Temperature Measurement System Design Lab

Objective:
We covered the idea of cascading operational amplifiers which are op amps that are connected in cascade or heat to tail to get a larger gain. We also discussed out Wheatstone bridges are which are circuits that are used to convert variations in resistance to variations in voltage and are used in measurements systems where there are sensors that have a variation of resistance due to external influence such as thermistors. We apply this idea into our temperature Measurements system design lab.

Group Practice:
1. The circuit below shows two operational amplifiers where we are told to find the voltage output and the current as seen in Figure . We will find the voltage output of the first non inverting amplifier in which we calculated to be Vo = 100mV. We then use that voltage as the voltage input for second non inverting amplifier. We would then calculate the voltage output using the equations Vout = (1+(Rf/R1)Vin. The result is Vout =350mV . The current is found by treating the current input in the positive terminal to be 0 and treating the resistors in series. We use the equations I=Vout/(R1+R2). The current is I = .025E-3mA.

Figure 1. 

2. The circuit in Figure 2. tells us to find Va and Vb. Since we have Vs which is the input voltage. We will calculate the voltage at a by using voltage division as seen below. We then calculate the difference between the points a and b.

Figure 2. Using voltage division to find the voltage at point a and b. 

Lab Procedures and Results:
1. The purpose of this lab is to build a simple temperature measurements system which outputs a DC voltage which indicates temperature. We will use a thermistor, potentiometer, and the idea of a Wheatstone bridge circuit as seen in figure 4 and 5 as its equivalent circuit.

Figure 3. Schematic of circuit which is planned to be built.

Figure 4. Wheatstone bridge circuit

2. We are considering a thermistor where the resistance is 10K when the temperature is at room temperature or 20C. When the temperature increases to 37C, the resistance increases to Rhot= 37K+/-.05. We measure the actual resistance of the thermistor Rth = 10.35K+/-.01 at room temperature.

3. When we connect all the components to the circuit, we notice that we must connect the potentiometer in series with the resistor at the bottom right of the circuit as seen in Figure 5. This will help us acquire a voltage difference between point a to b with 0 and measure the voltage Vab easier when the temperature changes for the thermistor.

Figure 5. Actual build for the Wheatstone bridge circuit.

3. We measure the resistance of the resistors, potentiometer, and thermistors that will be used in the Wheatstone Bridge circuit: Rth = 10.35K+/-.05 at room temp, R1 = 10K+/-.05, R2 = 10K+/-.05, R3 = 9.98K+/-.05, R(Potentiometer) = 9.07K+/-.05.  The potentiometer has the ability to change the resistance with a knob but in this case we are trying create a voltage difference between point a and b of 0 which can be seen in figure 6. This will help us measure the voltage when the thermistor increases in temperature. The voltage at body temperature is Vab= .25V+/.05.

Figure 6. Taking necessary steps to make the Vab equal to 0 at room temperature

4. We will use the voltage Vab for the input voltage for the operational amplifier. as seen in Figure 8. However, there is more work to be done since we need to find the necessary resistors for the operational difference amplifier as well as the voltage output. We note that the we must have a voltage output of 2V. In this case we will use the equation Vout = -(R2/R1)Vin. Our voltage input must be Vab=.25V+/-.05. Our calculation was wrong and acquired a theoretical resistors of resistance of R1= 11.87K+/-.05, R2 = 2.93K+/-.05, R3 = 9.98K+/-.05, R4 = 12K+/-.05. This means, based on our resistors used, that the voltage output should be 1V at body temperature which is shown as a video in Figure 9. The video shows that the voltage output increases as temperature increases. The voltage output increases to 1v in this case based on our error of calculations.

Figure 7. Difference operational amplifier

Figure 8. Schematic on how to connect the Wheatstone bridge circuit and difference operational amplifier.

Figure. 9. Video of successful experiment

Summary of Labs and Learning Objective:

The lab takes two components into consideration and they the use of building a Wheatstone bridge circuit and an operation difference amplifier. We also take thermistor and potentiometer into consideration since we are building a simple temperature measurements system. The reason why we build a Wheatstone bridge circuit is because its resistance change will convert to a voltage change. The Wheatstone bridge has a difference output that will be close to zero according to our set up. As a result we used an operation amplifier in order to increase the output voltage of the whole system to 1V. The lab require us to create a voltage output of 2V. However, we make a miscalculation when find the necessary resistor values for the operation difference amplifier. The resistance used and the resistance of the thermistor are Rth = 10.35K+/-.05 at room temp, R1 = 10K+/-.05, R2 = 10K+/-.05, R3 = 9.98K+/-.05, R(Potentiometer) = 9.07K+/-.05. This values helped us acquire a difference voltage between the point a and b of circuit from figure # to zero. We connect this Vab which is considered as out voltage input for the operational difference amplifier to the proper terminals. Yet we calculated the resistor needed in order to create a voltage output of 1 V while the voltage difference is in fact Vab= .25K+/-.05 when there is a temperature change. We use the equation Vout = (Rf/R1)Vin where Vin = Va-Vb. The resistors that were used in order to acquire a output voltage of 1v are R1= 11.87K+/-.05, R2 = 2.93K+/-.05, R3 = 9.98K+/-.05, R4 = 12K+/-.05. We tested our circuit by raising the temperature of the thermistor by squeezing it in order to represent a change of temperature based on our body temperature. The lab was a success and saw that the voltage output increases as temperature increased as seen in out video in Figure 9.