Course: Sensors & Instruments. Instructor: Prof. Viviana Vladutesu

Course No: EET 3120

REPORT FOLDER

NEW YORK CITY

COLLEGE OF TECHNOLOGY

OF THE CITY UNIVERSITY OF NEW YORK

Department Of: Electrical and Telecommunication Engineering Technology

Experiment No. 4

Title: Wheatstone Bridge and its Applications to Biomedical Engineering

FACULTY ONLY STUDENT ONLY

Returned for corrections .
Correction due .
Corrections handed in .
Comments: .
Grade:______Approved by______/ Date experiment completed:.
Date due:.
Date handed in: 05-26-15 .
List of members:Wilfredo Gonzalez
Momodou Njie

Table of Content

  1. Objective3
  2. Introduction3
  3. Theory/Background3
  4. Results7
  5. Conclusion8
  6. Reference9

Objective:

The objectives to this lab were to be able to troubleshoot a Wheatstone Bridge circuit by using Mulitisim, and to create a circuit on the NI ELVIS II and find the resistance of our own arms.

Required Components:

  1. Multisim 11.0
  2. NI ELVIS II
  3. (4) 1K Ω Resistors
  4. (2) Alligator to Alligator clips
  5. Connector Wires

Theory:

Samuel Hunter Christie invented the Wheatstone bridge in 1833 and was improved and became widely used after 1843 because of Sir Charles Wheatstone. It is used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. Its operation is similar to the original potentiometer.

The Wheatstone Bridge circuit is nothing more than two simple series-parallel arrangements of resistances connected between a voltage supply terminal and ground producing zero voltage difference between the two parallel branches when balanced.

The Wheatstone bridge (or resistance bridge) circuit can be used in a number of applications and today, with modern Operational Amplifiers we can use the Wheatstone Bridge Circuit to interface various transducers and sensors to these amplifier circuits.

Figure. 1

A basic circuit is shown in Fig. 1. Impedance R can either be active or reactive meaning they can be simple resistances like the piezoresistive gauges that senses vibration, or capacitors, inductors, or combination of both. For a pure resistor the impedance R for a capacitor, the magnitude of its impedance is equal to 1/2πfC and for inductors it is 2πfL where f is frequency of the current passing through the component.

The bridge output voltage is represented by

Formula 1

The bridge is in balanced state when

Formula 2

Under this balance condition the output voltage is zero. When at least one resistance in the bridge changes, the bridge becomes imbalanced and the output voltage goes either in a positive or negative direction.

This bridge circuit is commonly used with strain gauges, piezoresistive pressure transducers, thermistor thermometers and other sensors.

PROCEDURE

Part 1:Simulation in Multisim

Webuilt the following circuit by using a program calledMultisim.

Figure. 2

Then we open our NI ELVIS Instruments icon and open our digital multimeter (DMM). We connect the DMM's ports to the components to measure the voltages. These areas measured were: R1 and Ground, R4 and Ground, and R2 and R4 respectively. We proceeded by clicking the run button to run the circuit and simulate a real circuit. This allowed us to see the results on the DMM.

Part 2: Troubleshoot in Multisim

We couldn’t the wheatstonebridgeexample file. But we proceed with the actual circuit on the NI ELVIS II.

Part 3: Prototype on NI ELVIS II

Working with the NI ELVIS II; we built the first circuit that was created, on the NI ELVIS II board. We opened the instrument launcher, and chose the DMM. With the voltage measurement selected, we measured all the voltages as we did in the previous part, and recoded in a table. After, we recorded the resistances of R1, R2, and R3, and also recorded in the same table. For R4, we calculated it manually. Then checked this value by using the DMM. After, we removed R4, replaced it with 2 wires, and connected these to our left arm by using alligator clips. One connection is placed on the wrist, while the other in a straight line right before the elbow joint. We repeated the previous measurements for our arm. The pictures 3 represents on Wheatstone bridge on the NI Elvis circuit board, while figure 4are the electrodes on the arm and wrist.

Figure 3

Figure 4

RESULTS

Part 1:Simulation in Multisim

By using the DMM, we were able to measure voltages across:

  1. R1 to ground,
  2. R4 to ground,
  3. R2 to R4

R1 to ground: 5V

R4 to ground: 2.5V

R2 to R4: 5V

Part 2: Troubleshoot simulated

Table 1 / V across R1 to Ground / V across R4 to Ground / V across R2 and R4
Part 1 / 4.733 V / 2.374 V / 2.374 V

Part 3: Prototype on NI ELVIS II

Table 2 / V across R1 to Ground / V across R4 to Ground / V across R2 and R4 / R1 (Ω) / R2 (Ω) / R3 (Ω) / Calculated / R4 Actual
Circuit / 4.733V / 2.374V / 46mV / 985 Ω / 985 Ω / 980 Ω / 981.25 Ω / 986 Ω
Arm / 4.733V / 4.73 V / 2.3 V / - - Ω / - - Ω / - - Ω / - - Ω / - - Ω

CONCLUSION

We had trouble getting a resistance ready from the arm measurement, I initially thought it was but the resistance in our arm was really high, infinite higher than the to get a reading. We tried to get the reading several times, only to get the same result.

REFERENCES

EET 3120 Sensors and Instrument Laboratory Manual, Developed and Edited by Professor Viviana Vladutescu, Spring 2015

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