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Characteristics of an LDR

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This Instrumentation task is an independent project, aiming to demonstrate my practical, experimental and planning skills. I have the choice of three options in which to do so, and they are as follows…

* Building and testing a sensor

* Exploring the characteristics of a sensor

* Designing and assembling a measurement system, and using the sensor to make a measurement

From these alternatives I have decided to choose and investigate; “exploring the characteristics of a sensor.” I have chosen this criteria as I think I can relate my physics thoughts and experimental skills to this I can relate my physics thoughts and experimental skills to this aspect well. I will decide on a simple sensor so I can use its properties to perform an affective instrumentation task.

I contemplated on doing the other two tasks, as I feel they are a little impractical and I cannot perform them scientifically as well as the characteristics of a sensor.

How will I perform this task?

From the sensors that are available to me I have decided to choose an LDR (Light Dependent Resistor). This is a light detecting sensor in which can be set up simply in a circuit, to explore a characteristic of its properties. I have chosen this sensor as I think it can be used in several different experiments and can be performed straightforwardly and is less complex with standard apparatus.

Once I have decided on a sensor I now need to decide on what characteristic I can explore. I have decided to see how the distance away from the light source to the LDR affects the resistance. By doing so I can set up a simple but affective experiment which will give me accurate results to make calculations with. I suppose the experiment I wish to do could also double as one of the other options for this instrumentation task. By involving a distance away from the light source you could estimate “if the LDR is at 20cm away the resistance will be…?”. This could involve the third option ‘Designing and assembling a measurement system’, may be you could use this in a circuit to active something, maybe for machinery etc.

Variables in which I can incorporate in the experiment are as follows…

* Changing the distance from the light source to the LDR.

* Change the Light intensity of the light source.

* To change the resistance of the LDR by using a potentiometer.

For the experiment in which I wish to do changing the resistance of the LDR or light intensity of the light source, are irrelevant to my preferences. By simply keeping a fixed resistance, raw from the power pack and a fixed light intensity, I can change just one variable and see more clearly how the distance affects the resistance of an LDR. I wanted to keep the power supply straight from the power pack to the circuit as a set voltage, so I can get a more direct set up, also by keeping it simpler.

Apparatus and Diagram

Circuit Diagram


Set up apparatus as in diagram. To keep the experiment fair and accurate, some fair test procedures are required so the results are accurate. Firstly by keeping a fixed setting on both power packs and using the same equipment i.e. LDR, Ammeter, Voltmeter. This could affect the experiment and would produce anomalous results. To ensure a more accurate experiment, I have decided to incorporate an extra procedure in the experiment; ‘The black out tube’, as seen on the diagram, this is a black cardboard tube shape which is positioned between the LDR and the light source. This is so no background light (natural light) affects the experiment LDR’s are very sensitive and would pick up any another means of background light. Ideally a back out room would be better, and a lot more accurate. However, for the characteristic I wish to explore, this was not required.

Once the apparatus are set up and all fair test procedures are complete, I must obtain the results. I decided to start the LDR 40 cm away from the light source, and work down to 40 cm. This is so I can just simply cut down the black out tube, rather than re-make it. I will work down from 40cm – 10cm in 2 cm’s at a time. Any less would be a little impractical and irrelevant, but any more could affect the general pattern to the results. I will simply tape the LDR to the ruler as I move it down, and record the readings in a suitable table. The ruler and ray box will be taped down at all times, throught out the experiment.

The readings in which I will record are the voltage across the LDR and the current in the LDR circuit. By doing s o I can calculate the resistance using the formula: ? = V/I, after the experiment.


Generally, I predict that as the light intensity increases the resistance will decrease, resolving the current to decrease as well. I know this due to previous education in physics lessons. Basically as the LDR gets closer to the light source the light intensity increases. Therefore more light is exerted onto the LDR front and creates more energy. The more energy means the electrons from the LDR front become excited and are available to carry current. In result the resistance drops and the output voltage increases.


In conclusion to this instrumentation task I established that as the LDR moves closer to the light source the light intensity increases. Therefore, the resistance drops and the output voltage increases, this is due to the LDR being made from Cadmium Sulphide (Cds). When light falls on the Cadmium Sulphide, electrons break free from the Cds atoms. This is why the resistance drops, extra electrons become available to carry current. Therefore I was correct in my prediction in stating this finding. My previous knowledge and understanding of this topic helped me to influence my ideas, so I could produce an accurate experiment. With out this previous knowledge I mat not of been able to conduct an experiment that will show a clear characteristic of a sensor.

On the whole my results were quite accurate and I think the experiment was performed successfully. During the experiment I decided to do the task twice, to get a total of two sets of results. This way I can make sure that the results I did were not false or performed incorrectly. By doing so I can analyse the data better, find an average and confirm my previous results. If my second results were completely different to the first results, then I would perform the task again to try and determine were I went wrong. However, both my results were quite similar as I did both experiments with the same apparatus and preformed as fairly and as accurately as I could. Therefore I think the way I conducted the experiment by making sure the experiment was fair, I think the accurate results proved this.

During my experiment I encountered a slight problem, to begin with from moving the LDR from 40cm – 32cm, the current was to low for the normal ammeter to read. To solve the problem I switched to a milli-ammeter which can read milli amps (10^-3). So when the milli-ammeter reaches its maximum current reading (0.02 on a normal ammeter) I switched to a normal ammeter so this problem was resolved. This way I could complete the experiment with more accurate results.

After completing the second set of results I realised that in peculiarly the first set of results/graph were slightly inaccurate. Notice on the graph that there is a huge step up from 24cm-26cm this is only a 2cm gap and the resistance jumps up 113 ohms. I found this slightly strange that only 2cm could make such a difference. Once I had completed the second set of results I noticed the same problem, this time an increase in resistance in 2cm of 112.6. So why was this happening, I realised that with the normal ammeter only being able to read to 2 decimal places made the experiment slightly inaccurate. You needed an extra decimal place or perhaps 2 more, and then it would produce a more smoother graph line/curve.

To resolve this problem I decided to lower the Voltage to the LDR and so I could use a milli-meter for most of the experiment. This way when dividing the voltage by the current to get the resistance, the gaps between the results will not be as high. This is what Results 3 show, and as you can see by the graph it produces a clear equal increase of resistance in the LDR.


Overall I believe my experiment to be a success, the results were fairly accurate as they followed my prediction and conclusion. However, there was one anomalous result with in the second set of results. At 22cm away from the light source the current increased to 0.04 from 0.03, when in theory it such have decreased. But this little anomaly didn’t prove to be a problem and did not affect the experiment as a whole.

If I was to perform this experiment again I think I would perhaps investigate one other characteristic of an LDR and compare my results. Maybe I could use the LDR and a resistor to form a potential divider to make a light sensor, and see how the brightness of light affects the resistance. This way I could view two different experiments and see which will affect the resistance greater.

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