In this lab, we designed a circuit that would be able to provide a way to measure temperature using a solder-less bread board and a thermistor (a resistor that varies with temperature). Essentially, the circuit was built to vary the output voltage due to the changing temperature of the thermistor.
Before building the actual circuit, we analyzed the circuit depicted in the lab book. First, we determined V_out as a function of R_th and R. Next, we verified that V_out in the equation increases or decreases with the temperature. Last, we chose a value for R such that V_out changes by at least 0.5V over the specified temperature difference.
According to our analysis, V_out = (R * V_s)/(R_th + R). We tested the hot and cold resistances of the thermistor at 24°C by recording the resistance using a multimeter, then squeezing it between Andrew's fingers to obtain the resistance while at 37°C. At 24°C, the resistance was found to be 7.3 kOhms, and at 37°C, the resistance was found to be 11 kOhms. Plugging these values into the equation showed that there were two possible fixed resistance values that would give us a resolution of 0.1 °C per bit. We needed to choose either a 6.681k or a 12.018k resistor. Since neither of these resistors exist in real life, we had to choose a standard resistor that would be close enough to the calculated values to give us a good result. We had the choice between a 6.8k and 12k resistor, so bwe went with the 12k resistor because it is much closer to the respectie value obtained through calculation. According to the calculations, the voltage was predicted to be 2.6V at 24°C and 3.1V at 37°C.
The photos below depict our actual experiment building the thermistor circuit:
Cold voltage = 2.59V
Warm Voltage = 3.12V
As shown, our predicted values matched the values obtained from the actual circuit. Cold temperature had a percent difference of -0.4%, and the hot temperature had a percent difference of +0.6%.
In-Class Exercises:
1. Calculate the Node Voltages in the following circuit:
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