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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 pressuresensitive 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, pushbutton 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
Not very often used in control of models as
single or double pole, may have more than it often needs soldered connections and is
two positions.
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.