Light-dependent resistors as sensors

How is a light-dependent resistor (LDR) used as a sensor or transducer? A voltage is needed to drive the output device, such as a voltmeter, yet the LDR only produces a change in resistance. The sensor must use this change in resistance to generate the change in voltage. The solution is to place the LDR in series with a fixed resistor, as shown in Figure 11.8.
The voltage of the supply is shared between the two resistors in proportion to their resistance so, as the light level changes and the LDR’s resistance changes, so does the voltage across each of the resistors.
The two resistors form a potential divider whose output changes automatically with changing light intensities.

WORKED EXAMPLE

2) Using the graph in Figure 11.9, calculate ${V_{out}}$ in Figure 11.8 when the light intensity is 60 lux.

Step 1: Find the resistance of the LDR at 60 lux.
${R_{LDR}} = 20k\Omega $
Step2: Divide the total voltage of $10 V$ in the ratio $3 : 20$. The total number of parts is 23 so:

${V_{out}} = \frac{{20}}{{23}} \times 10 = 8.70V$
Hint: The answer on your calculator might be 8.69565. When you give your answer to three significant figures, do not write 8.69 – you must round correctly.

Thermistors as a sensors

The thermistors that we refer to in this course are known as negative temperature coefficient (NTC) thermistors. This means that, when the temperature rises, the resistance of the thermistor falls. This happens because the thermistor is made from a semiconductor material. One property of a semiconductor is that when the temperature rises the number of free electrons increases, and thus the resistance falls.
Figure 11.10 shows a graph of the resistance of a thermistor and the resistance of a metal wire plotted against temperature. You can see that the resistance of a metal wire increases with increase in temperature. A metal wire is not a negative temperature device, but it could be used as a sensing device.
A thermistor is more useful than a metal wire because there is a much larger change in resistance with change in temperature. However, the change in resistance of a thermistor is not linear with temperature; indeed, it is likely to be an exponential decrease. This means that any device used to measure temperature electronically must be calibrated to take into account the resistance–temperature graph. The scale on an ordinary laboratory thermometer between ${0^ \circ }C$ and ${100^ \circ }C$ is divided up into 100 equal parts, each of which represents ${1^ \circ }C$. If the resistance of a thermistor were divided like this, the scale would be incorrect.
The thermistor can be used as a sensing device in the same way as an LDR. Instead of sensing a change in light level, it senses a change in temperature.