# Sensors cwk. 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

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) and the resistance decreases as the brightness of light falling on the LDR increases.

There are two main types of light dependent resistors, the negative co-efficient, which causes the resistance of the device to fall as the light falling onto it increases. Whereas, some other LDR’s work in the opposite way i.e. their resistance increases with light, called positive co-efficient LDR’s. For, the purpose of this coursework we are only going to need to now about the negative co-efficient.

If light falling on the negative co-efficient device, is of a high enough frequency, photons absorbed by the semiconductor gives bound electrons enough energy to jump into the conduction band. The resulting free electrons conduct electricity, thereby lowering resistance.

The device can be either intrinsic or extrinsic. In intrinsic devices, the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire bandgap. Extrinsic devices have impurities added, which have a ground state energy closer to the conduction band; since the electrons don't have as far to jump, lower energy photons (longer wavelengths and lower frequencies) are sufficient to trigger the device.

Ogborn et al (2000), suggests that ‘Light dependent resistors are made of a high resistance superconductive materials such as cadmium sulphide (CdS), which conducts better when light shines on them. To get a large change in resistance, the resistance is increased by laying down the CdS in a long zig-zag strip. The change in resistance be detected using a potential divider. The output of the potential divider can be arranged to increase, or to decrease, when the sensor is illuminated.’

LDR’s are very useful and when put into a potential divider they can be used for a number of devices such as automatic night lights, clock radios, security , and outdoor clocks. The simplest way in which to show how a LDR works is by putting it in a potential divider.

A potential (or voltage) divider is made up of two resistors. The output voltage from a potential divider will be a proportion of the input voltage and is determined by the resistor values.

If we connected a voltage supply across a resistor and opened it up and measured the voltage at any point along we would find the voltage varied linearly along its length.

A potential divider basically uses the same principles; however, we can not measure the voltage as left because we can’t tap the voltage off at any point therefore we use two resistors instead. The voltage is then measured between the two resistors. The diagram below illustrates this:

V1 is the voltage in and V2 is the voltage out.

If we know V1, R1 and R2 we can use the following equation to work out V2:

V2 = (R2/ (R1 + R2)) * V1

This is because the ...