A "Minimal" Arduino/XBee/Pachube Sensor Network

One or two people have asked in online forums for code from my sensor network, and while I'm usually happy to share, it's full of a lot of extraneous stuff (RTCs, NTP, displays, thermocouples) that might not be of interest to everyone, but more importantly, that probably only obscures the fundamentals of XBee networking and communicating with Pachube.

So with that in mind, I cut out the extraneous stuff, and just left the bare essentials: A remote sensor unit which transmits a single reading from a photocell once per minute to a base unit which connects to the internet and forwards the data on to Pachube.

Bill of materials -- Base unit:
(1) Arduino Uno, Through-hole or SMD edition
(1) Arduino Ethernet shield
(1) XBee ZB low power Zigbee Module, Digi Product ID XB24-Z7CIT-004, XB24-Z7WIT-004, XB24-Z7SIT-004, or XB24-Z7UIT-004 (these differ only in the antenna, they are functionally equivalent)
(1) Adafruit Industries XBee Adapter
(1) LED for heartbeat (optional)
(1) 330-ohm resistor for heartbeat LED (optional)

XBee configuration:

• Load the Zigbee Coordinator API firmware (I'm using Version 2170).
• Set the PAN ID as desired, to match that of the sensor unit(s).
• Set API mode 2 (AP=2).
• Set baud rate to 9600.

Bill of materials -- Remote unit:
(1) Arduino Uno or similar (Through-hole, SMD edition, Boarduino, etc.)
(1) XBee ZB low power Zigbee Module, Digi Product ID XB24-Z7CIT-004, XB24-Z7WIT-004, XB24-Z7SIT-004, or XB24-Z7UIT-004.
(1) Adafruit Industries XBee Adapter
(1) CdS photocell
(1) 10K resistor

XBee configuration is same as above, except:

• Load the Zigbee Router API firmware (I'm using Version 2370).

Here are the sketches.  Unzip into the Arduino Sketchbook folder.  Before uploading, be sure to disconnect the XBee from Arduino pin 0.  Also be sure to enter your MAC, IP, and Pachube API key in the base station main module, and your Pachube feed number and the address of your network coordinator XBee in the main module for the remote unit.

Here is the Pachube feed. Pretty boring, but at least you can see that it does work. Not sure how long I'll leave it running, probably until I need the parts for something else!  Feedback and questions welcome!

Note that in the following wiring diagrams, the XBees plug into the Adafruit adapter, which in turn plugs into the breadboard. The XBee pins do not (in fact, cannot, spacing is different) connect to the breadboard.  The adapter plugs into the breadboard, this is represented by the 10-pin header below the XBee.

Wiring the base unit

Wiring the remote unit

My base unit

My remote unit (using a Boarduino)

Easy NiMH discharge curves

This post is a bit of an aside, or at least it started out that way.  I thought it worth noting though, because it's a good example of how quickly and easily a new Arduino/Pachube application can be put together, once you have some basics in place.

A friend was having some difficulty with some older NiMH rechargeable batteries (actually AA cells).  He'd charge them up, but when he put them in his camera it would immediately complain that the batteries were low, and it wouldn't turn on.  The batteries seemed OK when checked with a voltmeter, reading 1.4V or a little better (the nominal voltage for NiMH cells is 1.2V).  My friend was aware that batteries are best tested under load, and I gave him a few resistors for that purpose, and he went off to try again.

In the meantime, I was looking at my recently-completed XBee/Arduino/Pachube lashup (see prior post) and a light went on.

I wired up two NiMH cells with resistors for loads, connected them to two analog inputs on the Arduino, added a few lines of code to the Arduino sketch, and literally within 10 or 15 minutes I had Pachube collecting the voltage data once per minute (in addition to the temperature data that I was already collecting).  It's taken me a lot longer to write this post than it did to get the thing working in the first place!

Now I could produce nice discharge curves for two batteries simultaneously.  It was just a matter of popping them into the battery holders, then waiting for 8 or 10 hours while they discharged and Pachube collected the data.  In the end this gives a heck of a lot better picture of what's going on than could have been accomplished with a voltmeter, and with a lot less effort!

This first chart shows two cells that I had.  The first was relatively new (~8 months old), and the other a few years old, and while I no longer used it in my camera, it still seemed to work OK in my old iRiver MP3 player.  You can see Cell A held 1.2V for six hours or so, but Cell B dropped below 1.2V after about only an hour.  What surprised me is that the curves aren't that different, just shifted down for the older battery.  If you do the math on the mAh capacity, it comes out pretty close!

The second chart shows two of my friend's cells that wouldn't work in the camera.  Cell C looks similar to my Cell B, but Cell D drops below 1.2V almost immediately, and after an hour, is below 1.1V.  Note the sharp initial drop in voltage.  This leads me to suspect that as these cells age, their internal resistance goes up, while overall capacity may only decline modestly.  This is contrary to my gut feel; when a battery doesn't work I think it's dead, empty, out of capacity.  Actually these older batteries might work OK in devices that aren't terribly voltage-sensitive.  Obviously the cameras watch the voltage pretty closely and let you know about it when it drops only a few tenths of a volt.  But consider the challenge: You need to know you're running low far enough in advance of the knee in the curve, because things go south in a hurry then.  And the camera has to make some sort of assumption about the quality/age of the cells.  Or maybe the camera people are just in cahoots with the battery companies ;-)  A 5-ohm load was used on all cells.

A simple wireless sensor network

A lot of pieces come together in this project.  It seems to be working well; I'm pretty happy with it.  I'll jump right in and give a high level description of the pieces.

I started this blog with digital clocks and this project starts with one as the sensor unit.  It consists of a Arduino Pro Mini 328 (check the size of that guy!), a "Chronodot" Real-Time Clock (RTC) module from Macetech based on the Maxim DS3231 chip, and an XBee ZigBee module from Digi International (part no. XB24-Z7WIT-004).  Just for fun and to generate a little more data to collect, I connected a simple Cadmium-Sulfide (CdS) photocell to one of the Arduino's A/D inputs to measure light intensity wherever the sensor unit happens to be.  The readout is a 16x2 LCD from Sparkfun.  See the picture below.

Sensor Unit.  L to R: Arduino, RTC, XBee, Photocell and XBee status LEDs, power supply.

A few words on the parts.  The Arduino Pro Mini has the same ATmega328 microprocessor and basic specs as the Arduino Uno, but is much smaller!  I soldered on headers so it would plug into a breadboard.  What it doesn't have is a power supply or a USB interface.  Both Sparkfun and Adafruit make nice little breadboard power supplies that work great and accept the same wall wart input as the Uno.  As for the USB connection, you'll need an FTDI breakout board (about the size of a small postage stamp) and FTDI drivers to do the interface.  Once you have that (and it's pretty easy), the USB interface works pretty much the same as the Uno.

The DS3231 RTC is pretty cool too.  If you keep it between freezing and about 100°F, it will keep time within two parts per million, which is about a minute per year.  It accomplishes this by monitoring its own temperature (a major cause of oscillator drift) and switching capacitors in and out of the crystal oscillator circuit to trim the frequency.  The crystal itself is on-chip which also helps accuracy.  A very sweet piece of engineering.  In addition to the date and time, we can also read the temperature from it (in °C to the nearest quarter degree).

Last we have the XBee module, which uses the ZigBee specification, based on the IEEE 802.15.4 standard.  These modules are low power and don't offer terribly high bandwidth, which is just fine for sensor networks which tend to send very small amounts of data at any one time.  The really neat thing is that XBee modules form themselves (with just about zero help from you!) into self-healing mesh networks.  They can be configured either as network coordinators, routers, or end devices.  Every network must have exactly one coordinator.  Routers can do anything an end device can, plus forward messages between devices.  End devices cannot route traffic, and must do all their business through a parent node which is either a coordinator or a router.  Additionally, end devices can sleep in a low-power mode which would be very good for battery-powered remote sensors.  If an end device is sleeping, its parent node will store traffic for it.  The XBee in my sensor module is configured as a router.

Now we come to what I call the "base" unit.  This consists of an Arduino Uno, with an Ethernet shield added on top and connected to my internet router.  Also we have an XBee module, configured as the network coordinator, and a seven-segment LED display.

Base unit. Arduino and Ethernet shield at top. XBee, status LEDs, temperature display on breadboard.

The whole thing works like this.  Once a minute, the sensor unit sends the temperature and light intensity data to the base unit.  The base unit sends the data to Pachube (click here to go to the Pachube feed for this project) and displays the temperature on the LED display.  We can then download the data from Pachube, or have it create charts like below.  These are live charts and will refresh every few minutes (not sure how often exactly, haven't found the spec on Pachube yet).  So if you refresh this page, you will see current data, assuming that a cat hasn't chewed through any wires on my end in the meantime.





DS18B20 Temperature Sensor


It would be straightforward to add additional sensor units to this project.  One idea I have in mind is to measure the temperature of my wood stove with a thermocouple.  Maybe send a text message to my mobile phone if it got over a certain temperature.