom starting with a 3.7 battery through a 3.3V regulator than it would be with a standard regulator that has 1.0 to 1.3V dropout. With that combination I was able to get 14 hour runtimes out of this unit.



This is a side view of the sandwich. The flat, silver lipo battery is on the bottom and protected on top by a piece of lexan which forms the platform for the protoboard. On top of that is the GPS and XBee with the regulator.


This is a bottom view. I velcro'd the battery to the bottom of the case. You can clearly see the two end spars that I custom cut with a band saw to give support to the lexan platform of the GPS and to make a compartment for the battery


The XBee GPS removed from the case but mounted on its lexan platform. The black colored housing connector to the right of the XBee is a custom cable I made which adapted the tiny connector that came with the Sirf III to 0.1" spacing. I cut the Sirf III cable which had an identical connector on either end. I left the end connectors because those will mate with the internal connector of the GPS, but the opposite end I added a 0.1" housing and plugged it into a right-angle header that attached tot he board and provided the interface to the GPS.


The lipo battery with the overcharge/overdischarge cutoff circuit soldered directly to the battery leads.


The flip side of the protoboard.

I was recently asked how I dealt with the crazy small JST connector from the Sirf-III GPS. I searched the net for something like a JST to 0.1" molex or any kind of JST-to-something-else adapter that could be made to work. The only JST housing I could find that might form the basis of an adapter was a surface mount component. I did try to adapt that, but it was so tiny to work with in a non-surface-mount way, I gave that up.

Finally, after hours of searching, I gave up and cut the cable in two parts. Using 1/2, I soldered on some female pins that went into a 0.1" spaced 1X5 pin housing.

A close-up view of final connector looks like this:



It only took about 10 minutes to make this (get the right crimp tools and parts from Hansen Hobbies) and it finally made access to the Sirf III possible. I wish someone made something like JST to 0.1" cable commercially.

My next major phase of this project is to miniaturize it as much as possible by creating a custom circuit board and enclosure. I will also test a few different lipo battery configurations, add external DC charging jack to the case and generally turn it into something more useful.

This shows a quick and dirty wireless GPS I made from a Copernicus GPS and XBee Pro. I use it because my office doesn't get a good GPS signal, so I can set this little gizmo up on the south side where the GPS signal is good and spit the NMEA strings to my office. I catch them with another XBee that pipes them into my microcontroller....it's none the wiser.

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