Build a successful sensor that will measure the proximity of a light source.

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An instrumentation project is to be carried out for our AS coursework revolving around the key ideas covered in chapter 2- Sensing.

Sensors have to do with systems that measure or respond to changes in their surroundings.

The project is to involve one of the following tasks as a main objective;

-        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.

I have chosen to build and test a sensor. My aim is to build a successful sensor that will measure the proximity of a light source. I also hope to be able to improve the sensitivity of my sensor by using different forms of circuits. I have chosen this because I am confident of the resources needed being plentiful. I also believe that I will find it interesting and challenging, while still feeling that I am able to do it independently.



I intend to commence by building, a light sensor, to determine how light intensity changes at varying distances, e.g. every 10mm. The light sensor will enable me to ascertain the changing light intensity. It will consist of a basic potentiometer circuit. The voltage change across the sensor will be measured and recorded as the light intensity changes. Three circuits of the same form will be used, and then an average recorded, to rule out anomalies caused by faulty equipment.


To stop any other light affecting the results, (ruling out a risk of random light such as daylight, causing anomalies) the tests will be carried out in either an opaque box or the dark room. Both will be contrasted during my preliminary investigations. The same room or box will be used each time, because if a different room is used they may vary in size, so more light would be lost in a larger room as it would not be reflected back to the experiment, it would be scattered elsewhere. Also if a different box was used it may be a different colour or shade so would absorb or reflect a different amount of light from the light source, meaning the photodiode would detect more or less light. To limit this interference of light absorption or reflection of light, the same box would have to be used throughout.

       After discussing my equipment list with the technicians, they do not have a completely opaque, light stopping box, but do have a spare dark room, or an opaque, telescopic black tube in which a light source can be placed. It has another loose tube inside with an attached LDR or photodiode at the end. Therefore can be moved away from the light source. It is also equipped with a stuck on tape measure so it will be easy, quick and reliable to measure and alter the distance. The tape measure peaks at 120mm.

         To allow me to measure the sensitivity of the sensor, I will eventually measure the potential difference change across the light intensity-measuring device that I finally choose. In an ordinary circuit you can measure the voltage by using the following formula, and by then rearranging it to make voltage the subject.

Resistance (Ω)= Voltage (V)

                            Current (I)

It can also be used to measure an accurate measurement of the resistance of the photodiode. This can then be used to see if the resistance of the photodiode. This can then be used to see if the resistance of the photodiode can be calculated at any given distance from the photodiode, using any relationship found between the resistance and the distance from light, e.g. Light intensity.

The above formula relates directly to Ohm’s law. Ohm’s law states that ‘Conductance or resistance can be calculated at any given current or voltage. If the resistance or conductance changes the current and voltage will also change.’ As you increase the resistance of a resistor the current and voltage across the resister will decrease. This law is proven especially well by a potentiometer, (or a variable resister). The resistance can manually be changed. This is able to prove Ohm’s law, because in a circuit with a potentiometer, you can change the resistance and the voltage and current will also change in linear form.

         The chosen light source is to be a lamp, in the preliminary investigations it was discovered that an LED gave out insufficient light to collect a good range of accurate results;

        Distance from LED                        Resistance of photodiode (KΩ)

10mm                                        0.074                                        

        20mm                                        0.082

        30mm                                        0.086

        40mm                                        0.090

        50mm                                        0.095

        60mm                                        0.095


…. Alternatively, a lamp gives off enough light to gain a range of results to detect even small changes. For this, the more figures and decimal places received from the readings; there is a larger contrast in the overall averages.

Distance from lamp                        Resistance of photodiode (KΩ)

10mm                                        0.216

20mm                                        0.389

30mm                                        0.577

40mm                                        0.717

50mm                                        0.890

60mm                                        1.023

Therefore, the lamp is clearly the best instrument to be used as the light source in my experiment. It will now be placed in the black telescopic tube to enable me to test it, compare and contrast it to the use of the darkroom.

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         I noticed a small gap between the inner black tube and the outer tube, which is inevitable because they have to be able to move inside one another, but it could introduce a light interference anomaly. To prevent this the tube could be used in a dark room, however, I would need a small separate lamp to read the multimeter, however even this interference would be less than that of daylight in the classroom. I will use a LED to do this because it is the smallest, lowest level of light that I have access ...

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