foot pedal power

Ian Stirling olpc at
Sun Jan 23 06:37:12 EST 2011

Mike Dawson wrote:
> Hi All,
> Such a device would need adaptation for the OLPC to be possible for
> kids (particularly those in the developing world) to use.  We found by
> experimentation with weights that kids 8-12 average max sustained
> power applied against gravity was around 11W.  They would need at

This seems a bit pessimistic.
Say 25Kg, 11W will raise 25Kg about 4cm in one second.

Or about one step of a normal stair per 2 seconds.

When was the last time you saw a kid go up stairs that slow?

I have a scribble-blueprint of my ideal phone charger, but it's complex.

Imagine a box ~100*25*80mm about 300g or so.

This then unfolds into a device like a car footpump - an A frame.

You step on this, and it generates a pulse of 100W or so.

This is stored in a couple of LiFePO4 cells, which can take about
30W of charge each.

It charges enough to recharge a phone in perhaps 100-200 steps.

The gearbox and frame is challenging - it requires a light
robust gearbox capable of perhaps withstanding impacts of 200Kg,
and normal cyclic use of 120Kg or so.
The electrical design is similarly complex, you need a peak power
tracker to efficiently charge the battery from the variable speed,
as well as smarts to optimise the speed profile for ergonomics.
Then you need to use it as a motor to pop the pedal back up.
(this is lighter than a spring.)

This can be made a lot easier to make if you allow the weight to
grow to a kilo or two, and drop the required efficiency.
This also makes it a whole lot less usable.

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