Making, Calibrating and Testing a Sensor
The aim of my investigation is to produce a working sensor for use in any application of my choice. In order to do this you were required to choose a task for my sensor to carry out, and then choose an appropriate sensor to carry out the task. The sensors available for selection are an LDR, a linear resistor and a thermistor. However, for the sensor to be fully operation in its application it will be to be used alongside a resistor to give the right amount of sensitivity for the task, therefore I must also select a resistor.
Then you must conduct an experiment to allow me to calibrate my sensor to make a fully functioning sensor. Description The scenario for which I have chosen to design my sensor for is for use in a garage. When cars park into garages it is useful for the cars to know when they are getting close to the end of the garage, in order to allow them to stop safely without being to close to the end of the garage where they might crash or not inside of the garage enough thereby meaning the garage door will not close. Method In order to produce a fully functioning garage sensor an LDR is going to be used.
The way the LDR will work within my sensing circuit is it will be placed at the end of the garage and there will be a light mounted at the opposite end of the garage. Then as the car drives through the garage the LDR will sense how close the car is by sensing how much light is hitting the LDR. The more light reaching the LDR, the further away the car is. Once the car has reached the maximum point it can safely park a buzzer will sound and the driver should park his vehicle. The circuit will be used on experiments to select a resistor for optimum sensitivity, calibrate the LDR’s operation and measure the resolution of the sensing circuit.
In my experiment for selecting a resistor with the maximum range of sensitivity, many different resistors will be used, in conjunction with an LDR. I will then connect the LDR to a multimeter which will measure the voltage across it. Then to test the range of sensitivity i will measure two values, the two extremes an LDR will have to deal with. The first being the dark so the LDR will be completely covered, blocking out all light and this will give me a minimum value and then the light where the light will be allowed to shine fully on the LDR giving me my maximum value.
Then the range will be calculated by taking away the value of the measurement in the dark, from the measurement from the light. This will give me my range and the largest figure; will be the LDR which gives me the largest range. Next to calibrate my sensor the voltage will be measured on a multimeter from different distances from the LDR. I am going to set up the experiment in much the same way as testing for the resistor. However a fixed resistor will be used, that would have selected in my first experiment.
Then as opposed to testing only light and dark, the sensitivity will be tested from a distance of 30cm back using a piece of card to replicate the car and a lamp to replicate the light, then the piece of car will be moved forwards at 1cm intervals until it is 1cm away from the LDR, this will replicate the car moving forwards. I have chosen to start at 30cm because this is the point at which a driver will no longer be able to gauge how far away from the garage he is unless the driver is fully aware of the length of the car.
Then it will be measured at 1cm intervals because the car will reverse slowly and the more frequent the measurements the more aware the driver will be aware of just how close they are to the end of the garage, this will therefore be my resolution. Finally, I am going to a final distance of 1cm due to the fact that it is the safest possible before you risk hitting the end of the garage. The final area of my sensing circuit to be tested using my current equipment setup is the resolution.
Ideally for a parking sensor the time response should change frequently because the car will be moving at a steady pace and the sensor should be able to keep up with the speed the car is doing. If it fails to do this the foreseeable problem is that the beeps will be delayed and therefore the instructions to the driver will be delayed. To test the time response, the equipment will be set up in the same way as the previous experiment with the addition of a stopwatch. I will then time the time it takes for the LDR to sense a change in the position of the piece of car replicating the car.
Alternative Method The method chosen to measure the distance a car is away from the end of a garage is by having an LDR at the end wall and then a light at the opposing side and the LDR measuring the car is away. However this was not the only way to conduct the experiment, the LDR could have been placed on the floor at the furthest point a car can safely stop and a buzzer or light would come on when the car was on this point. Although, the reason this method was discounted is because it does not take readings as frequently as my method due to their only one point at which it measures where the car is.
Another problem to do with less frequent readings is a driver will not be gradually alerted as to their position of their car against the end of the garage. This problem can be overcome by having multiple LDR’s located along the floor of the garage and as the car passes over them the sound of the buzzer will slowly get louder. The problem with this though is that it uses multiple LDR’s whereas my method uses just one and it still alerts the driver more frequently of their position. Equipment list * Power pack * Wires * Fixed resistor * LDR * Resistor clamp Multimeter * Stop Watch Safety Notes In general, there are minimal safety notes about conducting the above experiments. However because of the use of electricity there are always risks. The major one is at the plug because the voltage coming out of them is 240V therefore you should avoid contact with any live points. Then within the experiment mostly the voltage will be safe, however it could accidentally be set at a higher voltage on the power pack therefore you should avoid holding both a positive and a negative wire or you will risk receiving an electric shock.
Also, when a resistor is connected to the circuit you should keep the voltage down because resistors can burn out if used with a voltage that is too high. If a resistor burns out it can potentially release harmful fumes, therefore if your resistor is at risk of burning out with the voltages you are using it may be advisable to use a face mask. Furthermore, because the resistor is burning it could get something else on fire from the heat produced so the use of a heat mat would again be advised.
The final safety note is not to handle any liquids in the experiment area because of the electrical items being used. Improvements If the experiment were to be re-run the changes that would be made in terms of how the experiment was conducted would be to take the measurements either within a garage or inside a cardboard box which would better simulate a garage. I would also mount the light at the back of the box again to give more realistic results. The reason these changes would be made is this part of my experiment is because it was conducted in the open which would not be the scenario in a garage.
The second area of my sensing circuit to be changed is the LDR. I found that the resolution on the LDR was too low which meant that my voltage was not changing quickly enough when the piece of paper was moved. The reason this would need to be changed is because the LDR would not change fast enough to keep up with a reversing car and therefore it would give the driver delayed responses. Errors Due to the fact the experiment was conducted within a classroom science lab, the accuracy would always have limitations.
Achieving zero error would not be possible in such as environment however it is possible to minimize error and maintain your variables to ensure the experiment as fairly and accurately as possible. Firstly, in order to minimize errors it is important to identify and highlight any potential area which could create error. In this particular experiment the main factor which contributed to error was the light source. In the experiment I used sunlight as my light source however this is not a variable you can control and because of this it can create error.
The reason this happens is because sunlight is not a constant and therefore on particular days it could not be as intense or there could be a cloud cover, the disadvantage of this is that the results may not be comparable because they will have different values dependant on the lights intensity. However, this was identified early into the experiment and a lamp was used instead because its intensity can be controlled and it is a constant, which allows for fairer, more accurate and more comparable results. Although on the whole errors in terms of light sensitivity were reduced, there was still noise.
The source of the noise came from other light sources such as the sunlight and the classrooms interior lighting. The effect this had on my experiment is the possibility to affect my results if on a certain day the light is more intense. To attempt to reduce this error, the experiment was conducted towards the centre of the room to avoid effect of the sunlight and I ensured all lights were turned on to create a constant. A better alternative would have been to close the blinds however this was not practically convenient because other students needed them open.
During the experiment there were two main systematic errors present. The first was in the time response experiment. The systematic error was created when I was operating the stopwatch the results which I collected were collected with my reaction time to press the stop watch included therefore there was a percentage of error included. The second systematic error produced was due to the calibration and accuracy of the measuring equipment. Firstly, the positioning of the ruler on to desktop was not always placed in the exact same place every time leading to a systematic error.
Then the voltmeter may also not have been correctly calibrated which could have led to further errors in the results. Conclusion The aim was to producing a working sensing system and this has been successfully achieved as the final calibrated sensing system serves it purpose well. Comparatively with other similar sensing systems it works just as well if not better. The main reason for this is because of its simplicity, the use of an LDR is more than adequate to sense a car approaching the end of a garage.