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 Membrane Switches
- Used
in oder calculators and
mobiles.
-
Useful when many switches need to be placed close together.
- Momentary
/ Non-latching
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S.P.S.T
Switches 
We
have come across these already. S.P.S.T. stands for
"Single Pole Single Throw". They have two connections and
can either allow current to flow, or not flow, in the circuit it
has been placed.
S.P.D.T Switches
S.P.D.T.
stands for "Single Pole Double Throw". 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. Manufacturers can place them near the safety guards on dangerous machines to detect if the guard has been put into position. It can be wired to keep people safe, by preventing the 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 down and up 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 the 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. Select from the list below, both the
name and the best technical description of the switch that could be used:-
rocker
- tilt - membrane - micro - normally open
- normally closed
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