Use an Op-Amp in a scaling and level-shifting circuit to change the output signal of an electronic temperature sensor to produce a temperature reading in degrees Fahrenheit.
Pre-Circuit Information/Calculations:
For the lab, the national semiconductor LM35 is used to represent the temperature in degrees Celsius. The conversion rate for the LM35 to degree Celsius is 10 mV/°C. The LM35 is tested to see if it is working properly by following the schematic below.
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| Schematic for to check LM35 |
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| Schematic for the Circuit |
TF = 1.8TC + 32 (Original)
TF = 1.8TC + .32 (LM35 equvialent)
The temperature conversion equation in terms of circuit elements:
VF = (1 + R2/R1)VC - R2/R1*Vref
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| Calculation for Circuit Components |
In the circuit element equation, the 1.8 from the temperature equation is represented by (1 + R2/R1). This yields the ratio value of R2/R1 which is 0.8. With the ratio obtained, the value of Vref can be determined by setting R2/R1*Vref = 0.32. This yields the value of Vref to be 0.4V. The values of R2 and R1 are determined by making sure that the ratio of the two resistors is about 0.8.
Values of Components:
Vref = 0.4 V
R1 = 2144 Ω
R2 = 1765 Ω
The Circuit:
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| The Circuit |
For the circuit, three power supplies were used. Two were set up at around 9V to power the op-amp and the LM35. The other power supply was to supply the 0.4 V for Vref. The voltmeter read about 77mV for the temperature of the room in Fahrenheit.
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| LM35 value |
The value of the LM35 was about .237 mV, which is about 24°C. Plugging this value into the Celsius to Fahrenheit conversion equation:
TF = 1.8(23.7) + 32 = 74.66 °F
The Fahrenheit temperature obtained by the voltmeter for the circuit is about 77.0 mV, which is about 77°F.
The percent error of TF:
Percent Error = (77-74.66)/ 74.66 *100 ≈ 3.13%
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