Temperature Sensor Amplifier Project

EET 150

Introduction to EET

Lab Activity 12

Temperature Sensor Amplifier Project

Required Parts, Software and Equipment

Parts

Equipment

Required

On-Campus Students

Adjustable Dc power supply

SolderlessExperimenters' Board

Digital Multimeter

Hookup wire (22 AWG)

Wire cutter/stripper

Optional

On-Campus Students

3 Banana jack leads red/black

Required

On-Line Students

2, 9 V batteries

2, 9 V battery snap connectors

Solderless Experimenters' Board

Digital Multimeter

Hookup wire (22 AWG)

Wire cutter/stripper

Optional

On-Line Students

Adjustable Dc power supply

Power brick DC-DC converter +5 to ± 9 Vdc

Software

MS Word

Objective

Construct and test an amplifier circuit that will increase the output voltage of a temperature sensor. The circuit output should be within the range of 0-5 Vdc. Determine how changing external component values effects the output of the circuit. Select a resistor value that gives a circuit output near the middle of the desired range.

Theoretical Background

The Operational Amplifier(OP AMP) is a building block of electronics. Adding external components turns this device into amplifiers and frequency filters. Other circuits allow the OP AMP to perform mathematical operations on input voltages. This project uses a single OP AMP of the type LM741 to amplify the output of a LM34/35 temperature sensor. The LM34/35 IC temperature sensors produce voltages proportional to the temperature in degrees F/C. The proportionality constant is 0.010V/degree (10 mV/degree).

In the next activity, on-campus students will connect the amplified signal to a computer data acquisition (DAQ) board installed on a lab computer. On-line students will connect the output to the Analog Discovery 2 (AD2) scope input and use the data logger in WaveForms 2015 to record measurements. In both cases, the input will be an analog signal that represents the measured temperature.

The amplification factor of an OP AMP circuit depends of the circuit configuration. Figure 1 shows the circuit configuration for a non-inverting amplifier circuit. The resistors labeled R1 and R2 determine the amplification of this circuit. The Zener diode labeled D1, limits the amplifier output to 5.1V to protect the input of the DAQ board/data logger.

The OP AMP requires bipolar power supplies. Previous videos show how to connect the lab dc power supply to create the desired power source. Two 9 volt batteries can also provide a convenient power source.

Formula 1 and measurements of the circuit input and output voltages provide a means of determining the circuit amplificationexperimentally.

Formula 1

Where:Av = Voltage amplification factor.

Vout = Measured output to DAQ board AD2.

Vin= Measured input from sensor.

Procedure

1.) View the video associated with this project. Construct the circuit shown in Figure 1 on a solderless experimenters’ board. Use the pinout diagram included in the IC data sheet to convert the schematic symbol locations to the IC pins.

2.) Connect the input to a 0.25 V dc source for circuit testing. This source can be derived from 0-6 volt source on the triple output supplies in the lab or from another dc supply. The project video shows an alternative method that uses a 10k Ohm potentiometer and the positive 9 V supply to derive this voltage.

Figure 1. Sensor Amplifier Test Circuit.

3.)Use a multimeter to measure the input and output voltages and record them in Table 1. Maintain the input voltage constant at 0.25 Vdc while making all the measurements required in the table.

4.)Turn off all power supplies. Disconnect all batteries if using them for a power source.

5.)Remove the dc source or potentiometer from the circuit input and replace it with the temperature sensor you are given. The accompanying project video shows how to make the following connections in detail. Figure 2 shows the connections for the temperature sensor. Use the pinout included in the LM34/35 data sheet to make the correct power, ground and signal output connections for this device. Use the +9V supply from the OP AMP to power the sensor. Determine if the sensor is an LM34 or LM35 by examining the markings on the device. Record the type in Table 2.

Figure 2. Sensor Circuit With Temperature Sensor Installed.

6.)Replace resistor R2 with a 100k value if you have an LM35 device. Change the value of R2 to 33k if you have the LM34 device. Turn on the power supply or reconnect the batteries for the OP AMP/sensor circuit. The output voltage should be 2.5-3.25 Vdc if the circuit is connected properly.

7.)Measure the circuit output and record it in Table 2. Use the measured value of Avm for the value of R2 then use Formula 2 to estimate the room temperature in degrees C or F. Record these values in Table 2.

Formula 2

Where:T = Measured temperature

Vout = measured circuit output voltage

Avm = measured circuit amplification factor

8.)On-Campus Students: Keep the power supply on. Obtain a soldering iron and connect it to the ac power. After the iron has heated to temperature, bring it near the temperature senor. DO NOT TOUCH THE SENSOR WITH THE HOT SOLDERING IRON. Observe the changes in the circuit output. Does the voltage increase with higher temperatures? Turn off all power supplies and disconnect the soldering iron after making your observations.

On-line Students: Keep the power supply on. Obtain a hairdryer and connect it to ac power. Turn on the hairdryer and direct its air toward the temperature sensor and away from the OP AMP chip and other components. Excessive heat can cause component failure. Observe the changes in the circuit output. Does the voltage increase with higher temperatures? Remove the batteries from the circuit after making your observations.

Temperature Sensor Project Measurements

Table 1 - Amplification Measurements

Table 2- Estimated Room Temperature

Fall 20171 sensor.docx