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I am wondering why some tactile switches have 4 terminals instead of two? For example, take a look at these switches.

What is the use of the two remaining pins? If the pins of the exact opposite side are always shorted then why don't they have just 2 pins?

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3  
It does actually say in the blurb "4 Pin layout can be used as jumper leads." Possibly a clue there. –  JIm Dearden Feb 21 at 20:55
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mechanical stability –  jippie Feb 21 at 20:56
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As @JImDearden says, it makes it possible to build an inexpensive matrix keypad using a single-sided PCB, without requiring a lot of wire jumpers. –  Dave Tweed Feb 21 at 21:00

5 Answers 5

up vote 37 down vote accepted

I'm going to put David Tweed's comment into an answer, which it deserves.

The dual shorted pins allow inexpensive single-sided boards to be used for X-Y matrices of switches without requiring jumpers.

Here (from an NKK datasheet) are a couple examples of such layouts:

X-Y matrix (This would typically be scanned by a microcontroller or ASIC):

enter image description here

Common line (one side of each switch common, typically it might be connected to Vss or Vdd and a pullup or pulldown resistor (perhaps internal to a chip) would be required for each switch.

enter image description here

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Mechanical stability. When soldered down with four leads the part is can't move in any direction. The shorted pins makes the pinout slightly more versatile when routing a PCB. When introducing 2 extra pins for mechanical stability is probably cheaper (and more rugged) to build shorted pins than to add unconnected pins.

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My guess is that 4 pins are for mechanical stability. Then since they are there anyway, someone in marketing had the brilliant idea to feature them as jumpers too. Think about it though, If you only had two pins there would be less of a need for a jumper.

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3  
No, you think about it: How would you lay out a large NxN matrix of buttons on a single-sided PCB without jumpers? It would require N-1 tracks to pass between the two pins of some of the switches. Having at least one split pin on each switch makes it trivial. –  Dave Tweed Feb 21 at 21:41

Think of a switch as being two staples (held in place by the plastic base) and something that can bridge between them. Such a design requires two non-moving pieces of metal plus a moving contact which doesn't have to flex much and a coil spring to keep that contact away from the pins. Such a design is more mechanically robust than using a flexible contact which is integral with one of the pins. Further, if one needs at least three solder connections on the board for mechanical stability, it's easier to use two identical "U"-shaped pieces of metal than it would be to have contacts which were only connected to one pin, and then need other pieces of metal for the other mechanical connections.

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Yet three pin and two pin switches are also common. –  Passerby Feb 22 at 3:29
    
@Passerby: Different switches have different requirements with regard to cost, compactness, key-bounce, reliability, etc. The design I described isn't ideal in all applications, but it's pretty common. –  supercat Feb 22 at 16:45

Some 4 pin tact switches do come in DPST or SPDT or even DPDT setups. http://www.e-sonic.com/aboutus/cat/S/switches%2029.pdf

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Other than an error (TL3301SPF260QG) in the distributor catalog, I don't see any. Also, DPDT in a 4-pin package seems somewhat unlikely to me. –  Spehro Pefhany Feb 22 at 21:02

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