I am hoping to be able to set up a gunge tank in the next few week, nothing huge, just a bucket over the head type of thing, released by pulling a string connected to a rubber bung in the bottom of the bucket. My question is, is there a really basic way of automatically pulling the string after a indeterminate period of time? Think the candle burning through the rope in a cartoon, which then releases a swinging axe.

P.S. no candles or swinging axes please