Investigation of a light dependant resistor.
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Introduction
Investigation of a light dependant resistor
Light dependant resistors or LDRs are resistors which alter their resistivity according to a particular light level. I am going to find out how these work and use them in a context so I can use them to locate a light source. To do this I must firstly find out how light effects the LDRs and then use this data to try and create something in order to detect a light source and locate it.
Results
Distance light is away from LDR | Voltage reading (supply-12v) |
5cm | 11.5 |
10cm | 10.9 |
15cm | 8.7 |
20cm | 6.4 |
25cm | 3.7 |
30cm | 1.9 |
As you can see from these results as the light intensity increases, the resistance decreases. I need to find the range of light levels the LDR can detect and distinguish how sensitive it actually is. The maximum resistance it is possible to get from the LDR is 550kΩ. This is when there is maximum light shining on the LDR. The minimum resistance that is possible is 0.90kΩ. This is when the LDR is submitted to complete darkness.
Middle
The two LDRs are going to be separated by a sheet of card straight down the middle of the card that the LDRs are mounted on. This is so the light falls on one or the other of the LDRs. It will cast a shadow so one of the LDRs will be in perfect light and the other will be in near darkness. As the light nears the middle, the light will fall equally on both. By doing this I am emphasizing the amount of light on the LDRs, creating a bigger difference and more emphasized results. I am taking the results of the voltage of one of the LDRs. If I took the voltage of both of them, it would always read 12v. The lamp that I am going to use is 12v, 90watt bulb and it is going to be placed 15cm away from the LDRs and it will be moving from right to left at 5cm intervals in front
Conclusion
Another reason for the irregularity of the results could be to do with the section of card that was placed between the two LDRs. This could have cast a shadow on one of the LDRs while the other was in full light causing the difference and unpredictable nature of the results. As the light moved closer to the middle, the shadow would have got shorter and then disappeared when it hit the middle (40cm). This is apparent by the way that at 40cm all of the reading are the same. When the LDRs are only 5cm away from each other, the shadow will cover it more and for longer, producing the more dramatic set of results.
Because the piece of card stayed the same and didn’t change, when the LDRs are further away from each other, the shadow doesn’t affect the results as it would the other two sets. This could explain why the results for the 25cm result seems inverse to the others, the shadow isn’t reaching the LDRs now because they are further away from each other.
This student written piece of work is one of many that can be found in our AS and A Level Electrical & Thermal Physics section.
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