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14th  November 2010
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Mechanical switches come in a variety of types.  A glance through a catalogue will reveal many – but the principle for them all remains the same –
                         joining wires in a mechanical way  by :
            moving a slider or lever
            using an electric coil to magnetise a contact
            switching by electronic means using a voltage divider and sensor
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A simple switch can be a ‘push-button’ type.  
           This can ‘lock-on’ or it can be a ‘push-to-make’ or a ‘push-to-break’ action.
   
All of these actions give a DEFINITE – ON or OFF signal and they could therefore be called ‘digital’ switches.  If there is some in-between state possible – as in a volume control of a radio – then the switch could be described as an analogue switch.
  
Switches like this can also be operated by a ‘toggle’
or slider action.
Although the second three contact points can’t be seen on this small sketch they could be present alongside the set we can see.  This would make this a DPDT switch.  If the contacts we can see in the sketch were the only ones on the switch,  then it would be of the SPDT type.  
A supply wire joined to the centre of the switch shown here could be made to link to a part of the circuit connected to the left contact or a part
It is easy to see what a  Double pole – Double throw  (DPDT) would look like –
or a double pole – single throw. (DPST).  Just be checking if there can be two ways or only one way for the contact to be made.  These kinds can be in Push-to-make and push-to-break too.
If only one set of contacts is available on the back then the switch is said to be

Single Pole,
Single Throw   -  SPST
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Two ways for the contact to be made .....
Centre to left or centre to right.
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DPDT
 
As shown above
Switch Circuit Symbols
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Think about theses switches -TV controls, webcamera, keyboard and monitor switch.  What kinds of switches have been used for them ?
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IWB Electronics Wordsearch
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  Download Wordsearch
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