Creating a sound-activated electromagnet

[ajsparx]

Hey everyone, first off, I am NEW to circuitry designing (but I understand all of the equations, units, etc), and so I'm at a loss right now. I'm making a sound-activated electromagnet for my end of semester inorganic chemistry project (so that I can make ferrofluid dance to music, or sounds), but I need help figuring out how to make a sound sensor that lets a current pass when it hears a noise. Perhaps I could integrate a voltage regulator in as well, to let more current pass for louder noises, and less for quieter sounds... what do you think?

Also, if you have any suggestions regarding how to make a strong, yet responsive electromagnet, I'm all ears. I have access to a pure iron rod, but I was wondering about wire-wrapping "etiquette:" number of wraps, gauge of wire, how to start a new layer of wraps, etc.

 

[unclejed613]

you could use an audio amplifier to drive your electromagnet, but i would suggest using a ferrite bar rather than an iron bar. the iron bar will generate a lot of heat because of eddy currents in the iron.

 

[ajsparx]

I'll see if my lab has one, otherwise, I have access to "supply money" if need be.

 

[colin55]

Just put a mini speaker in the water.

 

[Torben]

Just put a mini speaker in the water.

Water's a ferrofluid?


Torben

 

[ajsparx]

Water's a ferrofluid?


Torben

Nope, obviously. Actually, I'm making mine out of magnetic laser printer toner (fluid), and regular 'ole veggie oil.
Then I'll put a sound-controlled electromagnet under a dish of the stuff.

 

[Sceadwian]

Magnetic laser printer toner fluid?!

Laser printer toner isn't magnetic, it's plastic and carbon based generally, so I'm a little confused about your intended ferrorfluid makeup.

Your answer is self evident. Simply extract the voice coil out of the biggest speaker you can find. A core material will reduce frequency response, although you may not need or want it if a core material will give you a higher flux as the ferror fluid you're making might be very insensitive.

What is the goal of this? A single coil isn't going to allow you to do anything interesting to look at, nothing more than putting ooblekc (cornstarch and water solution) on the top of a speaker. To create any truly intersting patterns with a coils in a ferrofluid you'd need more than one coil fed from different waveforms.

 

[ajsparx]

Magnetic laser printer toner fluid?!

Laser printer toner isn't magnetic, it's plastic and carbon based generally, so I'm a little confused about your intended ferrorfluid makeup.

Your answer is self evident. Simply extract the voice coil out of the biggest speaker you can find. A core material will reduce frequency response, although you may not need or want it if a core material will give you a higher flux as the ferror fluid you're making might be very insensitive.

What is the goal of this? A single coil isn't going to allow you to do anything interesting to look at, nothing more than putting ooblekc (cornstarch and water solution) on the top of a speaker. To create any truly intersting patterns with a coils in a ferrofluid you'd need more than one coil fed from different waveforms.

It's "magnetic ink character recognition" toner, the ink that's used to print the numbers on the bottom of checks. And a single coil should do it, check out this vid of ferrofluid and a neodymium magnet

Also, I'd like to have a minimum of electromagnetism going on when there is less sound, would a speaker coil do that? (I don't think so, but I'm not too keen on the properties of speaker coils, so I guess I wouldn't know)

 

[Sceadwian]

ajsparx what do you think a speaker is? It's a wire coil that's electrically driven to provide an opposing magnetic field relative to a static magnet which it pushes or pulls on to make the actual sound at the speaker cone. To come up with similar field strength to a rare earth magnet you'll need a VERY powerful voice coil, IE one from a multi hundred watt speaker. The applied magnetic field will be directly proportional to current which if fed from an audio amplifier will be directly proportional to ambient sound... Use a single diode to clip one end of the AC waveform or you're going to get nearly no responce from the ferro fluid.

 

[ajsparx]

Where would I splice in such a diode, assuming I can get my hands on such a speaker coil? (I have a DC powered subwoofer under my desk; I'd have to check the wattage)
After removing the cone could I simply hook the speaker input to a microphone then, and leave the rest unchanged?

 

[Sceadwian]

Depends on the exact speaker construction, I've never tried to take anything too powerful apart before, just 1 watt or less speakers. Excising the voice coild could be a bit tricky, you won't know till you try. Even if you screw up and cut a wire though you can just peel off a little more wire from the coil and burn the tip quickly to remove the insulation to get new electrical contacts, just be careful not to damage the main body of the coil itself. The diode would simply be directly inline with one of the speaker wires, the direction is irrelevant you just want to keep the polarity from shifting to the negative or the net effect on the ferromagnetic will be almost non existent. Be warned, the amp might behave badly when run at power with a diode inline like that.

 

[tcmtech]

To get enough DC power into the voice coil you may be best off using a adjustable voltage regulator circuit with the audio signal being injected into the part of the circuit where the feedback potentiometer is so that the audio input creates a fluctuating DC output.
I suspect a simple capacitive coupled method would work well enough. The regulator IC would automatically limit your peak currents as well.

Its just a guess though.

 

[unclejed613]

speaker voice coils aren't a good choice for what you're trying to do. the amount of magnetic force around one is actually quite small, and the amount of current you need for what you're trying to do will burn up a 500W voice coil in a heartbeat, since the voice coil is in open air, and can't dissipate the heat, a voice coil resides normally in a very narrow air gap between the pole pieces of a speaker magnet. pro audio speakers also have a layer of ferrofluid (with a very high surface tension, so it doesn't flow out of the gap) around them to dissipate heat into the magnet assembly. the voice coils in an air filled gap are cooled by radiant and airflow cooling. if you could get a diamond saw, you could cut a toroid in half and make a kind of horseshoe electromagnet with it, or use half of a flyback transformer rectangular "O" core. wind about 50 turns of #18 wire around it and you have an audio electromagnet.

i'm familiar with the MICR toner you're talking about, i repaired banking equipment for a few years. you can also use that powder to look at the magnetic domains on recording tape under a microscope.