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^{3P : nvestigating sensors}

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Possible circuits	A different sensor circuit
Measuring resistance	Conclusions
Light sensor circuit

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Possible circuits

There are just two ways of constructing the voltage divider, with the LDR at the top, or with the LDR at the bottom:

You are going to investigate the behaviour of these two circuits. You will also find out how to choose a sensible value for the fixed resistor in a voltage divider circuit.

Remember the formula for calculating V_out :

What will happen to V_out if R_bottom gets smaller?

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Measuring resistance

What to do . . .

Start by switching your multimeter to a resistance scale:

    Using a multimeter as an ohmmeter

With the setting shown, the fsd, or full scale deflection of the ohmmeter is 200 . This means that the meter will measure resistances from zero up to a maximum of 200 . With this setting, you will be able to see how the resistance of an LDR changes with illumination.

The other resistance settings work in a similar way. When you select the 20 position, the maximum resistance which you can measure is 20 . However, the postion of the decimal point in the display changes, so that measurements in this range can be made more accurately.

With nothing connected to the meter, the display will look like this

This means that the resistance between the meter probes is too large to be measured within the range selected. When the meter isn't connected, the resistance between the probes is extremely large, giving a '1' reading on all the resistance ranges.

Insert a black meter probe into the COM socket and a red probe into the VmA socket. What happens to the display when you touch the probes together?

The meter reading should change to

If the meter reading remains as '1', the most likely explanation is that the internal fuse in the meter has 'blown'. The fuse protects the meter from incorrect connection. This can easily happen when the multimeter is used as an ammeter or as an ohmmeter. If there is a problem, either replace the fuse (usually a 200 mA 'quick-blow' type), or use a different meter.

If you moisten your fingers and hold the probes tightly, you should be able to measure your skin resistance. You might need to switch to the 2000  (that is, 2 M) range to see this effect.

Now measure the resistance of your LDR, as indicated below:

Write down the resistance value you measure when the LDR is exposed to the room lights:

You don't need to shine light directly onto the LDR. If the reading on the meter changes as shadows fall onto the surface of the LDR, write down a resistance value which you think is a fair estimate of the average reading on the meter.

Now cover the LDR with your hand so that it is in the shade. The resistance of the LDR will increase. Write down the new resistance value:

Once again, the meter reading may fluctuate. Estimate the average resistance reading.

Try to cover the LDR in a way which is easy to repeat. You are going to compare 'light' and 'shade' readings for sensor circuits using different values of fixed resistor and you want the comparison to be as fair as possible.

If the LDR is placed completely in the dark, its resistance will increase to 1 M or more. This isn't necessary for the experiment you are going to do. You will get more meaningful results by shading the LDR.

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Light sensor circuit

Follow the diagram below to build a test circuit for a light sensor:

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To start with, use a 100  resistor as the test resistor. Make measurements of V_out first with the LDR in the light and then with the LDR in the shade. Write these results in the table below:

Fixed resistor value	Vout in the light	Vout in the shade	Voltage change
100
1
10
100
1 M

In the final column of the table, subtract the measurement of  V_out in the shade from the measurement of  V_out in the light to work out by how much the voltage has changed as a result of the difference in illumination.

Repeat this whole process for each of the other test resistor values. You will find that some test resistors give V_out readings which are dramatically different in the light and the shade.

With this circuit, is V_out HIGH or LOW in the light?

Which test resistor gives the biggest voltage change between light and shade?

Which resistor would you use to make your light sensor most sensitive to changes in illumination?

(This is a different way of asking the same question.)

Check back with the resistance measurements you made for the LDR in the light and in the shade. The test resistor which gives the biggest change in V_out is expected to have a value about half way between these extremes. In fact, the voltage divider is most sensitive when the resistance of the fixed resistor is equal to the resistance of the LDR.

KEY POINT:

Voltage dividers are most sensitive when Rbottom and Rtop have equal values

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A different sensor circuit

Rebuild your prototype circuit so that the LDR is in place of R_bottom in the voltage divider:

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Repeat the experiment for this voltage divider, writing in the values of V_out measured in the light and in the shade:

Fixed resistor value	Vout in the light	Vout in the shade	Voltage change
100
1
10
100
1 M

With the second circuit, is V_out HIGH or LOW in the light?

Which test resistor gives the biggest voltage change between light and shade?

Which resistor would you use to make your light sensor most sensitive to changes in illumination?

Perhaps this circuit should be called a dark sensor, since it gives a HIGH  V_out when the LDR is covered.

KEY POINT:

The action of the voltage divider is reversed when the LDR is used as  Rbottom instead of as Rtop

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Conclusions

• It matters what value of fixed resistor you use in a voltage divider.
  
• The optimum value of fixed resistor gives the biggest changes in V_out . This happens when  R_bottom = R_top .
  
• You can decide how a voltage divider circuit is going to work by thinking about whether to use the sensor component as R_bottom or as R_top .

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