Help my neighbour's kid with his school project...

He's making a model of a tidal generation plant. The crucial bit for which
he asked me for help is this:

He's making a float that rises and makes electricity with one of those tiny
PM motors. He tells me he's getting about 300mV (cool; a 14-year-old not
only knows what a DMM is but how to use it) and he'd like to run an LED. I
though he might be able to charge a capacitor and dump it through the led
at the top of the float's travel. What I need to know is if there's a way
to get enough voltage to get the LED to light.

Make it spin faster is not reallly an option - it's run through a lego
gearbox now and he says it can't be geared up anymore. I have a few
similar motors around, and something in my memory aboput cycle dynamos says
that PM motors as generators have an output that depends at least in part
on the load, but I don't know what happens when the load resistance goes
up/down.

Is there a simple circuit that I could tell him how to build that would
charge a cap to 2v or so? Should he be spinning and measuring the motors I
have or is that a non-starter?

No this is not for tomorrow, he says he has a week and a half left; the
thing is mostly built, shows output with the meter but something that
lights up would get more points, he thinks.
.

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