Study of Light Dependent Resistor (LDR)

1. Aim

To study variation of LDR resistance with light intensity using SEN and GND.

2. Apparatus / Components Required

3. Theory & Principle

LDR resistance decreases when light intensity increases.

In this experiment, LDR is connected between SEN and GND. Internally, SEN is connected to 3.3V through 5.1 kohm. From measured V_SEN, LDR resistance is: \(R_{LDR}=5100\cdot\frac{V_{SEN}}{3.3-V_{SEN}}\)

Lower light -> higher R_LDR -> higher V_SEN trend depends on divider orientation; verify experimentally.

4. Circuit Diagram / Setup

  1. Connect LDR between SEN and GND.
  2. Open resistance/LDR measurement screen in software.
  3. Ensure ambient light condition is stable before taking readings.

Mobile App

5. Procedure

  1. Record reading in room light.
  2. Increase light on LDR (torch close to LDR) and record reading.
  3. Reduce light (cover partly or move source away) and record reading.
  4. Repeat for multiple light levels.
  5. Plot R_LDR vs relative light level (low/medium/high) or lux (if lux meter is available).

6. Observation Table

Light Condition Measured $V_{SEN}$ Calculated / Displayed $R_{LDR}$ Remarks
Dark / covered      
Room light      
Bright lamp      
Torch very close      

7. Advanced: Sensitivity Estimation

Estimate change ratio: \(\text{Sensitivity ratio}=\frac{R_{\text{dark}}}{R_{\text{bright}}}\)

Larger ratio indicates better light sensitivity.

If multiple points are taken, a log plot (\log R vs \log L) can be used to estimate empirical exponent: \(R\propto L^{-k}\)

8. Error Analysis

9. Precautions

  1. Keep light source distance fixed for repeated trials.
  2. Avoid hand shadow while recording.
  3. Wait 1-2 seconds after changing light before reading.
  4. Do not apply external voltage directly to SEN.

10. Troubleshooting

Symptom Possible Cause Corrective Action
No variation in reading Wrong wiring / faulty LDR Recheck LDR between SEN and GND
Reading very noisy Flickering light source Use steady DC light source
Saturated high/low value Too dark / too bright constantly Adjust illumination range

11. Viva-Voce Questions

Q1. What happens to LDR resistance when light increases?

Ans: LDR resistance decreases as light intensity increases.

Q2. Why is SEN used in this experiment?

Ans: SEN has a known internal resistor to 3.3V, allowing resistance estimation from measured voltage.

Q3. Why can two readings differ even at same lamp brightness?

Ans: Distance/angle changes, ambient light, and LDR response time can alter readings.