Discrete Sensors

This page contains specific information regarding implementing various discrete sensors associated with the Forest project.

Thermistor
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Type: THERMISTOR NTC 10K OHM 3% PROBE
Part #: Digikey 495-2172-ND
Cost: $3.94

The thermistor is attached to the BBB onboard ADC with a series 33K Ohm resistor to the 1.8V ADC voltage. This value resistor was selected to maximum the resistance change in the temperature region from -35 C to 40 C. It was discovered that the ADC circuitry on the BBB has its own parallel resistance and was cause an incorrect voltage to be read. For now this problem is being corrected with a quadratic polynomial correction based on three operating points. This corrects the voltage reading to what the sensor is outputting. Once the voltage is reading “correctly”, the voltage needs to be translated into a temperature. This was done with a 4th order polynomial fit based on the resistance change from the datasheet table and the circuit configuration. This then outputs a temperature and was verified with a reliable source to be accurate to ~ 1C. The exact accuracy would require further testing to discover.

Further work: Figure out where the resistance inside the ADC is coming from or reconfigure the circuit to deal with the problem (variety of ways to do this).
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