# Build an effective sensing circuit

He graph overleaf produced a straight line with a positive correlation, this showed that the relationship between distance of the bulb and the LDR resistance was linear. It also showed that by using a tube with the bulb and LDR the curve caused by natural light was removed. To try and find out with what circuit the LDR is most effective I changed the circuit to include a resistor. I then measured the change in voltage across that resistor, as the bulb was moved away from the LDR. Below is a circuit diagram of this experiment.

Again, the bulb wattage was kept constant at 48W, and the bulb and LDR were both inside a black tube to block out light from sources other than the bulb. The bulb was moved in 10cm increments, up to a maximum of 50cm away. This was a limitation due to the length of the black tube. After taking readings at each distance, I changed the resistor value to try and find out which resistor would make the most efficient light sensor with the LDR. Below is a table of results for this experiment. Throughout the experiment the results produced were all close to my predictions, and there were few anomalous results in the tests made.

Each test was repeated three times and then an average taken of the three values to improve accuracy of results. In almost all of the results, the values recorded never fluctuated more than 5%, although by using an analogue meter instead of a digital one this problem could have been eliminated. One of the main problems with the tests I made was the background light interfering with the results. Also, as some of the tests were repeated or continued on different days and at different times of day the background light levels would have changed, and so would have affected the results slightly.

If I were to do the experiments again I would conduct all of the tests in a room where there could be no background light to interfere with results. This would also mean that the level of light would always be the same whatever the time of day or weather conditions. Another problem with the first tests was that the range of bulbs was small, and the bulbs used varied in size. This meant that the distance from the LDR to the bulb could not be as accurately measured compared to if the experiment was done with same sized bulbs.

If I were to do the experiments again I would try to use a large range of bulbs that were the same length. Also, when measuring the voltage across a resistor in one of the circuits the value of the resistor was always taken to be the manufacturers specified reading. However, this meant that as the resistor values can be 10% +/- the manufacturer’s specification the comparisons between the different resistors may not have been accurate. If I were to do the experiments again I would use an ohmmeter to get the exact reading of each resistor to use to make sure that any relationships or comparisons made were completely accurate.