This video tutorial (Click "Play" to start) shows how to build a Lock & Key mechanism.

For those who haven't seen a Lock&Key before, this is how it works. It's essentially a modified Item Sorter:

1. You put in a unique item that only you have a copy of (unless you choose to share) - this is the "Key," it works very similarly to a Password for your computer / online accounts
   
2. When you put in this unique item, the mechanism sends a redstone signal output
   
3. If you put in another item (like a piece of dirt, or even an un-named version of the same item), nothing will happen.
   

This is not my first video on Lock & Key Mechanisms, but what makes this one unique is it's very small! (At least, compared to other Lock & Key Mechanisms, both from myself and other redstone engineers).

It's 2x4, and 3 blocks tall, not including the pressure plates (which are optional) and door / door frame.
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