Mechanical Magnetic Display

I'm looking at creating a dynamic display device consisting of a load of ball bearings "magically" being moved around by an array of hidden electromagnets. I've not done much with inductive loads before, so I have more questions the more I think about it, so I'll just lay out my plans and assumptions, and hopefully people will chip in with clarifications and corrections.

Step one involves making some electromagnets. I'm currently winding 3 layers of 0.315mm enamelled copper wire onto the 25mm long cut-off shank of an old iron screw - about 6mmØ. I'm not sure if this will create a strong enough magnet, so will also try making one with many more turns.

My test coil only measures at about 0.6Ω, so with my 12V supply and a target current of 1A, that works out at needing about 11.4Ω additional resistance. Let's round that up to 15Ω.

If I'm correct, 12V at 1A results in 12W through the resistor, so that means I need to buy much more expensive and bulky power resistors.

I would like to be able to control the magnetism of each element individually, but am not sure of the implications of PWM on a coil. I'm guessing it's the same as a motor.

I will be needing at least 9, but ideally many more, of these elements. I will be able to limit how many of them are 'on' at any one time, but I'm not sure yet how much power I'll need at any one time.

In case my homebrew coils don't work out, I ordered a bunch of these from Hong Kong, but they'll take a while to arrive.

"magically" :wink:

The coils you have ordered (12volt/250mA) could be operated by a TPIC6A595 shift register (eight per chip).
No other parts needed except for maybe a 100n cap across the 5volt supply.
Many of these chips can be daisy-chained, and still use only three pins of the Arduino.
Leo..

Hi, your circuit looks OK... Your homemade magnets with a single piece of magnetic material seem questionable.. The commercial ones seem more workable...

If only 9 circuits, you don't necessarily need shift registers to expand the output bits..

terryking228:
Your homemade magnets with a single piece of magnetic material seem questionable.. The commercial ones seem more workable.

Absolutely. Leave the painful stuff to others.

terryking228:
If only 9 circuits, you don't necessarily need shift registers to expand the output bits..

One TPIC6A595 saves 8 diodes, 16 resistors, and 8 logic mosfets.
Parts and building it could be more expensive than a TPIC chip.

"I will be needing at least 9, but ideally many more"
Leo..

That TPIC sounds great, thanks for the tip. I'll just use the home-wound magnet stuff for experimentation until the slow boat from HK turns up with my sensible magnets :slight_smile: I just hope the 20mm diameter of them isn't too big, but I should be able to work around that.

Can't find the TPIC on my usual UK hobby suppliers, but will have a look around for alternatives.

thanks!

Try RS components.
Were I live (NZ) they do free shipping for online orders.
http://nz.rs-online.com/web/p/counters-shift-registers/0528141/
Leo..

Ahh, good shout, I forgot about RS.

Will I be able to do any sort of PWM to vary the power of each magnet? I'd like to be able to experiment with 'passing' a ball (or ferrofluid!) between two cells of my display, which might be prettier if I can reduce the power of one whilst increasing the power of the other. Again, my limited knowledge of inductors makes this an unknown area for me :slight_smile:

thanks

Cylindric:
Will I be able to do any sort of PWM to vary the power of each magnet?

I'd like to be able to experiment with 'passing' a ball (or ferrofluid!) between two cells of my display, which might be prettier if I can reduce the power of one whilst increasing the power of the other.

  1. Not for each individual output. You could try to PWM the "output enable" pin for the whole chip.

  2. Maybe by running the even solenoids on one chip, and odd ones on another chip.

Another option is a PWM port expander. e.g. the PCA9685 with 16 PWM outputs.

This solution requires 16 logic level mosfets, kickback diodes and gate resistors.
Leo..

You can PWM the TPIC by shifting new data in sufficiently fast.

Note that using electromagnets and armatures (balls), PWM will generally be extremely audible!

Audible is good. I’m going for an interesting “kinetic sculpture” sort of vibe with this. I’ll also try to work in some sort of “quiet mode” if I can :slight_smile:

If I use the TPIC power SR that Wawa recommended, will I not still need the resistors for each coil? If I use the purchased units I assume they’re internally wired to not need the external resistance, as they’re just rated at a certain voltage to draw the rated current? But if I use my home-brew, very-low-resistance coils, I’ll have to add enough resistance to bring the power consumption down to a sensible level?

No resistor needed.
The coils you have ordered draw ~250mA at 12volt.
The coil resistance must be 12 / 0.25 = 48ohm.
Leo..

Cylindric:
But if I use my home-brew, very-low-resistance coils, I'll have to add enough resistance to bring the power consumption down to a sensible level?

Correct.

I've created a bit of a schematic based on your ideas, and with some information from CrossRoads I found on this "Solenoid Matrix" thread.

It may seem excessive to add a second SR just for one more mixel, but I'm working on the assumption that I'll need more, so am designing in for chaining in readyness.

What are the diodes for? Doesn't the TPIC have them?

The TPIC has active voltage clamps, so no diodes needed.
Leo…

They are in fact, active clamps using the output FET itself.

Fair point. I'll remove them.