7.Sensors

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Sensors

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A sensor is a device which detects and measures a

physical quantity and responds by producing anelectrical signal.

Light

Pressure

Sound

Humidity

Strain

Heat

Sensors

A sensor that detects fingerprints,

used in security systems

Sensors are used to take measurements or readingsfrom the environment, such as

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Sensors Are Used in Various Ways

Data logging - the use of sensors to collect data

automatically at regular intervals, e.g. collectinginformation on earthquake vibrations, collectingweather data.

Computer control - using a computer to control apiece of machinery, e.g. the temperature in agreenhouse, a robot arm.

Input devices - e.g. a scanner uses a light sensor todetect the light reflected back from the page. Thiscreates an electrical signal, which is processed by a

computer to create a file of data.

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Sensors can be either analogue or digital.

Sensors are Either Analogue or Digital

Most sensors are analogue, which means that theyproduce analogue signals, which look like this

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Sensors can be either analogue or digital.

Sensors are Either Analogue or Digital

Analogue signals need to be converted to digitalsignals using an analogue-to-digital converter (ADC),

before a computer can recognise them.

Most sensors are analogue which means that theyproduce analogue signalswhich look like this

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Digital Signals

Digital sensors produce digital signals, which look

like this

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Digital sensors produce digital signals, which look

like this

Digital Signals

A difference between analogue and digital signals isthat analogue signals can take any value within arange, whereas digital signals can only take certainfixed values. This makes a digital wave jump in

steps, whereas an analogue wave changes gradually.

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Digital Sensors

There are not many digital sensors, but one example

is a pressure pad which uses a pressure sensor. A pressure pad simply gives an on/off response. If

someone treads on it, it produces a signal. If no-onetreads on it, it does not produce a signal. The signal

is either on of off, so it is digital. It produces a verysimple stepped signal like this

Someone treading:

signal on

No-one treading:

signal off

The vertical lines arenot part of the signal,

but are usually drawnto join up the signal

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Digital Sensors

There are not many digital sensors, but one example

is a pressure pad which uses a pressure sensor. A pressure pad simply gives an on/off response. If

someone treads on it, it produces a signal. If no-onetreads on it, it does not produce a signal. The signal

is either on of off, so it is digital. It produces a verysimple stepped signal like this

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Types of Sensor

Pressure Sensor

How it is UsedType of Sensor

Detects touch: used in pressure pads toactivate automatic doors.

Light Sensor

Temperature Sensor

Strain Gauge

Motion Sensor

Detects light: used in the CCD of digitalcameras to convert light into electrical signals.

Detects force: used in weighing scales, and tomeasure the forces across earthquake faultlines.

Detects movement: used in alarm systems todetect the presence of an intruder.

Detects heat: used in alarm systems to detectthe presence of an intruder, used to controlthe temperature in a greenhouse.

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Types of Sensor

Sound Sensor

How it is UsedType of Sensor

Detects sound: used in microphones toconvert sound into electrical signals, and inalarm systems to detect noise from intruders.

Infra-red Sensor

Magnetic Sensor

Humidity Sensor

pH Sensor

Detects infra-red light: used in remotecontrols, and in alarm systems, where itdetects when a beam is broken.

Detects humidity: used in greenhouses and inextractor fans, to switch on and offautomatically according to moisture in the air.

Detects pH levels: used by farmers to detectthe acidity or alkalinity levels in soil. (Which

can also reveal pollution levels).

Detects magnetic fields: used in MICR:Magnetic Ink Character Recognition, themagnetic numbers on the bottom on cheques.

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When a patient is critically ill or just had a major

operation in hospital, they may be placed in anintensive care unit. They will be constantly monitoredto make sure that their condition is stable.

Sensors in a Hospital

Temperature

Blood pressure

Heart rate

Breathing rate

Oxygen level

The patient couldbe monitored for:

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F1 racing cars are continually monitored using

sensors during a race.

Sensors on the Race Track

Fuel use

Engine temperature

Oil pressure

Tyre pressure

Tyre temperature

Car speed

Car acceleration

Sensors measure:

Most modern family cars have sensors that collect

data and report faults to the driver.

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In this greenhouse, a temperature sensor collects

data on temperature every 5 minutes.

Sensors in a Greenhouse

If the temperaturefalls below 26C, the

computer closes thewindows and turnsthe heating on.

If the temperaturerises above 26C, thecomputer opens thewindows and turns

the heating off.

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In this weather station,

different sensors, suchas a pressure gauge(barometer), humidity gauge(hygrometer), rain gaugeand thermometer continuallyrecord changes in weather.

Sensors in a Weather Station

The data is downloaded to a

computer which is used toanalyse the data, and makepredictions about futureweather (weather forecast).

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Sensors in a Weather Station

Further long term analysisof weather data, can lead toa greater understanding ofclimate changes due to thegreenhouse effect.

This person is replacing the

weather vane on a portableweather station.

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Sensors in Security Alarms

This security alarm uses an infra-red sensor which

detects when a beam is broken, and sends a signalto the control box.

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Sensors in Computer Control

A sensor detects a physical quantity and produces an

electrical signal. This analogue signal is sent to aninterface box (control box).

The interface box converts the analogue data todigital data (using an analogue-to-digital converter)and sends it to a computer to process.

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A sensor detects a physical quantity and produces an

electrical signal. This analogue signal is sent to aninterface box (control box).

The interface box converts the analogue data todigital data (using an analogue-to-digital converter)and sends it to a computer to process.

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The computer processes the data and produces a

signal as output. Below, the computer is controllingthe temperature of the water in the tank. If thewater gets too hot, the computer sends a signal toswitch the heater off. If the water gets too cold, thecomputer sends a signal to turn the heater on.

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Calibration

To calibrate a sensor means to set it up, and make

sure that it is reading accurately. This ensures thatthe data produced will be valid.

To make sure that a sensor is reading accurately,you have to check it against a known standard, e.g.

You may have seen your teacher calibrate aninteractive whiteboard. This process ensures thatthe screen (a sensor) accurately detects theposition of the board pen, so that the cursor is

displayed in the same position as the pen.

A set of weighing scales uses a strain gauge.The reading on the scales will be tested byweighing known weights.

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Advantages of Sensors

Sensors can be used in remote, inhospitable and

dangerous places.

They can collect data 24 hours a day, 7 days a week

if necessary.

There is no need for a person to be present to collect

data.

Sensors are more accurate at collecting data as unlike

humans, they do not make mistakes.

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Disadvantages of Sensors

If a sensor breaks, it may be difficult to replace if

the sensor is in a remote, inhospitable or dangerousplace, e.g. on Mars or inside a nuclear reactor.

Some sensors can be expensive.

A sensor may be faulty, or poorly calibrated in whichcase it will produce completely false data. It is up to

the person in charge to check that the data is valid.

If a sensor breaks, no more data will be collected

until it is replaced, so important data could be lost.

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Keyword Activity 1

Drag the missing words to the correct place in the

sentences

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Keyword Activity 2

Drag the missing words to the correct place in the

sentences

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Wordsearch

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Crossword

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Quiz

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After viewing this presentation, you should

Be able to explain how a sensor works.

Be able to list several types of sensor, the physicalquantity they measure, and give examples of where

they are used.

Know that analogue signals need to be converted

into digital signals using an analogue-to-digitalconverter so that a computer can understand them.

Be able to list some advantages and disadvantages

of using sensors.

Know that sensors are used in data logging,computer control and inside other input devices.

Objectives

Know why sensors need to be calibrated, and be ableto explain the process of calibration.

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