This is a really good point, and in the future it would be really cool to implement it on anything. However, it's tricky to guarantee compatibility with every n

This is a really good point, and in the future it would be really cool to implement it on anything. However, it's tricky to guarantee compatibility with every num pad out there, and there are several pieces of software which try and do this already but come up against numerous problems. In the spirit of keeping it simple, I'm using an Arduino as the heart of it as they are simple to programme and people can hack and tweak to their own needs, rather than struggling with firmware upgrades etc. I do really like your idea of making it work with a range of hardware. I'm actually going to release 4 different designs ranging from a single button right up to the 12 mechanical key version you see in the link. That way people can choose their price range. Could be as low as £15 !