I chose to do my sensors project about sliding potentiometers because they are very common in every day life, and I thought it may be interesting to discover how they work.

Physics coursework

Introduction

I chose to do my sensors project about sliding potentiometers because they are very common in every day life, and I thought it may be interesting to discover how they work. There are potentiometers on lots of everyday household appliances, sliding and rotary potentiometer on hi-fis to change the volume or tuning, washing machines and dishwashers to change settings and other electrical equipment and machine. As the world becomes more computer reliant sensors will be used more often, especially potentiometers as they are easy to operate. Rotary and sliding potentiometers both work in a similar fashion. As the slider gets moved along, the voltage increases as the potential difference changes. A rotary potentiometer works the same way, but is twisted round, so rather than measuring displacement, degrees are considered. I will take this simple idea and look into true values and patterns.

Plan

Fig 1. Circuit diagram of the circuit to be built.(Not to scale)

I am going to build the diagram as shown above and slide the potentiometer along and see what the different readings of the voltmeter are.

The equipment I shall be using is:

Wires x 4
A 59mm sliding potentiometer (5kB)
A digital voltmeter, Rapid 212 DMM. Reading error: 0.005V
30cm Ruler. Accuracy: 0.5mm
Stopwatch, Unilab Accuracy: 0.005 secs
Power Pack (1kWIN L.V power supply (J0032)). Reading error 0.005V

I have chosen to use a power pack rather than a battery pack as I can guarantee that the voltage will be constant and not drained, as would happen with a battery pack.

Safety is a very important factor to consider as potentiometers are used by many people so safety is paramount. Safety is important in my experiment so I don’t get injured, and either do my classmates. I have considered safety by making sure that all wires are secure and bare wires are not exposed, not making a short circuit as that could make my equipment hot and dangerous to handle and making sure that all my equipment is connected correctly so the equipment is used correctly and safely, like the manufacturers intended. Safety was considered all the way through my experiments.

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Theory

To predict what results I will get for my experiment the theory of how a sliding potentiometer must be considered. A potentiometer is simply a potential divider. The position of the slider decides what the resistance is either side of it, like splitting up the potentiometer into two resistors. Moving the slider on a resistor taps off a potential difference proportional to its displacement. If we think of it in this way, the theory behind it is fairly easy to comprehend.

Fig 2. Potentiometer as two resistors.

In this diagram the voltage ...

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Theory

Fig 2. Potentiometer as two resistors.

In this diagram the voltage on the voltmeter would be half that of the voltage from the power pack. This would be the case if the sliding potentiometer was half way along. If the slider was three quarters of the way along, this would be the equivalent of having one resistor with three time’s greater resistance than the other. The voltage is then spread out in relation to the resistance.(1) With this idea I would predict the following results.

Method

Firstly I collected all my equipment, the wires, voltmeter, potentiometer, power pack, ruler, stopwatch. I then plugged in power pack and found out which of the three wires coming out of the sliding potentiometer was the ‘c’ wire.

Fig 2. The wires of a sliding potentiometer.

To find out which of the three wires is the c wire, I attached two wires at a time out of the three to the voltmeter, and slide the potentiometer up and down to see if there was a difference in the voltmeter reading. If there is a difference then one of the wires is the ‘C’ wire and you have to test it out both of those wires with the other wire to see which the C is. If there is not a difference, you know that the C wire is the unused one.

After establishing which of the wires the ‘C’ wire is, I attached the potentiometer’s A and B wires to the power pack, and I attached the voltmeter to the C wire and the power pack.

I set the digital voltmeter to the 2v setting and the power pack was set to 4.56V.

Preliminary Work

To prepare for experiment I decided I must do some preliminary work. I carried out the experiment as written above and measured the voltage at intervals of one cm work. The purpose of this was to get a rough view of what my final results would be like. After looking at these results I will decide at what intervals I will look at for my experiment.

The graph of these results is attached: Graph 1.

From my results I can see that as the displacement increases, the voltage increases fairly uniformly. To make my results more accurate I decided to do the experiment using 0.5cm intervals. This way I can be certain about the pattern and have a more accurate line of best fit.

The Experiment

As well as measuring in 0.5cm intervals I will also measure the response time as it is vital to know if this sensor is going to be used in an electrical appliance, how long it will take for the displacement to be measured. My second experiment was carried out without any problems. These are the results.

From these results I drew a calibration curve that can be used to reference the connection between displacement and voltage. With this curve you can tell by noting the voltage what the displacement is. This calibration curve is attached, Graph 2. Any anomalies were minimal, the calibration was near perfect. Anomalies were due to small inaccuracies in measuring displacement. When measured again these anomalies disappeared.

I also drew the curve linking the percentage of the displacement to the voltage. This way the calibration curve can be used with a potentiometer of any length.

After drawing these curves I tested them, to see my testing look at the attached annex.

Results analysis

From my results I can conclude many things. As I predicted earlier the displacement compared with the voltage gives us a linear connection. In fact it’s a very simple linear connection. As predicted half the voltage means half the displacement and other fractions of the voltage coincide with the displacement.

From the graph 2 I have worked out that the resolution is 0.146. This is the smallest change on voltmeter in terms of displacement.

By looking at the gradient of the calibration curve I worked out that the sensitivity was 0.814. There is constant high sensitivity.

The response time was worked out by measuring how long it took for the voltmeter to stay on a constant level. I did a number of tests, and although it is hard to measure accurately as it is such a short time scale, these were my results:

As there is a fairly large difference between the results it is hard to get an accurate result as an average. The actual average response time is 0.9858 secs. However this is too accurate and I think that 1 sec (2 sig. fig.) is a better value.

From all these results I can conclude that for a knob on a hi-fi a sliding potentiometer is ideal, it has a short response time, highly sensitive, and very easy to use. The only downfall is that the displacement is limited, but the volume and such like is limited also, so this is not a problem.

A sliding potentiometer is ideal for hi-fi knobs.

If I was to do this experiment again I would try out different lengths of potentiometers to check my percentage of displacement, voltage graph was correct.