Friday, July 30, 2010

Teaser: XBee Wireless Temperature Sensor

XBee wireless temperature sensor

Above: XB24-ZB with four TMP36 analog temperature sensors connected to XBee's analog inputs.

The reason four sensors are connected is: comparison and statistical analysis.

It remains to be seen how good or bad analog readings are, and how easy it is to compensate for these imperfections in software.


This design is based on a similar one, with a few minor differences:
  • TMP36 is used instead of LM34 (the only reason being that I had some spares laying around). Obviously, the result calculation is different. Wide selection of similar sensors (TMP35, TMP37, LM35), result would be just about the same, save for value recalculation;
  • Incidentally, this allowed to get rid of the voltage regulator because whereas LM34 requires +5V to +30V power, TMP36 is fine from +2.7V to +5.5V, hence, the same 3.3V power can be used to feed both XBee and TMP36;
  • Caveat is, at +70°C TMP36 output voltage reaches maximum allowable voltage for ZB series ADC, 1.2V. I don't think it's a problem because the intended usage is indoor temperature measurement, if your indoor temperature reaches +70°C, you'll have bigger problems to worry than a possibly damaged XBee. But keep this in mind if you're planning to use these sensors to monitor a temperature of, say, a water heater or air coming out from the furnace;
  • 330Ohm resistor added to ASSOC LED to restrict the LED's operating current (might want to use 1K and higher if your LED is too bright for you).

More to come.

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