Now we’re getting somewhere! My previous networked temperature monitoring systems were a little buggy. My first temperature monitor had its cable chewed up by a rodent and needed to be reset frequently. My second prototype failed on restart, making it problematic in low-power situations (a known issue for the Arduino’s Ethernet Shield), and eventually the probe itself succumbed to moisture because of a failed seal.
My new system relies on serial communication between an Arduino and a Motorola C168i cell phone, bridging data to Pachube through Twitter, and sending an e-mail to my server as a backup. I really like this setup because it is reliable, cheap, and deployable in far more locations than ethernet or WiFi would allow. I picked up a C168i at Goodwill for 10 dollars (and another on E-bay for $12) and it costs me $5 a month to add to my T-Mobile plan. The system runs off a single 6-volt source, and I’m currently piecing together a solar/battery system to go completely wireless.
I picked up a $6 5-quart dry bag from my local sports store, and am using that to house my arduino and cell phone. It rolls down on the cables, and is a great solution to the weather-proofing electrical connections. My sensors are connected through 1/8″ mini stereo jacks, because they’re ubiquitous. Tear them off of any broken set of headphones.
Rather than crimping the PEX tubing housing my sensor string, I’ve switched to screwing in 1/2” pneumatic fittings with some teflon tape. The stabbing end is filled with epoxy, and the cable end is sealed with several layers of shrink-wrap tubing and high-temp hot glue.
Code, and notes on communicating with a C168i:
The C168i has a wonderful feature that aught to be on all cell phones- you can talk TTL to the audio jack! It is presumed that internally the phone is a G-24 Lite GSM Module, and G-24 commands work on the C168i. On the forums there are some comments about sudden communication failures and dead phones. I think I’ve pinpointed the problem- The G-24 operates between 3.3V and 4.2V, and most USB devices, (including the Arduino) run at 4.6-5V. The phone is obviously running hot- when you plug a USB-mounted arduino directly into the C168i its screen brightness jumps and flickers. I knocked the voltage down to 3.7V by placing four 1N4007 rectifier diodes (two pairs of two , one facing each way) on each of the communication lines (RX & TX). I know it’s inefficient, but it’s quick, and only for when the system is being programmed/debugged.
In the fielded system I removed the diodes and just run the power source at 6V, at which point the Arduino communicates at 4.1V.