# I Generating light by using the energy from a yoyo

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1. Apr 18, 2017

### hnbgm

I want to design a yoyo that upon spinning, the energy from the spinning, can turn a mini generator inside and store that energy into a battery which then powers a LED inside.

From basic calculations, I have seen that I could probably generate about 10W of power per 'toss' due to the high rpms and the low friction in the ball bearing that spins. If the generator is only capable of 5W, would the wasted power affect the yoyo and overheat excessively? If so, how would I go about protecting the rest of the circuit such as the battery and LED.

I am a noob in electronics so please be kind.

2. Apr 18, 2017

### Staff: Mentor

Welcome to PF!

10W sounds waaay high, but it depends on for how long. Could you post the calculation you did? In either case, no, 5w of heat for a few seconds shouldn't be a big deal.

3. Apr 18, 2017

### hnbgm

Thank you for the warm welcome and your help :)

Correction, I made a stupid calculation error haha. It can only generate 0.8W.

From what I did, I estimated a yoyo bearing to be 20mm wide (outer diameter) and 10mm (inner diameter) with a mass of 100g which is larger than a typical yoyo bearing. I thought, the larger and heavier the bearing, the longer it would spin and generate torque for a longer period of time.

I calculated the inertia to be
1/2 X mass X (radius(outer)^2 - radius(inner)^2)) = 1/2 X 0.1 X (0.01^2 - 0.005^2) = 0.00000375m^4
Then I estimated rpm to be 6000rpm from all the forums I've seen, yoyos can generate this much speed
I calculated the angular velocity to be
(6000/60 secs) X 2pi = 628.29rad/s

Then I calculated the kinetic energy to be
1/2 X Inertia X (angular velocity)^2 = 1/2 X 0.00000375 X (628.29)^2 = 0.8W.

From knowing this, if I wanted to power a 5W led for example, my thinking is I would need to spin the yoyo a few times into my 5W generator which charges the battery, it would take a fairly long time to charge correct? It would almost become a scenario where I am playing with the yoyo for a longer time than getting actual light. Is there any way I could boost this process a bit?

4. Apr 18, 2017

### davenn

use a bigger and heavier yoyo

5. Apr 18, 2017

### Staff: Mentor

Well, there's still a few issues...
You tripped over the units a bit there: they should be kg-m^2. The number is correct though.
Again, the math is right, but the units are not. Watts are not a unit of kinetic energy -- the correct unit is joules. 0.8 Joules. And since a Watt is a unit of power equal to a Joule per second, you can expend that 0.8 Joules for only 1 second.