Using simple switches in electronic circuits - Attractrive guide to Basic Electronics - An introduction to basic electronics for GCSE students and KS3. SPST, SPDT, DPDT, PTM, PTB and other acronyms of switches unravelled !

14th November 2010

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

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