They are around $69 and I would enjoy making one on my own.
Yeah you can use a voltage divider to scale it down. But I'd still put input protection, see this page.http://www.kevinmfodor.com/home/My-Blog/microcontrollerinputprotectiontechniques
Another way to do it would be to use an optoisolator; this would also have the advantage of isolating your Arduino from any potential high-voltage spikes on the electrical system.
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