ET 150
Temperature Sensor Amplifier Project
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) The amplified signal in input to a computer data
acquisition (DAQ) board installed in a lab computers. 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
an 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. The OP AMP requires bipolar power
supplies. Previous videos show how to connect the lab dc power supply to create the
desired power source. Formula 1 and measurements of the circuit input and output
voltages provide a means of determining the circuit amplification experimentially.
R1
10k
R2
10k
9V
+
+
0.25V
Vs1
U1
LM741/NS
DAQIN
Connect this point to
acquisition board
-9V
D1
1N4733
Test circuit
with 0.25 Vdc from power
supply. Remove supply and
connect temperature sensor
after testing.
Figure 1. Sensor Amplifier Test Circuit.
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Formula 1
Vout
Vin
AV
Where:
Av = Voltage amplification factor
Vout = Measured output to DAQ board
Vin = Input from sensor
Procedure:
1.) Construction 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 Vdc 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.
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.
5.) Remove the dc source from the circuit input and replace it with the temperature
sensor you are given. 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.
R1
10k
9V
R2
10k
9V
+
Install
temperature
sensor
LM34/35
U1
LM741/NS
DAQIN
-9V
acquisition board
D1
1N4733
Figure 2. Sensor Circuit With Temperature Sensor Installed.
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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 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 A vm
for the value of R2 used then use Formula 2 to estimate the room temperature in
degrees C or F. Record these values in Table 2.
Formula 2
Where:
T
A vm
Vout
0.010 V/degree
T = Measured temperature
Vout = measured circuit output voltage
Avm = measured circuit amplification factor
8.) Keep the power supply on. Obtain a soldering iron and connect it to the ac power.
After the iron has heat to temperature, bring in 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?
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Temperature Sensor Project Measurements
R2 Value
(ohms)
10k
27k
33k
47k
56k
100k
270k
470k
Vin (Vdc)
Table 1 - Amplification Measurements
Vo (Vdc)
Theoretical Avt Measured Avm
2.00
3.70
4.30
5.70
6.60
11.00
28.00
48.00
Table 2- Estimated Room Temperature
Value
Sensor type
(LM34/LM35)
Measured Avm
Measured Vout
Computed Temperature
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