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Forum Home > Tips, Tricks and Tutorials! - Builder's Guides for GuitarPCB Boards! > Sticky: Understanding switches

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This topic: Courtesy of Tonmann.

Wiring switches can be confusing mainly because there is no standard method of labeling the contacts or showing the switch in a schematic.  The following methods are used throughout GuitarPCB (or at least on my and Bruce's schematics) hopefully you will find them as simple to follow as I do.

The first part of the diagram shows a Single Pole Double Throw (SPDT) switch where the connections (solder lugs) are labeled from top to bottom 1, 2 & 3.  If you look at the schematic representation of a SPDT switch it looks like a lever (the pole) that flips between two outer contacts (the throws) and that is exactly what physically happens on the switch; in one position lug 2 is connecte to lug 1 and in the other postion lug 2 is connected to lug 3.  It should be obvious that lug 2 must always be used for the switch to function.

The Double Pole Double Throw (DPDT) switch is basically two SPDT switches in one enclosure.  They are mechanically dependent and electrically independent of each other which is a fancy way of saying that when the switch is in one position lug 2 is connected to lug 1 and lug 5 is connected to lug 4, in the other position lug 2 is connected to lug 3 and lug 5 is connected to lug 6; there is no electrical contact between the first switch (lugs 1,2 & 3) and the second switch (lugs 4, 5 & 6).

The lugs are always numbered (looking at the back of the switch) from top to bottom, left to right. Although there are other methods of numbering and describing the lugs we find this the easiest - I would rather describe a problem as being on lug 4 than being on the top throw of the second switch.

This method of labeling the lugs also extends to other types of switches; a 3PDT (which is basically three SPDT switches in one enclosure) would have lugs 7, 8 & 9, a 4PDT would have lugs 10, 11 & 12 and so on.

One point worth mentioning is the physical action of the switch.  Taking the SPDT as an example, when the switch is in the up position lug 2 is connected to lug 3 and when the switch is in the down position lug 2 is connected to lug 1.  Although this "see-saw" action might be counter-intuitive it shouldn't be a problem as you will ultimatedly decide what is up and down (or left and right) when you mount the switch on the enclosure.

All double throw toggle switches come in two flavours: ON - ON and ON - OFF - ON.

The ON -ON switch has two postions as already mentioned in the up postion lug 2 is connected to lug 3 and, in the case of a DPDT switch, lug 5 is connected to lug 6.  In the down position lug 2 is connected to lug 1 and lug 5 is connected to lug 4 for a DPDT switch.

The ON - OFF - ON switch has three postions, as well as an up and down position where the connections are exactly th same as for an ON - ON switch, there is a middle position where lug 2 (and lug 5 in the case of a DPDT switch) is not connected to either of the outer lugs.  The difference between the two types of switches should be obvious when looking at one of our schematic diagrams.

Following switching arrangements on a schematic can be a problem for some people, in order to make the circuit design program EAGLE more flexible I had to come up with a method of labeling switches which  would be easy and logical (to me at least !!!!).

The diagram above shows three switches connected to a circuit board.

SW is used to denote a switch.

The number after SW denotes the physical switch.  In the above example there are two switches, SW1 and SW2.  If there were a third switch it would be labeled SW3, a fourth switch SW4 and so on.

Letters, or absence of letters, after the number denote the type of switch:

Single pole switch - no letters (one set of solder lugs)

Double pole switch - letters A and B (two sets of solder lugs)

Three pole switch - letters A, B & C (three sets of solder lugs)

Four pole switch  - letters A, B , C & D (four sets of solder lugs)

and so on.

The lever (pole) of the switch which is the M(iddle) lug always points to the T(op) lug and never the B(ottom) lug.  I've added T, M & B to the example above, these letters never appear on a proper schematic.

For all of the information above it should be easy to see that the circuit has two switches:

SW1 - a SPDT ON - ON switch

SW2 - a DPDT ON - OFF - ON switch

plus the wiring from the board to the solder lugs of the switches.


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February 12, 2014 at 12:21 AM Flag Quote & Reply

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