Contents Page

Aim ……………………….………………………………………………………1

Hypothesis ……………………….……………………………………………….1

Science ……………………….……………………………………………..….1-4

Preliminary Experiment ……………….….………………………….……..….5-6

Method …………………….…………..………………………………………7-8

Results and Graphs ………………….………………………………….……9-10

Analysis of Results ………………….……………………………….…………11

Inverse Square Law ………………….…………………………..….………12-13

Response Time ………………….……………………………………………...14

Evaluation ………………….…………………………………………………..15

Conclusion ………………….………………………………………………….16

Bibliography ………………….………………………………………………..17

Aim:

The aim of this coursework is to construct a potential divider circuit with a light dependent resistor (LDR), and observe how light intensity affects the voltage output. Furthermore, I will have to calibrate the sensor which is achieved by the production of a graph with input plotted against output. From the resultant curve, expected output can by interpolated for a given input, and vice versa.

By the end of this experiment I hope to assemble a circuit which can be used either in street lamps to switch them on in the evening and off in the morning or in an electrical system which opens curtains when light intensity reaches a set optimum. The feature of the system which I am going to test is how the light intensity during the course of the day will affect whether street lamps will be switched off or on, and why.

Hypothesis:

I predict that as the light intensity increases the resistance of the LDR will decrease due to the extra number of charge carriers. The Voutput however will depend on how the LDR is connected in the potential divider. If the LDR is to be connected on the top of the potential divider the Voutput will be high as the resistance of the LDR is low, whereas if its connected at the bottom of the potential divider its vice versa.  

Science:

The light dependent resistor, LDR, is known by many names including the photoresistor, photoconductor, photoconductor cell or photoconductive sensor. However it is simply an input transducer (sensor) which converts brightness (light) to resistance and eventually voltage. It is made from cadmium sulphide (CdS)

This is a model of what my circuit will be like:

After connecting my circuit as above, I will go to a dark room and by using a 10V light bulb, I will try to obtain results as follows:

• Firstly, make sure that the LDR is in direct sight of the light bulb.
• Mark out 10cm points, starting from 10cm to 200cm.
• Make sure the variable resistor is fixed at a resistance of 1kΩ.
• Place the bulb 10cm from the LDR (at your first point or mark).
• Record the Voutput  shown on your voltmeter and tabulate this.
• Repeat the last step for each different marked point up to 200cm.
• Tabulate the results and then draw graphs of them.  

After drawing the graphs, I will be able to calibrate the Light Dependent Resistor. Once I have drawn my graph I should end up with a ‘calibration curve’. This is important for the sensor because a calibration curve tells you how to look up the input to a measurement system if you know its output.

Results and Graphs:

Using, the results above I will plot a line graph in order to calibrate my sensor.  By doing this I can estimate the Voutput  when the light bulb is placed 13cm from the graph of 155cm. On the other hand, I can also predict how far the light bulb needs to be from the Light Dependent Resistor, when the Voutput is 1.75 volts.

Calibration curves are very useful as we can find out the Voutput  from a given distance of the light bulb and we can work out the distance of the light bulb from the LDR if we are given a Voutput.

As we can see form the graph, the results all follow a general trend, which causes the line of best fit to be a curve. This further reinstates that the results obtained are accurate and contain no outliers in the data.

In my hypothesis I assumed that as the light intensity increases the resistance of the LDR will decrease due to the extra number of charge carriers. In addition I noted that Voutput will depend on how the LDR is connected in the potential divider. If the LDR is to be connected on the top of the potential divider the Voutput will be high as the resistance of the LDR is low, whereas if its connected at the bottom of the potential divider its vice versa

The prediction was correct; I connected the LDR on bottom which meant that the Voutput increased as the light intensity decreased; this is due to the Vout receiving a larger ratio of the voltage supply.

In conclusion, the experiment went as planned and using my previous knowledge of potential dividers and light depending resistors in science I can assume that my results were accurate. Furthermore, the results matched my hypothesis and I can conclude despite there being errors the experiment went as planned and the results obtain were accurate.

Bibliography

• Thompson C, Wakeling J (2003) AS Level Physics. Coordinate Group Publication.

• Ogborn et al (2000) Advancing Physics AS. Institute of Physics

• http://www.doctronics.co.uk/ldr_sensors.htm (23 March 2008)

• http://www.technologystudent.com/elec1/ldr1.htm (25 March 2008)

• http://www.physics.iitm.ac.in/courses_files/courses/eleclab03_odd/light_dependent_resistor.htm (25 March 2008)

• http://www.reuk.co.uk/Light-Dependent-Resistor.htm (30 March 2008)