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# I aim to find out how distance affects the light intensity emitted from an LED.

Extracts from this document...

Introduction

Aim

I aim to find out how distance affects the light intensity emitted from an LED.

Introduction

To do this I will have to use a sensor of some description.  There are a few different components that are currently available to sense for light, these are an LDR (light dependent resistor), a photodiode, a photo voltaic cell or a photo transistor.

The LDR as its name suggests is a resistor that is affected by light.  The darker it is the higher the resistance and obviously the more light the lower the resistance.

A photodiode is a p-n diode with a window that allows the p-n junction to be exposed to light. “The diode is reverse-biased. Incident light creates charge carriers in the junction region enabling current to pass through the p-n junction.”

A photovoltaic cell is a cell designed to generate an electromotive force when light shines on it. A photovoltaic cell, sometimes referred to as an active solar cell, consists of a semiconductor such as copper oxide or selenium with a very thin metal film on its surface.  These cells are used to generate environmentally friendly electricity for example a calculator that doesn’t have batteries has one.

I am going to use the LDR as it is easily available and has a reasonable degree of accuracy.

As I said earlier the LDR is a resistor that changes resistance when the light intensity changes.

Middle

2.60

40

2.90

45

3.15

50

3.39

On the next page I have done a graph it is to show these results.  I have put a line of best fit on it and most of the points are not far off that line so it looks as though it was fairly accurate.  It shows what I already knew that as you move the white disk away the light intensity will decrease as it has to travel further, I know this because the resistance goes up as the light intensity goes down.

Now I have these results I can estimate the results to the next experiment.  I also know that I need about a 1.5MΩ for the fixed resistor.

Prediction for second experiment

Using the formula that I wrote earlier I can now work out the v out simply by substituting the figures in.

 Distance of white disk from LED Calculation Estimated V out 0 (9x1.5)/(1.5+0.21) 7.89 5 (9x1.5)/(1.5+0.17) 8.08 10 (9x1.5)/(1.5+0.46) 6.89 15 (9x1.5)/(1.5+0.88) 5.67 20 (9x1.5)/(1.5+1.25) 4.91 25 (9x1.5)/(1.5+1.56) 4.41 30 (9x1.5)/(1.5+2.18) 3.67 35 (9x1.5)/(1.5+2.60) 3.29 40 (9x1.5)/(1.5+2.90) 3.07 45 (9x1.5)/(1.5+3.15) 2.90 50 (9x1.5)/(1.5+3.39) 2.76

I would expect the values to be around these however not exactly as these are theoretical values and they don’t take in to account the resistance of the wire, heat changing the resistance of any thing, electrical noise which also means the circuit is in efficient and loses energy, and that is assuming the voltmeter has an infinite resistance, however in practise the voltmeter can not have an infinite resistance as then it wouldn’t be able to conduct and it would not get a reading.

Conclusion

After doing this experiment if it wasn’t sensitive enough or it was a bit of a slow reaction time, I could go on and do another experiment this time with a slightly different circuit, still measuring the V out of the potential divider circuit only this time another two resistors both 1.5 MΩ (joined in series with each other) joined in parallel with the existing circuit and then the other end of the volt meter to the v out of the extra resistors. Like so;

And if this gave too lower reading then there is always the possibility to use an Op amp to amplify it.

However I don’t think it is necessary to do them as this sensor is as sensitive and as fast as its going to get in the current circuit.  It also has a fairly good range I tested it from 0 to 50cm how ever because the light is reflected that should be double so that’s a metre.

I think the experiment went well, the aim was met, but if I did it again I would like to explore the possibility of some of the other original ideas like the solar cell and the other photo sensitive components.

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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