Measuring Resistance

1. Aim

To measure an unknown resistance connected between SEN and GND using SEELab/ExpEYES.

2. Apparatus / Components Required

3. Theory & Principle

Inside ExpEYES, SEN is connected to 3.3V through an internal 5.1 kohm resistor. When an unknown resistor R is connected from SEN to GND, a voltage divider is formed.

\[V_{SEN}=3.3\cdot\frac{R}{R+5100}\]

Hence, \(R=5100\cdot\frac{V_{SEN}}{3.3-V_{SEN}}\)

Best accuracy is obtained when R is of the same order as 5.1 kohm.

4. Circuit Diagram / Setup

  1. Connect one terminal of unknown resistor to SEN.
  2. Connect other terminal to GND.
  3. Open “Measure Resistance” in software/app.

5. Procedure

  1. Start with a known resistor (for example 1 kohm) for verification.
  2. Note displayed resistance.
  3. Repeat for 470 ohm, 2.2 kohm, 10 kohm, 47 kohm.
  4. Compare measured and nominal values.
  5. For unknown resistor, repeat measurement 3 times and average.

6. Observation Table

Nominal Resistance Measured Resistance % Error Remarks
470 ohm      
1 kohm      
2.2 kohm      
10 kohm      
47 kohm      

7. Series and Parallel Combinations

Use two known resistors (example: R1 = 1 kohm, R2 = 2.2 kohm) and verify equivalent resistance.

For series: \(R_{series}=R_1+R_2\)

For parallel: \(R_{parallel}=\frac{R_1R_2}{R_1+R_2}\)

Procedure:

  1. Measure R1 and R2 individually.
  2. Connect R1 and R2 in series between SEN and GND, then measure.
  3. Connect R1 and R2 in parallel between SEN and GND, then measure.
  4. Compare measured equivalents with calculated values.
Combination Calculated Equivalent Measured Equivalent % Error
R1 alone      
R2 alone      
R1 + R2 (series)      
R1 || R2 (parallel)      

8. Advanced: Manual Calculation from SEN Voltage

If software shows V_SEN, calculate R manually using: \(R=5100\cdot\frac{V_{SEN}}{3.3-V_{SEN}}\)

Example: if V_SEN = 1.65V, \(R=5100\cdot\frac{1.65}{1.65}=5100\ \Omega\)

9. Error Analysis

10. Precautions

  1. Keep resistor leads firm and clean.
  2. Do not apply external voltage directly to SEN in this experiment.
  3. Use dry fingers / insulated clips; finger touch may alter high values.

11. Troubleshooting

Symptom Possible Cause Corrective Action
Reads nearly 0 ohm Short circuit in wiring Recheck connections
Reads very high / overflow Open circuit / loose lead Tighten clips
Reading unstable Poor contact Clean terminals and reconnect

12. Viva-Voce Questions

Q1. Why is resistance measured between SEN and GND?

Ans: Because SEN has a known internal resistor to 3.3V, making a divider with the unknown resistor to GND.

Q2. Why is measurement most accurate near a few kohms?

Ans: Maximum sensitivity occurs when unknown resistance is comparable to the internal 5.1 kohm resistor.

Q3. What is the approximate measurable range?

Ans: Roughly 100 ohm to 100 kohm for useful accuracy.