I'm building a sensor net for our house; I'm using Arduino and XBee and these thermistors: http://www.sparkfun.com/products/250
. They work relatively well when connected directly to the board and because I'm only measuring current to compute temperature, I can connect them directly to XBee on the sensor side (don't need arduino there). But when I connect them using a long wire (10 - 20 feet) things get really weird. For people who have more experience with electronics this might not be surprising, but these are the things I'm seeing:
- sometimes, the measurements over a long wire have very little noise (<0.1 oC)
- sometimes what I get looks like actual noise (ie, random oscillations around a certain value), but sometimes the measurements form a "zig-zag" line. I'm measuring every 5 seconds, and for 30 seconds, the values increase, then for 30 seconds decrease (the difference between top and bottom is 2oC)
- I have a sensor connected over a long wire, but the actual sensor is right next to the board (so I'd expect same temperature) and I'm getting (average) 18oC, then I replace it with sensor connected directly to the board and I'm getting 22oC. this seems like too much error to come from the resistance on the wire
- finally, I have a board with 2 directly connected sensors and 1 over a long wire (but same location). the two directs are giving me both 21oC, but the wired one is 18oC. then I replace the wire with another directly connected sensor and this actually changes temperature of all sensors -- now all sensors are showing 20oC
Anyway, there's just too much unpredictability in this to use for actually measuring temperature.
Any suggestions? Is there a way to handle the noise, but still use these thermistors? I'm thinking about buying different sensors, but the advantage of these is that I can connect directly to XBee and send measurements without Arduino on the sensing end.
I appreciate any feedback!