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555 Timer : Universal PWM Controller
If using a fixed loft bed isnt possible, I would recommend adding some hefty pins to lock the bed in place while overhead to take the load off of the rope.
I know this is an old post, but most garage door openers can't lift much weight. That is why garage doors have springs on them. You'd be better off using a powered winch. You can get one from Harbor freight for about $50 on sale.