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# Aim: Build a sensor circuit to test the proximity of an object using a light detector to detect light from a bulb reflected from an object.

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Introduction

Aim: 'Build a sensor circuit to test the proximity of an object using a light detector to detect light from a bulb reflected from an object.' In this experiment I am going to build a sensor circuit using an LDR, I will then calibrate the sensor and use my results to test and improve the sensor circuit. The LDR works by having a very high resistance when the light intensity is low, which does not allow current to flow through the potential divider circuit. When the light intensity increases significantly, the resistance reduces dramatically and current can flow through the circuit. The resistor within a circuit can be changed to make the circuit more sensitive. The first thing I had to do was to test two different sensors to see which one had better sensitivity. ...read more.

Middle

The LDR was fixed onto a block of wood so it was level with the bulb. A wooden block was also set between the bulb and LDR to stop light being detected directly from the bulb to the LDR. A wooden board was also used as the object that I would measure the proximity of. A one metre scale was measured out and the board was moved 0.05m for each measurement from 1m down to 0.15m. The Vout was recorded at each point on the scale. The experiment was carried out in a dark room. These are diagrams for the above description: Diagram 2 Diagram 3 My final calibration results are shown in table B: Distance/m Voltage Output/V 1.00 1.77 0.95 1.74 0.90 1.76 0.85 1.75 0.80 1.82 0.75 1.80 0.70 1.85 0.65 1.86 0.60 1.93 0.55 1.98 0.50 2.08 0.45 ...read more.

Conclusion

From my data I could see that the error varies with distance. This could be because of the LDR's limited sensitivity when light is reflected from longer lengths. This means that the percentage error increases with distance. To test this systematic error, I could calibrate the sensor for longer lengths, then test it and compare the percentage error to my first results. The effective range of my sensor is around 0.15m to 0.60m. Important properties of a sensor are good sensitivity resolution, rapid response time, small random and systematic errors. A sensors response time is the time it takes a sensor to respond to a change in it's input. Random error could be caused by a fluctuation in the input or from noise generated by the sensor, these errors are always present in experimental data. Systematic errors occur due to disturbing influences usually from the environment around the sensor and are not easily detected. ...read more.

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