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'[EE] Grid of bipolar electromagnetic indicators'
2008\05\25@194109 by Peter Todd

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Just a sanity check before I spend a bunch of time building this...

Attached is a circuit diagram I did up for a grid of bipolar
electromagnetic indicators, also known as dot-flip displays, that I want
to build. The attached grid is a 3x3, but what I will actually build
will be 56*13

Electrically speaking you apply a roughly 12V pulse of a few ms in
either direction. This magnetises two bits of ferrite which hold the
colored dot in either orientation, yellow side up, or black side up.

I just want to make sure my multiplexing scheme will actually work, and
is the simplest possible solution, never done multiplexing before with
anything other than leds. Of course, the high-side FETs will have to be
PMOS, but other than that I can't think of any gotchas.

- --
http://petertodd.org 'peter'[:-1]@petertodd.org
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2008\05\25@194905 by Peter Todd

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part 1 1113 bytes content-type:text/plain; charset=us-asciiOn Sun, May 25, 2008 at 07:39:57PM -0400, Peter Todd wrote:
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> Just a sanity check before I spend a bunch of time building this...
>
> Attached is a circuit diagram I did up for a grid of bipolar
> electromagnetic indicators, also known as dot-flip displays, that I want
> to build. The attached grid is a 3x3, but what I will actually build
> will be 56*13
>
> Electrically speaking you apply a roughly 12V pulse of a few ms in
> either direction. This magnetises two bits of ferrite which hold the
> colored dot in either orientation, yellow side up, or black side up.
>
> I just want to make sure my multiplexing scheme will actually work, and
> is the simplest possible solution, never done multiplexing before with
> anything other than leds. Of course, the high-side FETs will have to be
> PMOS, but other than that I can't think of any gotchas.

The attachment...

Also FWIW this is the project:

http://petertodd.org/art/entropy-oscillator/

--
http://petertodd.org 'peter'[:-1]@petertodd.org


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part 3 35 bytes content-type:text/plain; charset="us-ascii"
(decoded 7bit)

2008\05\25@200859 by Richard Prosser

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Peter,

Looks OK to me (P channel fets excepted).

You might want to add some sort or current limit protection in case
something gets "stuck".  - Series resistor with a capacitor bypassing
it? Or just current limit the power supply?

RP

2008/5/26 Peter Todd <spam_OUTpeteTakeThisOuTspampetertodd.org>:
{Quote hidden}

> -

2008\05\25@221022 by Apptech

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> Looks OK to me (P channel fets excepted).

Looks OK.
Feels like you should be able to reduce the FETs to row /
column but my brain refuses to reveal how.


       Russell


2008\05\25@222346 by Peter Todd
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On Mon, May 26, 2008 at 02:10:16PM +1200, Apptech wrote:
> > Looks OK to me (P channel fets excepted).
>
> Looks OK.
> Feels like you should be able to reduce the FETs to row /
> column but my brain refuses to reveal how.

I know 'eh? I've been thinking about this one for months myself. I did
find some rather broken commercial flip-dot modules, which had two
dioded per element, but the actual driver circuitry was missing. One
module I found had no diodes at all, but I suspect that it must have
used some magnetic hysterius tricks, like how core memory works, and
besides was constructed entirely from bundles of magnet wire woven
through the ferrite elements, very different than the sub-modules I
actually bought.

Using a duel-rail power supply might help, but I'd rather avoid that
complications really anyway.

- --
http://petertodd.org 'peter'[:-1]@petertodd.org
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2008\05\25@234005 by Spehro Pefhany

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At 10:10 PM 5/25/2008, you wrote:
> > Looks OK to me (P channel fets excepted).
>
>Looks OK.
>Feels like you should be able to reduce the FETs to row /
>column but my brain refuses to reveal how.
>
>
>         Russell

If they're all *tristate* drivers and you only flip or flop one actuator at
a time.

Same deal as Charlie-plexing. I don't think it's worth it.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



2008\05\26@002743 by Peter Todd

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On Sun, May 25, 2008 at 11:43:08PM -0400, Spehro Pefhany wrote:
{Quote hidden}

Fortunately, I will only be flipping or flopping one flip dot module at
a time.

> Same deal as Charlie-plexing. I don't think it's worth it.

So that would essentially replace each of the half bridge PMOS/NMOS
pairs I've created with a single tristate driver?

Isn't the trick with Charlie-plexing that you are exploiting that the
combined forward voltage of two leds in series is high enough that a
single led in parallel will take all the current? My 12V will be
significantly higher than the forward voltage of the diodes used.

- --
http://petertodd.org 'peter'[:-1]@petertodd.org
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2008\05\26@015337 by Spehro Pefhany

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At 12:27 AM 5/26/2008, you wrote:
{Quote hidden}

It would be about the same, I'm wondering if the diodes can be eliminated.

>Isn't the trick with Charlie-plexing that you are exploiting that the
>combined forward voltage of two leds in series is high enough that a
>single led in parallel will take all the current? My 12V will be
>significantly higher than the forward voltage of the diodes used.

I think with the sneak paths you'd get 12V/3 = 4V across the coils that are
not supposed to be energized. If that's too much to guarantee *no* flipping
then it won't work reliably without all those diodes.

A 3-cent BAV99 each isn't so bad.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffspamKILLspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com




'[EE] Grid of bipolar electromagnetic indicators'
2008\06\25@110912 by Peter Todd
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On Mon, May 26, 2008 at 01:56:24AM -0400, Spehro Pefhany wrote:

Doh, this ended up in my sent-mail, but never got past that... weird...

> >Isn't the trick with Charlie-plexing that you are exploiting that the
> >combined forward voltage of two leds in series is high enough that a
> >single led in parallel will take all the current? My 12V will be
> >significantly higher than the forward voltage of the diodes used.
>
> I think with the sneak paths you'd get 12V/3 = 4V across the coils that are
> not supposed to be energized. If that's too much to guarantee *no* flipping
> then it won't work reliably without all those diodes.

I did some quick experimentation, the coils can be reliably flipped with
as little as 3V with a unknown by hand pulse-width, and that same 3V
accross three dots fails to flip them.

That said, in a private email someone found the supplier of my modules.
As far as I can tell they just put their website online within the past
few months:

http://www.flipdots.com/EN/electromagnetic_displays/products/page-5/dot_stripes.html

The 2.7" modules are the exact ones I have, which they are now making
with original tooling bought from the defunct MARK IV FP Electronics.

They specify a 350ma minimum current, achievable with 1ms pulse at 7.5V
given the coils inductance.


Anyway, I may build up a pulse tester and see how well things work in
temperature extremes, and from unit to unit, although diodes would be
safer.


FWIW this is a quite nice work of art using these things:

http://www.troika.uk.com/cloud.htm

> A 3-cent BAV99 each isn't so bad.

Yup, $64.74/1000x and $200 worth of labour. :)

- --
http://petertodd.org 'peter'[:-1]@petertodd.org
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