Home made electromagnet

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headingwest

Member
Hi All,

I'm build a tiny LED strobe and disco ball for my daughters barbie house.

I want to make the mirror ball move slightly, it doesn't need to rotate, just twitch a bit to move the reflected lights.

I thought of having a Picaxe 08M go to a home made electromagnet that pulls a paperclip about 2mm around when it's activated.

So pin out --> diode --> 220ohm --> u shaped metal nail with copper coil --> gnd

Is this a bit nuts trying to run an electromagnet from the picaxe pin? I don't need to move the paperclip far.

Thanks for any help and suggestions.

Jim.
 
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inglewoodpete

Senior Member
headingwest said:
So pin out --> diode --> 220ohm --> u shaped metal nail with copper coil --> gnd

Is this a bit nuts trying to run an electromagnet from the picaxe pin? I don't need to move the paperclip far.
Click to expand...
To get much magnetism you will need quite a lot of current and/or turns of wire.

You will need to use a transistor on the PICAXE output the provide enough power to the electromagnet and to provide some protection for the PICAXE. You will need many, many turns of very fine wire. To get something small enough, I suggest you butcher a small relay if you have one - they are ready-wound!

Alternatively, get an ultra miniature solenoid from JayCar, since you're in Oz. These ones need 12 volts.
 
H

headingwest

Member
I'm trying to keep it very small. It will run off button batteries and be built into a bottle lid about 30mm diameter and 30mm tall.

Can you get tiny gear motors? Maximum 15mm in each dimension.

I'm thinking a butchered relay as suggested could work.
 
A

AllyCat

Senior Member
Hi,

Yes, my thoughts as well, perhaps even cannibalised from an old 'phone. You will need to slow the speed, perhaps using PWM (and at a low frequency to give tiny "kicks"), adding inertia with a heavy disc such as a foreign coin. Or if you can remove the offset weight, the bare shaft might directly drive the edge of small disc (a coin again) at a sufficiently slow rate (using a rubber or plastic sleeve/band on one of the surfaces).

But if you want to persevere with a home-made electromagnet, then consider reacting it against a tiny "button" neodymium magnet (typically 2mm x 8mm). Magnetically very strong and just a couple of Pounds/Dollars for 10 or 20 from ebay. Great fun to play with (and some serious applications).

Cheers, Alan.
 
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boriz

Senior Member
Idea:

Hack a cheap earphone. The sort of thing you get with MP3 players.

They are moving coil, with integrated magnet and support. Very small too. You might be able to persuade one to push 2mm. Can't be done direct from Picaxe though. Will need current amplification. Easiest way is a MOSFET. Only needs to push in one direction, (it will spring back when current is removed), so a single MOSFET will do.

A Picaxe will be able to push repeatedly with preset time intervals / repeat rates.

Also:

Anyone remember the first generation of slotcars? Forget what they were called, but they had unusual 'motors'. It was a sort of ratchet-on-a-spindle type system, with a solenoid driven 'pusher' pin. Repeated pushes with the pin against the ratchet would turn the spindle, which was on the axle. You might be able to do something like that. I have a photo somewhere...
 
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headingwest

Member
Thanks for the great suggestions. I've gone ahead and purchased a vibration motor for a whopping $1.81 from Ebay.

I went ahead and built a home made magnet to test the concept, it was a fail. Basically the nail remained magnetic, and adding 4.5V / 100mA didn't make a huge difference to the amount of magnetism.

I wrote it all up here and put in a photo.

I'll let you know how I go with the vibration motor.
 
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boriz

Senior Member
The wire in the coil has resistance. Putting any other resistance in series with it will form a potential divider. EG: If your coil is 1ohm, then using a 10ohm series resistor will mean that about 91% of the voltage will be developed across the resistor and only about 9% across the coil. You may also notice the resistor getting hot. It's dissipating about 91% of the power as heat.

Based upon the photo, I'm guessing that coil is a lot less than 1ohm, making the situation much worse. Measure the voltage across the coil. I bet it's less than 200mV when driving it and the 10ohm series resistor with 4.5v.

You need to study up on ohms law.

I like the clock idea.
 
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bluejets

Senior Member
An interesting thing I saw Inside the guts of a safety switch. (elcb...earth leakage circuit breaker)

It was much like a standard relay coil with an armature under spring tension to open it. The quirky thing was, inside the end of the coil near the armature was a small rare earth magnet.

This acted to hold the armature in the closed position against the spring tension. When a small current flowed through the coil, it counteracted the effect of the magnet on the armature and the spring snapped the armature into the open position, tripping the adjacent (rather solid) circuit breaker mechanism.
 
H

headingwest

Member
boriz said:
The wire in the coil has resistance. Putting any other resistance in series with it will form a potential divider. EG: If your coil is 1ohm, then using a 10ohm series resistor will mean that about 91% of the voltage will be developed across the resistor and only about 9% across the coil
Click to expand...
Thanks Boriz, you were right. There's no measurable resistance in the magnet coil. I measured the current directly from a AA 1.5v battery at 3A, with the transistor I reached 100mA. So basically I can't have a resistor in series to limit current and I would need a relay or maybe mosfet to drive the 3A.

I'll stick with the clock or vibration motor for now.
 
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boriz

Senior Member
headingwest said:
...There's no measurable resistance in the magnet coil...
Click to expand...
There is. It's just very small. A few milliohms perhaps. [Rcoil]

Also, there is internal resistance inside the battery [Rbat]. This too will form a potential divider. You measured the current at 3A, but did you measure the voltage? Bet it's much less than 1.5v. The voltage measured at the battery terminal will be the voltage at the potential divider junction between Rcoil and Rbat. EG: 1ohm Rcoil, 1ohm Rbat @ 1.5v = 0.75v at the terminals (across the coil). Realistically, Rcoil will be far lower than Rbat, so the terminal voltage will be very low and most of the power (>90%) will be dissipated across Rbat inside the battery, warming it up.

This kind of battery terminal voltage drop catches out many people, causing Picaxe resets and such. And remember, series batteries have series Rbat, increasing the effect.

It comes down to the difference between an ideal voltage source and a practical battery.

One option is to increase Rcoil. Usually done by adding many more turns and using very thin wire. (have a look inside a relay). Another option is to provide very low (source) resistance power. Difficult and potentially expensive.

Bottom line: If your load resistance is much less than your source resistance, you prolly need a redesign.

(It's no coincidence that the biggest and most expensive component in a decent stereo amp is the power supply)
 
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boriz

Senior Member
If you go for the motor, the problem will be how to convert many thousands of RPM into the slow rotation you need. There's many tricks for this. Especially for a low torque app like yours. I have some ideas...
 
techElder

techElder

Well-known member
Excellent explanation!

Very well articulated explanation of the effect of battery "resistance", Boriz!
 
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