Pushbutton Question

I have been taking apart Gameboys and keyboards. These devices have buttons or keys that you push to play a note. When I take it apart, I dont see pushbotton switches like id expect. Instead I see a rubber form with a little black dot on it, and when you press it, the black dot touches a black surface on a PCB. Anyone know how this works?

Comments

[steveastrouk]

Yes, the dot is conductive rubber. it makes a circuit with the inter-digitated traces on the PCB.

Steve

[NachoMahma]

. +1

[frollard]

Agreed with steve -- conductive rubber. My old NES pads had graphite pads on bare copper (hence they wore out so fast).

Nowadays they're painted with that black conductive paint to prevent corrosion on the metal pads.

[crocboy]

So if i wanted to replace those switches with pushbuttons I would have to follow the traces back to a solder point and connect a switch there?

[steveastrouk]

No,look carefully at the board and solder from one set of fingers to the other via your switch.

Steve

[crocboy]

Ok I have it figured out now. Thanks everybody!

[Kiteman]

Is the black surface on the pcb at all flexible?

It might be a membrane switch.

[crocboy]

No its not flexible it seems to be some sort of paint? And on the keyboard there is two of them, they both have to be pressed in order to play a note. Any ideas?

[Kiteman]

In that case they are patches of something conductive, and touching them together completes the circuit.

[crocboy]

Well I thought the same thing until i found that the two black dots that have to touch the patches arent connected in any way. They are isolated by rubber. So I am baffled at this point.

[Kiteman]

See Steve's comment: conductive rubber.