Using induction for charging a car's battery I'm a student and I have the project to study the downloading by induction. I would like first, in order to discover experimentally how the induction works, to build a system which will be able to load a phone.
For that I use low frequency generator (LFG), 2 copper's coils, wires and an oscilloscope to analyze the signal.
I think it is going to be complicated to light the LED, because of the very low intensity i, that delivers the LFG.
What I did:
I build two coils, the first one the transmitter and the second the receptor, both having antennas, then I connected the transmitter coil with the LFG by using two wires : this is the transmitter part of the system. Then I connected the receptor coil with the oscilloscope, without forgotten to connect the LED to the coil.
After I put the coils closer, but unfortunately, the LED didn't light...
Here is the signal:

What can we say about the signal ?
And also if  you have some interesting stuffs about load by induction or wireless power transfert system it would be great for me.
Thank you
 A: The device that, by your description, you have built and are testing is a transformer.
For the purpose of lighting an LED you would usually connect the 'primary' coil of your transformer to a supply of a few volts at a frequency of somewhere around 50 Hz. I wonder what your 'low frequency generator' is producing?
You will not, in my experience, be able to transfer much power by electromagnetic induction from one coil to the other (from primary coil to secondary coil) unless you wind the coils on the same iron core. Even this may not be good enough; the core ought to be in the shape of a closed loop (so that almost all the magnetic flux produced by the primary is linked with the secondary) and made of soft iron (that is iron that is easily magnetised and demagnetised) and laminated (that is built up of insulated layers). I'm afraid that you'll find such cores difficult to source – unless you take an existing transformer apart! But your experiment might work if you wind your coils on a nail or, better, a sheaf (fascis) of nails.
And what about the coils themselves? For a transformer built on a proper core, the rule is
$$\frac{\text{voltage across secondary coil}}{\text{voltage across primary coil}}=\frac{\text{number of turns on secondary coil}}{\text{number of turns on primary coil}}$$
This is because, for a given change of magnetic flux, each turn of either coil gets the same voltage induced in it!
You need to put at least a hundred turns on the primary. So for example, if you put 150 turns on the primary and applied 6.0 V across it, you'd get 2.0 V across a secondary of 50 turns – in theory at least. But if you're using a straight core made of nails, you'll get much less than this; you could try putting more turns on your secondary coil.
Note that the voltage across the secondary coil will be alternating, like the voltage that you are applying to the primary. An LED needs about 2 V to work. It should have a current-limiting resistor (150 ohm might suffice) in series with it (and ideally should be wired as Ed V describes in the comments below). Although LEDs will produce light only if the voltage is one way round, they will still give light with an alternating voltage – because on alternate half cycles it is the right way round!
These were a few practical points. For a proper understanding you need to read up about electromagnetic induction, and transformers in particular. Note that effective and efficient transformers, such as you'd need for a car battery charger, are not easy to construct. But you'll learn a lot by persevering with your present project to make a transformer that will light an LED. Good luck!
