This instructable is on a functional countdown timer in Minecraft that goes from 9 to 1. It's also my first instructable, so keep that in mind. The actual redstone is relatively simple, but it needs to be precise for the machine to work properly.
Step 1: The Display
This is the display for the timer, make sure to use sticky pistons, and make sure that the redstone blocks stay one block away from the screen when the pistons are retracted.
Step 2: Wiring the Display
Next, we need to make sure that each digit activates all at once, as in all three pistons of one digit. The digits on the sides only need two redstone torches to activate, but it is important that the repeater on the bottom goes off before the one on the top.
Step 3: Beginning the "Towers"
Next, make a wire from each of the digits so that they all end up on the same level, and are parallel. After that, build a tower like the one in the picture for each digit, with a block facing backwards on each number that the digit will be used on. Each level represents a number, with the lowest being 9, and the highest being 1. (I made a mistake in the picture, I should have used a repeater instead of the redstone that is there.) Also, placing signs to keep track of the numbers helps a lot. It is also important that the towers are at least one block away from each other.
Here's a chart to help you with which digits are used where. This is from the point of view of someone in front of the screen.
Lower left: 8,6,2.
Step 4: The Connectors
Once the towers are finished, make lines of blocks connecting to the repeaters on the backs of the towers. This is where you can test each number by placing a redstone torch at the end of any line. If any number doesn't work properly, follow the redstone line, and see if anything is wrong. Common mistakes would include: Two redstone lines accidentally connecting, forgetting to connect something, making too long a distance between repeaters.
Step 5: The Timer
This is what makes the machine work. There should be one for each connector. Also, I took this picture before adding another repeater into the line going to the next timer circuit, so add one more to your timers. At the bottom, add a line going to a start button next to the screen. All you need to do now is build a timer circuit for each connector, and you should be done!