After using the ATMega328 from my Arduino UNO in the ZX81 USB Keyboard, I needed to replace it. The chips themselves are only £3-4, so is much cheaper than buying a whole new Arduino. The only issue is the chip needs to have the bootloader burned into it before it can use the USB Serial programming in the Arduino. You can actually by them with the bootloader already programmed for about £5, but I was looking to burn it myself to give me more options in the future.
This is an old post, preserved for reference.
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.:oomlout:. in the UK, so this is just a quick review of that.
step by step build instructions are online,