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This is a picture of a metal dome switch taken off of the PCB it's mounted on.

I can see traces that connect the corners of where the button is mounted, but it seems to me then that the contacts should be always connected via the metal dome button. There's nothing in the middle on the PCB where the button makes contact whenever it's actuated. Could someone explain to me how these particular switches work?

Thanks!

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    \$\begingroup\$ There must be a contact within the button itself that the dome contacts when it is pressed. \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 7 '18 at 21:21
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    \$\begingroup\$ Those aren't simple domes. They are dome shaped buttons, and the pads on the board are just the connections. The dome doesn't have to make contact with the board. \$\endgroup\$ – JRE Mar 7 '18 at 21:24
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    \$\begingroup\$ it's a part \$\endgroup\$ – τεκ Mar 7 '18 at 21:27
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Notice that the top two contacts (1, 3) in your picture are not connected to the bottom two contacts (2,4). Presumably the switch connects between those two points.

They could be surface mount membrane switches. A membrane switch is basically two conductors separated by a thin membrane that keeps them apart until they are pressed together by an external force. This is what is used by most devices that have flat plastic keypads, like a microwave oven.

Another possibility is that they are capacitance based, and pressing down the dome increases the capacitance, thus causing the trigger. You might be able to figure this out by looking at the chip they are connected to. Some MCUs have built in capacitance switch circuitry and there are also dedicated capacitance switch controllers.

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