Build an effective sensing circuit that will be able to measure changes in light levels using an LDR.

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Sensors Project

Aim

To build an effective sensing circuit that will be able to measure changes in light levels using an LDR.

Safety

* All wires should be checked that they are properly insulated to prevent electric shocks.

* The voltage should not exceed the recommended manufacturer's limit for the resistors and bulbs used to prevent over heating or component damage.

* Careful observation should be made to ensure no incorrect connections that could possible cause a shorted circuit.

* Power pack should be fitted with a fuse and safety cut-out button.

* Clear work area

Apparatus

* 12V Power Pack

* 3V Battery Pack

* 6 Leads with crocodile clips

* Voltmeter

* Ohmmeter

* Ammeter

* 5W, 24W, 36W and 48W bulbs

* LDR

* Rheostat

* Black tubing

Development

To see how much the resistance of the LDR varies as light levels change I first made a small experiment to look at the actual resistance value changes.

The distance between the LDR and the base of the bulb was kept constant, at 25.6cm. The bulb length for the majority of the bulbs was 5.8cm, although the 5W bulb had a much shorter length and so the difference in distance had to be accommodated for. I first took a reading without a bulb to see what the resistance value of the LDR would be in normal light levels. To check the readings I repeated each test three times and then took an average of the three values. Below is a diagram of the first experiment.

Below is a table of results from the experiment:

Bulb Wattage (Watts)

Resistance of LDR in k? (1)

Resistance of LDR in k? (2)

Resistance of LDR in k? (3)

Resistance of LDR in k? (Average)

0

76.6 +/- 0.1

76.2 +/- 0.1

76.8 +/- 0.1

76.5 +/- 0.1

5

.61 +/- 0.1

.59 +/- 0.1

.63 +/- 0.1

.62 +/- 0.1

24

.2 +/- 0.1

.2 +/- 0.1

.25 +/- 0.1

.22 +/- 0.1

36

0.96 +/- 0.1

0.95 +/- 0.1

0.95 +/- 0.1

0.95 +/- 0.1

48

0.6 +/- 0.1

0.57 +/- 0.1

0.58 +/- 0.1

0.58 +/- 0.1

As you can see from the table the different wattage bulbs produced a significant difference in resistance of the LDR.

The graph produced a negative correlation, showing that as the wattage of the bulb increased, the resistance of the LDR decreased.

Here I expected a linear relationship because if the bulb wattage doubles, then the energy outputted from the bulb in joules would double also and so the light intensity would double, therefore halving the resistance value. However, a good relationship couldn't be produced as the amount of values was very limited due to a small range of bulbs with different power ratings.
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As the choice of bulb wattage was limited, I decided to add a rheostat to the circuit to enable me vary the voltage across the bulb and so the amount of light being produced. I also added an Ammeter and a Voltmeter so that I could calculate the bulb power using the equation:

Electrical Power Used by Bulb = Voltage X Current

Below is a circuit diagram of the changed circuit.

Here, the rheostat was moved up and down to change the resistance in the circuit, this allowed me to have many different power ratings ...

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