Hello!
Today i went bike riding and came up with an idea to get electricity out of riding. As i always forget to install/buy new batteries for my original light, i was thinking to make an emergency light that would always be mounted to my bike. I dont want any old fashion dynamos as this:
I was rather watching:
But, as always, this is not sold in our country and i dont want to order it also. So i came up with the idea:
Will it work? i dont want any hi-power leds, just one or two low powered led.
Maybe instead of rechargeable battery there should be a capacitor.. then i dont have to use a switch to turn the light off when not riding
Well, you're going to need a bit more circuitry to get that running. Right now if you wired that up your battery would likely explode or your wire would catch on fire as you have the battery shorted through the coil.
One or two low power leds would use so little power that you could run them entirely without the battery. With the proper application of high-capacity capacitors you could wire your (correctly built) coil with some diodes (keep that energy flowing into the led's and not back into the coil) and a voltage regulator. If you want to go the battery route you're going to need a simple charging circuit, check out Adafruit or Sparkfun, I'll bet you'll find something useful there.
Wrapping the coil onto the post of the bike probably won't work that well; you want your coil to be as close as possible to the magnetic flux (so the gap between the magnet and the coil needs to be as small as possible). Also, the axis of the coil should be perpendicular (IIRC) to the flux of the magnet (right hand rule and all that). The way you've drawn the coil (in relation to the magnets - but you didn't draw the poles of the magnets, so...?), it probably wouldn't work well at all...
You might want to study how motors, generators, and dynamos work, first - before you start playing much. Plenty of info out there, the technology is nearly 200 years old now.
Another option (and probably would be much more efficient) - use a small DC motor as a generator (you would need to run it through a bridge rectifier and some caps to smooth out the DC), with a small fan blade on it mounted inside a shroud (with the opening facing forward); as you pedal, the fan spins the motor, generating a small amount of electricity; use that for charging the battery/caps.
To add to Crosh's idea... use it on the back wheel/chain rather than the front... add a deraillur gear to drive the genny off the chain drive (even a small electric motor would do it) and run is off pedal power with capacitors for those moments that you're not pedalling..
Or add in a battery and charging circuit. You could hide it all under the seat too, which is LOADS easier than faffing about on the front end of the bike.
A hub dynamo or bottle dynamo would work much better. I have used bottle dynamos for years and the latest ones are very good. Your idea theoretically could work but it is really not worth your time for the possible power output of it.
Even if this idea works it will most probably be much less efficient then a real dynamo. Especially less efficient while the light is off. This is because you magnets will induce current not only in the coils but in the iron close to them. Thus you will get losses that will act like a brake. These losses will be highest when you draw no current. That is it will brake more if the light is out
