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Testing an LDR

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Introduction

Physics Coursework – Testing an LDR based sensor

I have devised a project to test the effect of a change in distance between the light source and LDR in a potential divider circuit. Ideally, it would be possible to determine the distance of any light source given it’s light output in Lux, however, practically, I only expect to be able to predict the distance of a particular bulb.

Background

Before performing my experiment, I needed to carry out some background work to decide how I should carry it out. The following documents my background research, and the conclusions it led me to. All background research was done independently, and I came to my decision without outside help.

Types of Sensor

The first, and most obvious, choice for my experiment is the LDR, or Light Dependent Resistor. Light dependent resistors work by having a small area of semi-conducting material whose resistance decreases as the light intensity increases. There is an inverse relationship between the two. The majority of LDRs use a cadmium sulphide cell as the semi-conducting material. Cadmium sulphide cells are cheap, have quite a large range of resistance, and are sensitive to a variety of wavelengths of light, from ultraviolet through to infrared. When cadmium sulphide (CdS) is subjected to light, the absorption of photons excites electrons to a higher energy state. When in this higher energy state, the electrons are able to flow as a current.

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Middle

image02.png

The first test I conducted was with the lamp at the maximum distance of one metre from the LDR. Following are the results. Unless otherwise stated, all results are two 2 d.p.

R1 / Ohms

0V

4V

6V

8V

10V

12V

100

10.11

10.08

9.89

9.59

9.15

8.77

220

10.11

10.05

9.67

9.11

8.36

7.68

470

10.11

9.95

9.26

8.26

7.07

5.99

1000

10.10

9.82

8.50

6.90

5.34

4.17

2200

10.10

9.50

7.16

5.02

3.42

2.46

4700

10.10

8.93

5.41

3.20

1.96

1.33

The results highlighted in bold are anomalous. As the voltage from the power supply was set to 10.0V, I can only assume they are either due to random error (such as power fluctuations) or systematic error (i.e. a incorrectly calibrated multimeter or power supply). As the voltage across the lamp was increased, the range of values also increased. This suggests the best data would be collected with the maximum rated voltage of 12V. After testing at the maximum distance, I took the other extreme, and performed the same tests, but this time with the bulb touching the LDR. I did not

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Conclusion

Voltage

6V

8V

10V

12V

Average Range

4.78

5.26

5.295

5.225

As can be seen, the 10V reading has the highest average range, albeit by a very small margin. Thus I chose 10V as the voltage to use on the lamp circuit. I chose to use 1000 Ohms as the value for the static resistor in the potential divider, because it seemed to offer a good balance of a wide range of values without approaching the limits of the range too quickly.

Overall, I thought the preliminary testing went well, and so decided to keep the same method, equipment, and setup for my final experiment.

Implementation

Due to the success of my preliminary test, I decided to use the same method in my final experiment. I chose a value of 10V to power the halogen lamp, 10V across the sensor circuit, 1000 Ω for the fixed resistor, and 1 metre in 10 cm intervals for the input variable.

Procedure

Following is a brief procedure for each reading taken:

  1. Check voltages on both power packs
  2. Measure distance between glass front of bulb and protruding ‘bubble’ of LDR
  3. Take reading
  4. Wait 5 seconds
  5. Repeat

Risk Assessment

Risk: Electric Shock

Severity: Mild, 1/5

Likelihood: Unlikely, 1/5

Risk Factor: 1/25 – safe

Precautions: take care when dealing with electrical wires.

Risk: Burns from hot lamp

Severity: Mild, 1/5

Likelihood: Quite Possible, 3/5

Risk Factor: 3/25 – safe

Precautions: Avoid touching lamp bulb when adjusting distances. Instead move using the retort clamp.


[1]http://en.wikipedia.org/wiki/Photovoltaic, 27/01/06

[2]http://en.wikipedia.org/wiki/Cryogenic_detectors, 27/01/06

[3]http://www.emant.com/316002.page, 08/03/06

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