It's been a while since I played Minecraft, but magically I'm still just as good at redstone, if not better than before :D

So here's a little device I made, surprisingly little... A counter for minigames, adventure maps, redstone devices, etc... Doesn't include command blocks and anything else you can't get in survival mode. The main innovation about this one is the display mechanism, it is very simple, compact and so cheap you could build it in early survival game.

The world download contains a step-by-step tutorial :)

This counter has a few extra capabilities that I'm sure most people who are going to use this device will never need, but it doesn't mean you can make the device smaller by removing functionalities you don't need (like adding / subtracting powers of 10) because they are all used by the device internally, and since they are all there anyway, I made these functions trigger-able from outside. The only thing you can remove to make it smaller is the display, which you won't need if you use the counter as a part of a bigger device.

Features:

1. Extendable - you can stack layers on top of each other as much as you want, every layer is another digit so you can easily stack 6 layers on top of each other and have a device that can count and display numbers up to one million! (note that you need to repeat the middle layer of the device, not the top and bottom layers, they are missing some unnecessary blocks that aren't needed only on the edge of the device)
   
2. Compact - no matter how much you extend this device, it will not change it's width and length, and it's height will depend on the amount of digits you add to it's capacity. It stays 9x9 all along, no matter how much you increase it's capacity. Removing the buttons from the screen and hooking the counter to a custom control room can decrease the width of the redstone by 2. When building a 2 or 1 digit version of this device, it's compactness increases A LOT. A 1 digit counter like this would only be 9x2x5.
   
3. Mostly silent - no pistons, nothing noisy except for occasional click of an empty dropper firing. The only exception is resetting the counter, it can make a bunch of empty dropper clicks. (but it's worth the compactness)
   
4. Customizable - You can place the control panel for the counter anywhere you want and include buttons for any actions the counter can do, which is a good thing because this counter can do more things than what you want to have buttons for. Also it comes with buttons on the screen itself, which make the device 2 blocks wider, but you could remove them easily. If you don't need to display the number at all you can remove the display circuit completely and make the device 4 blocks thinner.
   
5. Complex operations - If you use all inputs and outputs to their full potential you can have some functionality beyond simple counting: add and subtract powers of 10 (1, 10, 100...), reset any digit you want to 0 without changing the rest of the number, and the best part is that you can use the value of the digit as an output to any device you want - you can get the exact value from the comparator side or get a circuit to trigger when a digit reaches 9 or resets to 0 by using the signal from the far edge of the display circuit. Integrating these outputs allows doing all kinds of complex logic checks on the number, like checking if it passed a size limit, waiting for a specific number to be reached, checking if the number is in a specific range of numbers, check if the number is odd or even, and so on...
   
6. Toggle-able display - You can turn off the display without turning off the device itself, so you can keep using it as a counter without showing the number until you decide it's time to turn the screen back on.
   

Disadvantages:

1. If you send too rapid pulses to the counter it will fail since it contains redstone torches that can burn out, so be careful about that.
   
2. When subtracting, the device will change the value of only one digit at a time, which means incorrect result in specific cases.
   

How it works:

• Counting: each digit's value is stored inside a pair of droppers that face each other, containing 9 unstackable items in total. When activating one of the droppers it will move an item into the other dropper, which increases or decreases the value of the digit they represent. When the digit is 9 and another attempt to increase the value of that digit is made, the pulse is redirected to the next digit (layer above) and a 9-pulse loop is triggered to empty the dropper of the current digit. The input to external "add" commands is a pulse shortener to prevent errors in the circuit due to too long input.
  
• Display: A comparator attached to the dropper that represents the current digit, outputs a redstone signal into a 9x3 redstone dust path that is made in such a way that for every extra item in the dropper, the redstone signal will be able to reach another redstone lamp and turn it on. For example, if there are 5 items in the dropper of that digit, the redstone signal will reach the first 5 redstone lamps in this row. (there's a screenshot of the display circuit on the top)
  

I wonder if someone will use it in survival, maybe to count the mobs killed by his mob farm or something?