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ICT in the science department 1636 © CLEAPSS 2005

On/off sensors

There is another type of sensor, of use in computer control and robotics investigations,

that gives a two-state response to a parameter. The simplest example is a switch,

which can be on or off, and similar on/off sensors are available which, for example,

indicate whether a liquid is above a certain level or whether the temperature is above

a threshold etc. Table 16.11 and the diagrams overleaf illustrate such digital sensors.

There is no problem of compatibility; any of these digital sensors work with any digital

input.

Some datalogger systems make available purpose-made on/off sensors, eg, the Harris

e-Log light, reed, tilt, push and magnet switches. Data Harvest supplies pressure-

sensitive timing mats and reaction-timer switches which are useful in studying the

speed of pupils running and in testing their reaction times.

Table 16.11 Types of switch

Name(s) Action Uses

Bell push, push-on, push-

button switch

Contacts close on pressing, break on

release.

Manual start of sequence.

Push-off switch Contacts break on pressing, close on

release.

Simple NOT action (as used in refrigerator

doors).

Miniature tumbler, lever

switch, single throw switch

Contacts close in one position, open in

other.

Lighting models.

Reed switch, proximity switch A magnet causes the magnetic ‘reed’ to

touch a fixed contact.

Switch in door frame operated by magnet

in door.

Infra-red switch, opto sensor Invisible beam reflected back at the unit

causes switch to ‘close’.

Detects moving reflector (needs computer

interface).

Mercury tilt switch

Contacts connected by mercury inside

plastic container when upright.

To sense the position of level-crossing

barrier or car-boot lid.

Pressure switch, pressure mat Contacts close when pressed, open on

release.

Hidden under mat as burglar alarm.

Two-way, double throw Changeover switch, two fixed contacts for

each moving one.

Two are used for switching on stairs.

Toggle switch

Snap-action switch, may be single or

double throw.

Used to switch mains because contacts

move fast.

Micro switch, touch sensor

Operated by small movement, single or

double throw.

To sense that a door is closed or that a

buggy has hit a wall.

Single pole One moving contact. One circuit only.

Double pole Two moving contacts. Two circuits controlled by one switch.

SPST Single-pole, single throw. A simple on/off switch.

DPDT Double-pole, double throw (sometimes

has centre-off position).

Two changeover switches operated by

one control.

Reversing switch

A DPDT switch connected in a particular

way (sometimes has centre-off position).

Two wires in, two wires out, connections

reversed by switch operation.

Knife switch A switch with a blade moving between leaf

contacts may be SPST, SPDT or DPDT.

For models of Frankenstein’s laboratory! A

big, simple switch for a dramatic switch on.

Slide switch A switch with a sliding action, may be

single or double pole, may have more than

two positions.

Not very often used in control of models as

it often needs soldered connections and is

hard to switch.

Rotary switch A switch with a rotating action, may have

many poles and many positions.

Not very often used in control of models

but can be used for controlling several

lights.

Light-dependent resistor, LDR Has low electrical resistance in the light,

high in the dark.

Used to make light sensors.

Temperature-dependent

resistor, thermistor

Has an electrical resistance which varies

greatly with temperature.

Used to make temperature sensors.

Moisture sensor

Has an electrical resistance which varies

with wetness.

Rain sensor or water-level sensor.

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