omino code blog

About a year ago, I was vaguely wondering if there were any low cost 802.11-ish WiFi modules, to easily add a net-presence to my dinky electronics doodles. There wasn’t; WiFi modules seem to run around $150 or more. (Which is the same price as a WiFi Kodak picture frame, but that’s parts for ya.) But I came across these $19 “XBee” modules and was intrigued…

XBee modules, from www.digi.com (not digikey), implement an RF protocol called ZigBee, 802.15.4. It can run at various power levels, up to about a mile. At 1mW, you get a few rooms coverage. An XBee transmitting at 1mW consumes around 50mA when either receiving or transmitting.

Just recently, on a whim, I got their starter kit which includes two modules mounted on boards, $99.

I have to say, it gave great Out Of Box experience! Just plug one into the USB port, and plug a serial loopback into the other, and fire up their little terminal program, and they’re talking!

So here’s what their $19 module can do:

• Automatically form a repeater mesh
• Default mode is a transparent 9600 baud serial party line
• Can enter “command mode” by sending +++ onto the serial port
• Can configure the module into different modes with AT-commands (snicker)
• Can configure a module’s address, take it off the “party line”, and such
• Has a half-dozen available I/O pins, which can be AT-configured to act as “virtual wires” between modules
• Use ATWR to save the configuration… powers up identically next time.
• Requires 3 volts.

Interlude

The starter kit comes with two boards. The first uses USB for the serial comm port and for power. The second uses a DB-9 with official RS-232 on it, and a 12v adapter. The general purpose I/O lines are just sitting there, without even a header.

Some necessary modifications were in order. I added headers to access the I/O lines. (Pesky wave-solder on the PC board filled in all the holes; clearing them out with a soldering iron, melt-and-slam, works except for the pesky GND, I guess the four layer board’s ground plane is sucking away all my heat.) I contrived a 9v battery to power the remote board. I added some female headers to an LED and a microswitch.

Look, new toys! Some Panavise holders to replace the stupid “Helping Hands Magnifier” from Radio Shack. Sheesh I remember when that looked sooo useful. Also a new soldering iron, with digital readout. ($90 on eBay, search for “tmc soldering iron”, came with two irons and 6 tips, happy with it so far.)

Back To Work

I configured the two boards with a variation of the example in the datasheet:

And ho ho! They can talk.

Good clean fun.