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I'm building this circuit, which converts a toggle switch to a momentary one, which sends a key to a USB keyboard PCB by connecting 2 contacts together:
I just wanted to check, how do I know the keyboard PCB won't get fried when I flip the switch to 12v DC? Or worse my motherboard?
The momentary buttons make a physical connection when pressed. The relay's actuating part, does the same. And unless you short a wire out or physically connect them wrong, you won't fry the computer or keyboard. The relay provides physical isolation between the coil and the actuating part.
If nothing but the contacts goes to the PCB you won't hurt the computer.
If you want to be super-cautious, add a 0.1uF capacitor across the relay coil. That will ensure that when the switch opens, no serious spikes get coupled to the keyboard scan lines.
It seems a resettable polyfuse would do the trick:
USB Overcurrent Protection
The Arduino Mega2560 has a resettable polyfuse that protects your computer's USB ports from shorts and overcurrent. Although most computers provide their own internal protection, the fuse provides an extra layer of protection. If more than 500 mA is applied to the USB port, the fuse will automatically break the connection until the short or overload is removed.
