Electricity Tutorial 6

*Device*	*Action*	*Where used*
Light Dependent Resistor	Resistance falls with increasing light level	Light operated switches
Thermistor	Resistance falls with increased temperature	Electronic thermometers
Strain gauge	Resistance changes with force	Sensor in an electronic balance
Moisture detector	Resistance falls when wet	Damp meter

The picture below shows examples:

This picture shows a strain gauge, which is found in an electronic balance:

Light dependent resistor (LDR)

The light dependent resistor consists of a length of material (cadmium sulphide) whose resistance changes according to the light level. Bright light releases electrons so that the material conducts better. Therefore the brighter the light, the lower the resistance.

The characteristic graph of the LDR is shown below:

The graph shows us the variation using a linear scale. However, the measurement of light intensity is not an easy scale to work with.

Here is a list of typical intensities:

Light Source	Illumination (lux)
Moonlight	0.1
60 W light bulb at 1 m	50
1 W MES bulb at 0.1 m	100
Fluorescent lighting	500
Bright sunlight	30 000

LDRs are used for:

Smoke detection
Automatic lighting
Counting
Alarm systems.

Thermistors

The word thermistor comes from the mixture of thermal and resistor. So it changes its resistance in response to a temperature change.

The most common type that we use has a resistance that falls as the temperature rises. It is referred to as a negative temperature coefficient device. A positive temperature coefficient device has a resistance that increases with temperature.

Question 1

Explain the difference between a positive temperature coefficient thermistor and a negative temperature coefficient thermistor.

Below is a picture of typical thermistors and their symbols:

We can use apparatus like this to measure the way that the resistance of the thermistor changes with different temperatures:

The oil is heated with the 50 W heater and the temperature recorded. The multimeter shows the resistance. The data are plotted as a graph that looks like this:

If we plot the vertical axis as log₁₀ (resistance) we get a straight line, indicating that the temperature and resistance are linked by a logarithmic function:

Potential Divider Circuit

Resistive transducers like LDRs or thermistors are often put into a potential divider circuit.

This result can be thought of as the output voltage being the same fraction of the input voltage as R₂ is the fraction of the total resistance.

If the light level rose, the resistance of the LDR would fall. Therefore the voltage V_out would rise. If the output were connected to a transistor, the transistor would switch on as V_out rose above 0.7 V.

Electricity Tutorial 6 - Passive Transducers

Passive transducers are often referred to as resistive transducers. They detect a change in the environment (e.g. light level) by changing resistance (hence resistive). They do NOT generate an electric current. That is why they are called passive. The table shows some passive devices.

Active transducers generate a current. A pick-up cartridge on a record deck is one such.

Here are some passive devices:

The picture below shows examples:

This picture shows a strain gauge, which is found in an electronic balance:

Light Source

Illumination (lux)

Potential Divider Circuit