This is a fully tileable combo lock that uses repeaters for the combination. Yes, there are a lot of more easier and compact ways of building a combo lock, but I thought it would be cool to have one that utilizes repeaters for it's combinations.
There are two main parts of the machine, the BUD-detector that sends an output when a repeater is ticked and the Counter, which tallies up the amounts of ticks on the repeater.

The BUD:



BUD stands for Block Update Detector. The name is pretty much self-explanatory. When ever a block is placed, destroyed, or changed next to the piston (green wool), it gives out a redstone signal.

The next section is the Counter:


The counter takes a short pulse, such as from a button or BUD detector, and outputs a stronger signal for each pulse.

When combined they look like:



What this now does is count every block update and send out a higher signal. To make this useful, we have to add onto the counter:



What this will now do is keep a signal on unless the output is exactly 2 blocks. This can be edited to any length to change the amount of ticks required to turn the signal off. Now, if you invert the signal, you can hook it up to an iron door to open at the desired number of ticks.



Only one of these is not secure, as you can just click until the door opens. That's where the tile-ability comes in. You can put as many of these next to each other as desired to create infinitely long combinations.



All that is needed to link them is to connect the redstone off of the torches in the most efficient manner. The final line can be ran over to the door, making sure to use repeaters when necessary. To make a reset, you can just run a quick button/lever-controlled clock over the second set of droppers to put the items in the original dropper.

Thank you for checking this out, this is hopefully only the first of a longer line of redstone innovations.
-DragonGaming