Ah, after looking at it more I see what he's doing. Ultra conservative design (in terms of current draw through the Arduino). You really could simplify it and just use the NPN. You'd have to drop the value of R3 to 5-10K butit'd work just the same.
Thanks for posting this. It'd be great to see people doing more DIY electronics. It's intimidating at first but easy once you get into it. Stuff like this really helps!
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