Day 12 Temperature Measurement Design System

The objective of this lab was to design a simple temperature measurement system which outputs a DC voltage that indicates temperature. We again used a thermistor to indicate the temperature based on the variable resistance provided by the thermistor as temperature changes. Today, we introduced a Wheatstone Bridge circuit that allows us to take a change in resistance and turn it into a change in voltage. The final stage of this circuit will include a difference amplifier (OP 27) to increase the overall sensitivity of the temperature measurement system.

Pre Lab: Balancing the Wheatstone Bridge

The following is a drawing of our Wheatstone Bridge circuit. Like mentioned before, this circuit is useful for taking a change in resistance and outputting it as a voltage. Resistances R2 and R3 are equivalent. We have a Potentiometer labeled Pot and our thermistor's room temperature resistance as Rth. The first step of this lab was to balance the bridge so that there is no voltage measured at our output nodes across a and b. To balance it, we attached our multimeter to measure Vab, and adjusted the resistance of our potentiometer until there was no potential difference.

In practice, our 10K potentiometer did not have a high enough resistance to balance out the circuit. We were required to add another resistance in series with it to simulate a higher POT value. Below is our finished and balanced Wheatstone Bridge circuit. We were able to balance it to 0.02 V of output.

Difference Amplifier

The second half of our temperature measurement design system is an op amp. Here we utilized a difference amplifier in order to measure the change in voltage from our input voltage to the change in voltage due to our thermistor and Wheatstone bridge.

Above is a scheme for our difference amplifier. Having 10 k Ohm resistors for R1-R4 simplifies the output of the voltage measured: the ratio of Vout to the input voltages is simply,

Vout = Vb - Va

Now we are ready to put the entire circuit together.

Temperature Measurement System
Together, the Wheatstone Bridge and the Difference Amplifier work in conjunction to form a simplified temperature measurement system. The complete circuit is pictured below.

The output voltage from our Wheatstone Bridge (+Vab and -Vab) are the voltage supplies for the difference amplifier. We have three different voltages throughout this circuit: our original supply, our Wheatstone Bridge Output, and then the output voltage from our difference op amp.
At this point, this temperature measurement system is the most practical use of op amps we have seen so far.

Below is our system in action.

Our design objective of an increase of a minimum of 2V was not entirely achieved. Instead, we observed a change in our voltage closer to 1.3 V. I'm still uncertain about what could be changed to achieve the minimum of 2V. Perhaps our resistance values could be changed. All in all, it was a new and challenging experience learning about the Wheatstone bridge and the Difference Amplifier.