So today I built a pretty cool vault, I ended up having to harness the power of YouTube and learnt a few things:
Redstone Piston T-Flipflop:
http://youtu.be/LzYkd6KbkC8
Vertical Signal Transmission:
http://youtu.be/A2-SwoM5RgA
And the really cool part, a mightily compact 3x3 vault door!
Super Compact 3x3 Piston Door:
http://youtu.be/fOPGlU2M29U
I linked the videos above as there are SO MANY awful videos out there and many of them don't go into the detail I was looking for but these vids are all great and hopefully inspire you to build something cool :)
Oh and if you DO want to check out what I built today make your way to the Vault building basement :)
http://mc.tomasu.org/#/557/64/-74/max/0/0
Looks pretty darn cool! I ended up watching a bunch more videos after that first one that were quite helpful.
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Sand and gravel should be renewable by grinding stone into gravel, gravel into sand.