Electromagnet

[tangentzero9]

Working on a simple project:

 

I understand that if you wrap a wire into a coil and supply a current through that wire, a magnetic field will be created.

 

What I am trying to do is hook up an LED to a wire which is wrapped around a magnet. Then when the device is placed into the presence of anything metal the current produced will light up the LED.

 

At least, thats what i expected to happen...not even a flicker. Maybe I have my concept of electromagnetism wrong. And if it's right maybe its just not making enough current to light up the LED.

 

Any thoughts or ideas?

 

Using this LED: http://www.superbrightleds.com/cgi-bin/store/index.cgi?action=DispPage&Page2Disp=%2Fspecs%2Fr2_specs.htm

 

I'm using a harddrive magnet like this one :

[swansont]

A current is produced when a loop sees a changing magnetic field; the potential difference of a circuit will be proportional to the rate of change of the flux.

 

This means and oscillating magnetic field gives you AC. Not good for an LED. Even if you have a monotonically increasing or decreasing field to give you the right sign, it needs to be above the threshold to turn on the LED.

[JamesNBarnes]

To simplify what has been said, the magnetic field needs to be changing, not static. i.e the magnet needs to move relative to the coil to produce a constant current. However it will be an AC current, which will require extra circuitry in order to drive the LED which needs DC current.

 

Theoretically you could get the LED to light without additional circuitry, but it would pulse, not be a constant brightness. Assuming the oscillation was fast enough however you might be able to get it to appear as if there was constant light.

 

In doing so however, you would need to ensure that the current and voltage was enough to light the LED, but not so much that the "backwards" current was enough to damage the LED.

[tangentzero9]

I see, so placing it by anything metallic doesn't do anything. (Nor does moving it by anything metallic... to get a changing magnetic field?)

 

Looks like i'll have to make the device a bit more complicated. Perhaps wrapping the string around a hollow tube of plastic (like a straw) and placing a tube magnet in that straw. The magnet can be held in place by springs at either end. Then when an end of the "straw" is placed near something metal the magnet will move...possibly lighting the LED (gotta figure out the current and voltage and possibly AC/DC issues). I guess a voltmeter can help me with that. The greater the magnetic field disturbance the greater the current correct?

 

I understand that in this description above, the LED won't stay lit but only for the amount of time it takes for the magnet to move a bit. It's just a first idea. I'll sit and think about this.

[ewmon]

The electric current in a wire builds a magnetic field around the wire, and a collapsing magnetic field creates an electric current in the wire. So, a changing magnetic field creates a current in the wire.

 

A coil merely concentrates this magnetic field. For example, instead of the magnetic field spread out over 1,000 feet of wire, a coil would concentrate it in the space around the coil. And instead of a cylindrical magnetic field around the straight wire, a coil forms the magnetic field into a torus (ie, doughnut shape).

 

There's a few important factors. Coil: number of turns. Magnet: strength, proximity to the coil, and speed past the coil.

 

Other than making a generator with a spinning rotor, a magnet mounted on the tip of a pendulum that swings past a coil can provide the needed speed and closeness. This is basically a generator with an oscillating rotor.