Wireless temperature sensor (using a thermistor)

We built some temperature sensors for monitoring the performance of air conditioning units for a customer. The nodes use the WiSense WSN1101L sensor node. The temperature is measured using a thermistor (103AT-4 from Semitech). The node and the sensor interface circuit is enclosed in a weather proof enclosure with just the antenna and the thermistor sticking out. We used a resistor divider circuit and an op-amp (as a buffer). The op-amp output is tied to one of the 10 bit ADC channels on the MSP430G2955. The whole node is powered by a couple of AAA batteries (within a battery box glued to one side of the enclosure).

To reduce power consumption, voltage supply to the resistor divider and the op-amp is gated through a mosfet switch. The mosfet is switched on/off by the MSP430. The driver code for this thermistor will switch on the mosfet, wait for a few milliseconds and then measure the ADC channel.

Here is some ASCII art showing the mosfet.

The 103AT-4 is a negative coefficient thermistor (resistance decreases with increasing temperature). Wikipedia has a nice introduction to thermistors here – http://en.wikipedia.org/wiki/Thermistor.

We use this equation to get the temperature from the measured resistance.

loge(R25/RT) = B * (1/T25 – 1/T)

T = 1 / ((1/T25) – ((loge(R25 / RT)) / B))

Here T25 is the temperature in kelvin corresponding to 25 deg C. So T25 is (273.15 + 25 =  298.15 K).

R25 is 10K for this thermistor. That is, the resistance of this thermistor is 10K at 25 deg C.

RT is the resistance at “unknown” temperature T. RT is obtained from the circuit shown in the figure above.

The value of B (as specified in the datasheet) is 3435.

R25 is 10 Kilo-ohms. This also gives the value of the resistor to use in the resistor divider circuit shown in the figure above.

Voltage (V_R25) across the lower resistor (call it R25) is given by –

V_R25 = (Nadc * ADC_refV) / 1024

V_R25 = (VCC * R25)/ (R25 + RT)

(Nadc * ADC_refV) / 1024 = VCC * R25 / (R25 + RT)

R25 + RT = VCC * R25 * 1024 / (Nadc * ADC_refV

– Finally –

RT = (VCC * R25 * 1024 / (Nadc * ADC_refV)) – R25

Note that R25 is 10000 ohms.

Here are some pics of the sensor node.

In this pic below, you can see two LEDs next to the antenna. These blink when the node transmits/receives.