High school Science and Technology project

Thread Starter


Joined Nov 24, 2016
Dear readers,

i'm a high school Science teacher in Belgium and i've entered my students in a Science and Technology fair.
They have a good idea but my knowledge about electronics has its limits...
I've got 2 general questions first:
1) i've started with Arduino projects but do you guys know good books, tutorials, courses, etc so that i can make my own "simple" projects? I've been following the manual untill now.
2) What equipement is essential when starting with project?

and now about the project.
They would like to hook up a dynamo to a hometrainer to do 3 things:
1) Charge a phone
2) Power a small fan (12V)
3) Charge a battery pack (4cell NiMh)

Can it be done? I've been reading alot about it but i find alot of contradictions.
What i think so far (please correct me if i'm wrong)
A normal bike dynamo delivers a current of 6V AC. I need to convert it to 6V DC to charge the phone, charge the battery and power the fan.
If i use 2 dynamo's in serie i can ad up the current to get around 12V AC converted to DC. I need a rectifier and a capacitor to do this (don't know the correct name in english to convert AC to DC) If i try to power the fan and charge the battery pack it won't work really well so i'll need a switch to chose between the 3 functions. I should check the output current to see how many mAh it delivers. When i multiply the capacity by 1.4 (why 1,4? loss of current?) and devide it by the output current i know the time it takes to charge the battery pack.

Hope you guys can help me so i can start asap.

I'll post some pics if we are making progress.

Thank you in advance!

greeting from Belgium
Sander Hungenaert


Joined Jun 22, 2012
Hi, to convert Ac to Dc you need a Bridge Rectifier, and a smoothing capacitor about 470 to 1000uF, a dynamo usually gives out DC in Volts.

USB dynamo charger.gif

You can put the rectified voltages in series to create a bigger supply, but the Current will be limited to the smallest dynamo output.

Post some pictures of the dynamo and any voltage readings.
Last edited:


Joined Aug 1, 2013
My experience with bike generators is that they put out much higher voltage when unloaded. Much as in exceeding the 7805's 24V input rating. But that is a minor thing to investigate and adjust for.

Next, the AC frequency. I've run into problems using "normal" rectifiers in 400 Hz applications. Bike generators make lotsa Hertz, so best to find some high speed rectifier types to prevent excessive heating. Someone not in the US probably can recommend types commonly available in Europe.

Other than that, the project is pretty basic. The 7805 circuit shown will drive anything that has USB port for charging. A similar one based on an LM317 can be adjusted for 6 V or 12 V (assuming the peak voltage out of the dynamo is high enough. What is the current rating for the fan you want to drive?


Thread Starter


Joined Nov 24, 2016
Wow thank you for the schematics it really helps!
So if i understand it correctly. The bridge rectifier turns the AC into DC (is it also 12V or do you lose some voltage?).
The capacitor smooths out the current and it has to be strong enough so above 12V. Does the 2200uF matter alot?
The resistor is to control the current/voltage going to the output and then the 5V regulator to make sure the output is 5V.

The current fan we want to power is just a pc 12V cooling fan. I starts running at 3V but not so fast.
So if i want 10V i need a second circuit in serie to get the desired output? or do i need the LM317 circuit?

i'll post the specifications of the dynamo and fan later today.



Joined Oct 10, 2014
The bridge rectifier converts a sinusoidal AC signal to a somewhat pulsating DC 1, hence the mention of the smoothening capacitor to bring this waveform to a clean DC signal. The bridge consists of 4 diodes, each of which induce a volt drop (usually ~0.7V, you will have to see a datasheet for the specific 1 that you use). Also never expect your input voltage to be reflected at your output, components aren't perfect and there are always unexpected losses in a system. Also note that 12VAC does not equal 12VDC!

Simple voltage regulators can be used to power your fan or charge the phone, but know that this is not the most efficient way to step down voltage and will produce a lot of heat -- proper heat sinks could solve this or you could switch to a more efficient supply like a buck converter - check out the LM2576. You could get efficiency around 78% as opposed to around 20% with a simple regulator.

Charging of the battery isn't so simple, there are different stages of charging, fast, trickle, etc. The LM317 is an adjustable reg that can be used to trickle charge your battery.