Switches
- Taking Control
We have seen
in the last section how switches give us
control. They are used to interrupt the current flowing in a
circuit. There are many different types of switch and in this
section we will look at some of them and describe how they
work.
There are different
switches for different situations. They
have to be chosen carefully to suit the user and environment
they are in. Please go down the page to view some of the
different types we commonly use.
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Rocker
Switch
Quick
and easy to operate
- used
for room lighting
- once
operated they stay in the position they have been put.
(latched)
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Push
Button Keyboard
Switches
Easy
and comfortable to use.
- Used
in standard keyboards
for P.C's.
-once
operated they return.
(momentary
/non-latching)
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Membrane
Panel Switch
Waterproof,
hygienic and
safe to use in damp areas.
-
Used for cookers,vending
machines
photocopiers or
for flat
surfaces.
(momentary /non-latching)
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Push
Button Switches
- Used
for calculators,
mobiles etc where many
switches need to be placed
next to each other.
(momentary
/ non-latching)
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S.P.S.T
Switches 
These
we have come across already. S.P.S.T. stands for
"single pole single throw". They have two connections and
can either allow current to flow or not to flow in the circuit it
has been placed.
S.P.D.T Switches
S.P.D.T.
stands for "Single Pole Double Throw" and a
good
example of
this type of switch is the Micro switch. It has a
small sprung lever on the top which can be used to detect an
object or item pushing against it. Quite often it is placed near
the safety guards of dangerous machines to tell you if the
guard hasn't been put into position. They can be wired to
prevent a machine from turning on.
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Click
on the micro switch lever to operate it.
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Before
it has been pressed, the switch contact on the left
(known as the pole) is normally conected to the top
connection as shown.
After
it has been pressed the contact on the left now swaps
over and connects to the bottom connection on the right.
When you let go it springs back to its previous position - this
is another example of a momentary or non-latching switch).
Latching
switches
A
rocker or toggle switch doesn't usually spring back after
you remove your finger from the button. It requires a second
touch to return - this is known as a latching switch and is
therefore useful as an on/off button on electronic products.
Click
the latching switch twice to see how the current is redirected.
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Current flow inside
an S.P.D.T. switch
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Q16,
Read
the following description of an intruder
alarm:
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point at door
to close it
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A
simple alarm circuit, with battery and buzzer,
is turned on and off by a lever
operated switch (see
diagram above).
When the door is closed it pushes
firmly against the switch and the buzzer is off. When
an intruder opens the door, the switch "makes" & the
buzzer sounds. Now select from the list below, the
name and best description of the switch that could
be used:
Rocker
- Tilt - Membrane - Micro - normally open
- normally closed
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