When I create really big circuits and I go some distance away, the redstone mechanisms stop working and freeze. And if I come back they are still frozen. I have to click on a wire or repeater and place it again to get it working again. My circuit is too big for me to keep all of the chunks loaded by myself, and I can't open minecraft ten times and connect all clients to the server and place all players around the circuit.
Can I expand the area where redstone works?
By default, a 21 chunk by 21 chunk area (with the player at the center) is loaded for each player on a server. The simplest solution is to build your circuits so that they use both vertical and horizontal space to fit them within that area. If this is not possible, then you will need either a plugin (bukkit server) or a mod (vanilla server) to manually load the chunks. It is important to remember however, that this will cause an additional load on the server, so the first solution is still preferable.
Here is the mod that I use to load areas:
Chicken Chunks - http://www.minecraftforum.net/topic/909223-162-smp-chickenbones-mods/
A simple search on the bukkit plugin site will yield a number of options. I cannot suggest one over another, especially without knowing what version you are running.
In Vanilla Minecraft you can use loops of hoppers to force chunks to stay loaded. More details are at this Arqade answer: Hoppers as Chunk Loaders
