Switches
Whenever we turn on a computer, house lights or select an icon with a mouse we are making use of switches.
There are a large number of different switches available to suit all sorts of applications. For our purposes
we need to consider how five forms of switch operate.
Toggle Switch
A toggle switch has an upright ‘arm’
which is moved backwards and
forwards.
The pictures show some toggle
switches in reality, whilst on the far
right we see how toggle switches are
used on a radio control transmitter.
Here a toggle switch might be used to
lift the landing gear on a plane up and
down.
Push Button Switches
As its name suggests these are switches which are pressed or pushed to
make them work. They are a very common type of switch.
The are available in two main types, a Push To Make (PTM) switch which
switches on when pressed, and a Push To Break (PTB) switch which
switches off when pressed.
The picture shows just some of the many push button switches available.
Micro Switches
Micro switches are switches that need very little pressure to turn them on or off. Unlike other switches they
can be supplied with a range of fittings on them to turn them on and off, from levers to rollers and buttons.
The pictures top and middle left show micro
switches with lever arms of various lengths. The
longer the lever arm becomes the less pressure is
needed to control the switch. Top right shows a
roller micro switch. These are usually used when an
object that moves horizontally backwards and
forwards needs to control the switch. A good
example is in the control of Pneumatic pistons.
A computer mouse uses button micro switches.
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Rotary Switches
Rotary switches are used when we need to turn on or off a number of different items using just one switch,
the switch being rotated to the correct position.
The pictures show what one type of rotary switch looks like in reality together with a example of their use
in a car windscreen wiper control. The rotary switch (just visible under the panel) is joined to the circular
knob with the word ‘wiper’ on it. The switch has three positions and when rotated can switch the wipers
off (O position), switch them to low speed (LOW position) or high speed (HIGH position).
Reed Switches
Reed switches are different to the other types of switch we have looked at in that human intervention is not
needed to switch them on or off. Instead the switch is controlled using magnetism. When a magnet is
moved close to the reed switch it controls the switch. Reed switches are used to sense proximity, that is
how close an object has come to the switch, or movement.
The picture top left shows a reed switch enclosed in
a glass case. Metal contacts inside are pulled
together when magnetised closing the switch.
Where a glass case is not suitable they are enclosed
in metal cases. The picture top right shows such a
reed switch (the unit with the wires coming from it)
together with its magnet, whilst on the left is a reed
switch that its manufacturers state is ‘bombproof’ so
can be used in very harsh environments.
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Circuit Symbols for Switches
No matter which type of switch we use they all use the same circuit symbols. These are shown below.
Normally Open (NO) Switch
NO Switch with Spring Return
Normally Closed (NC) Switch
These switches have two contacts, these are called the POLE and THROW.
POLE
THROW
By identifying which contact is which it means that we are able to connect the switch correctly into our
circuit. The switch above has a SINGLE POLE and a SINGLE THROW, therefore it is called a Single Pole
Single Throw (SPST) switch. This is the most common arrangement and is used in many forms of simple
on/off switches.
A Single Pole Double Throw (SPDT) switch has one pole and two throws, for example.
THROW This type of switch is used when we want to switch between two different
circuits. For example a switch that enables us to swap between FM and AM on
a digital radio. In this case the switch is either on FM or AM.
POLE
THROW Sometimes we need to be able to move the switch to an OFF position as well.
In this case the switch is called a SPDT Centre Off switch. For example a switch
on a control panel that can move a robot forwards, backwards or stop.
A Double Pole Double Throw (DPDT) switch has two poles, each of which has two throws, for example.
The most common use of this type of switch is in reversing the
direction of a motor. To reverse a motor it is necessary to
swap over the power connections, i.e. the positive battery lead
THROW connects to the motor negative and the negative battery lead
connects to the motor positive. By using a DPDT switch we
THROW can make the motor move in either direction.
THROW
POLE
POLE
The dotted line joining the two poles together means that
THROW when one pole moves the other pole moves with it.
Battery positive
Battery negative
M
The circuit diagram on the left shows how to
connect up a DPDT switch to reverse a
motor. The diagram shows the motor
running in one direction. You need to
picture it when the switch has moved down,
now which end of the motor is joined to the
battery positive and which end to the battery
negative?
Switches like this in which we change between one set of connections and another are often referred to as
Change Over (CO) switches. Switches can also have one or more of their connections joined to the same
point. Where this happens the particular switch contact is referred to as a Common.
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Questions on Switches
Shown below is a photograph of Michael Schumacher’s steering wheel as used for a Brazilian Grand Prix
at Interlagos. The steering wheel has mounted on it thirteen switches of various types.
Using the work you have learnt about different types of switch, use labels to identify the types of switch on
the steering wheel. Write about why each type of switch has been used for that particular application.
You do not need to try to work out what each switch does, focus on what type of switch it is and why it has
been used. Apart from the obvious switches on the wheel itself, the two silver coloured ‘paddles’ behind
the steering wheel on the bottom left and right are also switches. These are the gear shift paddles that enable
the driver to change up and down gear without needing to take his hands off the steering wheel.
Although not switches, try to think why there are six gray rectangular display panels in the top centre part
of the steering wheel. What sort of display might they be and what sort of information might they be
presenting to the driver?
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Relays
Like the reed switches we looked at earlier, relays are another type of switch that does not require human
intervention to make it work. Relays operate by making use of ELECTROMAGNETISM. Pictures of what
some relays look like in reality are shown below.
In this relay the coil of wire is
on the right hand side and the
switch contacts on the left
hand side.
As you already know from your Physics lessons, when a current is passed through a coil of wire an
electromagnet is created. In a relay this electromagnetism is used to pull together two metal switch contacts.
As the relay operates by using electromagnetism there is no physical connection between the coil of wire
and the switch contacts. This enables a circuit that runs from say a 9V battery to switch on much larger
voltages, i.e. 240V mains lights without the mains current damaging the other circuit in any way.
The circuit diagram below shows how a transistor is used to switch on a relay controlling house lights.
+ Voltage
(+9V say)
240V
Fuse
Relay
Coil
House Lights
0 Voltage
Primary Circuit
Secondary Circuit
When the transistor is switched on, current flows through the relay coil creating an electromagnet. This
closes the two metal switch contacts which completes the secondary circuit turning on the house lights.
When the transistor is switched off, current stops flowing through the coil, the electromagnetic field
collapses and the switch contacts open turning off the house lights.
However, when the electromagnetic field collapses it generates
a large back emf (electromotive force), this can destroy the
transistor. To prevent this from happening we need to place a
diode across the relay coil.
+ Voltage
The circuit diagram on the right shows how to connect the diode
across the relay coil. If the diode were to be placed the wrong
way around, no current would pass through the relay coil and
the relay switches would not close.
0 Voltage
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