Featured application: WiFi GPS

IMPORTANT! Pin Numbers (P$nn) on schematic above are for the RN-131 G component. The pins for the RN-121 through hole board are DIFFERENT. My schematic drawing software fought my efforts to remove them. Use the WiFly 121 module documentation for the proper pin-outs.

ALSO, the GPS module shown above is generic and those are NOT the pin outs for the LOCOSYS module I used. The signal names are correct. Note that the TX of one is connected to the RX on the other, etc. Again, consult the data sheets for the module you use. I just picked a generic module available in my schematic drawing software.

So here's the parts list:

1 - Roving Networks WiFly RN121 Module, Digi-Key part # 740-1037-ND

Note that you can get these from Mouser as well. part #765-RN-121G. Either place $69.00

2. LOCOSYS LS20031 GPS Module, SparkFun part # GPS-08975 $59.95

Note that you can substitute almost any 3.3V GPS module you like here.

3. 3.3 V regulator. Mousr Part # 595-TLV-1117-33CDCY $0.77

Any 3.3V regulator will do. The two modules draw less than 100ma when running. You could probably skip the regulator and just power this from a couple of AA Alkaline cells in series but I don't know how long they'd go before the voltage would "sag" below that needed for operation of the GPS. I run mine off 4 AA cells and it will go for almost an entire day (+12 hours) of use. I use rechargeable AA's. The WiFly seems to keep operating long after the GPS has quit as the voltage sags.

How you package it it up to you. I used a Radio Shack part# 276-150 General Purpose Circuit Board for my prototype. I am in the process of designing a commercial quality board based on this prototype.

Dec 5, 2010 - WiFi GPS prototype is successful!

You mght be wondering why this is under the iGeoCacher section. I'm glad you asked. I've long wondered why Apple hasn't added the GPS to the iPod line. They have put it in the 3G version of the iPad. I suspect it is because they are using a combined radio/GPS chip so that they only put it in devices with the cellular chip.

The other "puzzler" for me as a developer is why Apple never implemented the BTSPP (BlueTooth Serial Packet Protocol) or if they have, they haven't made it available to developers. Bluetooth on the iOS4 is locked up tighter than a drum.

It seems prettly clear that they intend for ad hoc communications to be via TCP/IP and so I've been following the progress of one or two commercial companies that published "plans" to have a WiFi GPS. Alas, most of these never made it to the market for some reason. The one that did is pricey at $200!!.

So I decided to "roll my own" and the schematic you see to the left is the first fruit. Leaving out the GPS module of your choice, the other parts will cost you no more than $75 if you buy them all. The main one is the Roving Networks RN-121 WiFly module. It lists for $69 and I got mine from Digi-Key Electronics. I'll provide a complete parts list for the one I built this weekend later.

See YouTube Video: http://www.youtube.com/watch?v=S-DGuxuDpy4

The short story is this: This is EASY to build if you can solder. There are just FIVE connections to the WiFly module, FOUR to the GPS module and THREE for the power regulator chip.

I chose to use a LOCOSYS LS20031 GPS module from SparkFun because it has an on board battery to remember last location (helps for quick startup) and it had good specs and a lot of DIY GPS folks seem to like it based on some of the forums I've looked into (like DIY Drones et al.)

The bottom line is that this unit allows you to connect using an "ad hoc" TCP/IP socket connection and get the industry standard NMEA GPS data stream. I'm in the process of modifying iGeoCacher to be able to use this alternative data stream which will enable those of you with iPod Touch and non-3G iPads to play and you will be able to enjoy the full features of iGeoCacher that your iPhone brethren have enjoyed out in the field.

I find WiFi is nice because there are no pesky "tether" cords to get tangled up. This means that there are all kinds of possibilities for taking this into the field. I have a plastic clipboard/case that I take with me in to the field to hold pens, stickers, swag and to have a nice place to write log entries. This GPS fits nicely inside and one of the sub compartments fits the battery holder nicely. Use your imagination.

The GPS antenna is built into the RN-121 but there is a connector for an external GPS antenna and Roving Networks sells a pig-tail cable to connect the small connector on the RN-121 to the usual GPS antenna connectors. I will probably test this out to see if it helps those "deep woods" signal situations we all encounter. I've even thought of building the whole thing into a walking stick with the antenna on top. When you control the packaging, all manner of things come to mind (GPS hats?)

Have fun. I'll write more as I learn more but feel free to call on me if you have questions or suggestions.

d o n [at] b a b c o c k . o r g

I'm happy to correspond.