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9.9 Option - The Age of Silicon: 3. Transducers and other non-linear devices

Syllabus reference (October 2002 version)
3. Sensors and other devices allow the input of information in electronic systems	Students learn to: define a transducer as a device that can be affected by or affect the environment explain the relationship in a light-dependent resistor (LDR) between resistance and the amount of light falling on it describe the role of LDRs in cameras explain why thermistors are transducers and describe the relationship between temperature and resistance in different types of thermistors distinguish between positive and negative temperature coefficient thermistors explain the function of thermistors in fire alarms and thermostats that control temperature	Students: gather, process and present graphically information on the relationship between resistance and the amount of light falling on a light-dependent resistor solve problems and analyse information involving circuit diagrams of LDRs and thermistors gather and analyse information and use available evidence to explain why solar cells, switches and the light meter in a camera may be considered input transducers

Extract from Physics Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.

[Edit: 21 Aug 08]

define a transducer as a device that can be affected by or affect the environment

Aspects of the environment that people might want to monitor for a range of purposes include: temperature, light intensity, air pressure, humidity, chemicals present in water needed for drinking and chemical pollutants in air, to name some. Transducers are specially made devices that respond to a chosen energy source in the environment and transform it into a continuously varying electrical signal, usually detected as a voltage, that is an analogue of the environmental condition that produced it. Transducers of various types are often used in the probes we attach to data loggers.

Background

The relationship between the environmental variable (say temperature) and the amount of voltage it induces in the transducer is found, usually by experimental methods. When we graph the independent variable (the environmental aspect of interest, say, temperature) against the electrical output (voltage or resistance), the relationship between the two is not usually linear. Thus we cannot say that when the temperature doubles, the voltage from the transducer output doubles or the resistance doubles. In some cases the relationship is an inverse oneincreasing the light intensity decreases the resistance. Again that relationship is not usually linear.

Some transducers accept an electrical signal as an input from electronic systems and produce an effect (output) in the environment. These output transducers will be dealt with in section 4.

Transducers may be passive or active. Passive transducers require only an energy input to produce an electrical output. Active transducers require a separate source of electrical energy to produce an electrical output when they are exposed to the environmental energy source of interest.

Examples of input transducers include:

microphones (convert sound to electric current). They can be made to respond to sounds well outside the range of human hearing which ranges from 20 20 000 Hz. Microphones can be made of moving coils and fixed magnets (or the reverse) or they use a piezoelectric crystal or piece of piezoelectric film to convert sound into a voltage analogue.
photovoltaic cells (solar cells) made from semiconductor material convert light energy into an electric current. They can also be made to respond to electromagnetic radiation wavelengths outside of the range (400-700 nm) our eye can respond to.
thermistors made of semiconductor material (heat decreases their resistance, thus increasing the current flow in a circuit in which they are located)
light dependent resistors or LDRs made from Cadmium Sulfide (the greater the light intensity, the lower the resistance)
phototransistors react to incident light (the greater the light intensity, the lower the resistance)
piezoelectric crystals or film (physical stress from sound or heat results in the production of a voltage between two different surfaces)
the LM35 is an IC that responds to temperature changes in a very predictable way (it produces, or drops, 10mV for every one degree C change in temperature)
aerials in a number of devices such as radios, TVs and specialized equipment used to search for metallic ores (large scale exploration) or even coins lost on the beach (electromagnetic radiation induces an electric current in the circuit containing the aerial).

gather, process and present graphically information on the relationship between resistance and the amount of light falling on a light-dependent resistor

It is important to recognise that the relationship between light intensity and resistance is not linear. All the syllabus requires for this point is that you are able to present the relationship graphically. A typical graph can be seen at Resistive Input Transducers Antoine Education, UK. Scroll down until you see the graph.
If you are really keen, use Google and do an advanced search using the names of major manufacturers (if you know any) or use light dependent resistors (exact phrase) and English (Language)
When you have gathered enough information, process it by deciding which information presents the graph best and is the clearest.
Present the information in graphical form with notes of explanation.

explain the relationship in a light-dependent resistor (LDR) between resistance and the amount of light falling on it

A light dependent resistor (LDR) is made of a high resistance semiconductor material (such as Cadmium Sulfide). When exposed to electromagnetic radiation (emr) of a particular frequency, the electrical resistance falls. The brighter the radiation (more photons per second), the lower the resistance. Resistance falls because photons eject electrons from the semiconductor into the conduction band. The more electrons ejected, the lower the resistance.
Cadmium Sulfide is sensitive to emr from 515 nm to 730 nm (which almost covers the range of frequencies our eye can detectbut is most sensitive at 515 nm which corresponds to green, the colour our eyesight is most sensitive to).
LDRs are analogue devices because the change in electrical characteristic is as continuously variable as the light falling on the device.

Some more information about LDRs can be found at Technology Student.com by V Ryan, UK and some more at Chemistry Daily, USA.

describe the role of LDRs in cameras

LDRs are used in the light metering circuits in typical cameras to control the total amount of light that falls on the film. The LDR converts the light intensity it records into a voltage. That voltage is interpreted by the electronics in the shutter and/or aperture control circuits. Those circuits set an appropriate shutter speed and/or aperture to ensure that the amount of light falling on the film (or charge coupled device in a digital camera) will reproduce an image of the scene just photographed, usually as the eye would see it.

gather and analyse information and use available evidence to explain why solar cells, switches and the light meter in a camera may be considered input transducers

Gather information from websites, journals, CD ROMs or digital or hard copy encyclopedias
You could analyse the information by drawing together information from different parts of this material. You then may arrive at the following conclusions:
- Solar cells convert sunlight (electromagnetic radiation input) into electrical energy in output circuits to which they are connected.
- Switches convert mechanical energy (the act of manipulating the switch) into electrical energy (when the circuit is completed, electrical energy flows in the output circuit.
- The light meter in a camera is usually an LDR that varies the electric current flow in a circuit according to the amount of light falling on it. That output variable current is linked to shutter and/or aperture control circuits in the camera.
All three have in common the production of electrical energy as output. This satisfies the definition of an input transducer.

explain why thermistors are transducers and describe the relationship between temperature and resistance in different types of thermistors

Thermistors (from thermal resistors) are semiconductor based devices that respond to heat by changing their resistance in predictable ways. Thus an environmental input to the device produces a corresponding electrical output which satisfies the criteria for an (input) transducer.
There are two types of thermistors. One type responds to heat by increasing its resistance (a positive thermal coefficient or PTC devices) the other responds by decreasing its resistance (a negative thermal coefficient or NTC devices). Thermistors are analogue devices.

distinguish between positive and negative temperature coefficient thermistors

The capacity to respond to heat by increasing or decreasing resistance is a function of the way thermistors are made.
NTC thermistors may be made from crystallised silicon and germanium (semiconductor material). The resistance of NTC thermistors decreases proportionally with increases in temperature and their resistance changes gradually as the temperature changes.
PTC thermistors are made by adding small quantities of semiconductor material to polycrystalline ceramic. Their resistance rises rapidly over a narrow range of increased temperatures which means that they can operate like an on-off switch. The critical temperature range over which the resistance rapidly increases is determined by the composition of the thermistor at the time of manufacture.
Some NTC and PTC thermistors are made using other than semiconductor materials.

For additional information about thermistors see the wiseGEEK, USA site.

explain the function of thermistors in fire alarms and thermostats that control temperature

A fire alarm needs to respond to a rapid rise in temperature. An NTC thermistor in a fire alarm circuit can be used in an electronic circuit to trigger a siren once the temperature in a room rises above a predetermined level.
Thermostats that control temperature (in say a hot water system or in a room) may use NTC thermistors in electronic circuits to switch on heating devices when the temperature falls below a certain point or switch on cooling devices when the temperature rises above a certain point.

Background information

PTC thermistors are useful in protecting devices from overheating such as windings that may be overheating in transformers, coils in loudspeakers or electric motors. The thermistor can be linked to a circuit that cuts off the power once a predetermined temperature is reached or exceeded.

NTC thermistors are also used to limit current flow in situations where a current surge might be experienced, such as at the switch-on of an electric motor. An NTC thermistor in the input circuit of an electric motor has a high resistance at room temperature. This restricts the initial current flow to the motor at switch on, thus preventing it being overloaded. As the motor begins to work, the temperature rises and the resistance of the thermistor decreases thus allowing the current flowing to the motor to increase to an optimum amount.

solve problems and analyse information involving circuit diagrams of LDRs and thermistors

Because phototransistors, LDRs and thermistors are resistive transducers, the examples worked in the above section involving potential (voltage) dividers are appropriate here too.
As the resistance changes in response to input from the environmental aspect affecting the phototransistor, LDR or thermistor, the voltage across that device in the circuit changes in proportion to the current flowing through it. Thus circuits containing LDRs and thermistors are used to provide input voltages to other circuits designed to respond once the output voltage reaches or exceeds (or falls below) a predetermined level.