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Prisoners’ Escape

In an effort to make this blog more interactive, I’m switching the riddles to a new format. I’ll first post only the riddle, without the solution, so you guys can have a crack it. After a week or so, I’ll post the solution. Here goes…

prisoners are planning an escape. They bribed a guard to let them all meet once. Each prisoner was given a task to complete, such as digging or stealing some key. Once all the tasks are complete, the prisoners will notify the corrupt guard who will turn a blind eye while they escape. The problem is, the prisoners can’t talk to each other because they are all in solitary confinement.

Once a day, a prisoner chosen at random enters a storage room that has a single light bulb in it. The prisoner can either flick the switch (turning the light bulb on or off), or leave it as it is. At the time of the initial meeting, as they plan their strategy, the state of the switch is unknown.

Using only switch flicking, how can the prisoners coordinate their escape? Specifically, one of the prisoners must know at some point that all the tasks are done, so he can notify the guard and lead them all to freedom. If the guard is notified before all tasks are complete, the plan will fail and the prisoners will be caught.

(ready for the solution? here it is)