PWM LED Array

On Sunday, May 22, 2011 7:20:02 PM UTC-4, linnix wrote:
> BTW, why PIC32?  There must be other reasons than just driving LEDs.

It will also have a key matrix and a 16 bit lcd attached. There won't
be a whole lot of pins left, ergo my thought about using a driver
via i2c/spi or similar, perhaps.

Hi Gene,

On 5/22/2011 3:34 PM, Gene wrote:
>> Is this a 1x63 array?  3x21?  How many lamps do you intend to
>> have lit concurrently?  how many "intensities" do you seek?
>
> The array configuration is one of the things that I'm uncertain on.
> I would think that it'd be configured based on the end method of
> driving said array. I'd need all on at the same time, with at least
> a few intensity levels (5 or so would be plenty) for all LEDs.

Wait -- you want to treat them all AS IF they were a single lamp?
I.e., two states:  all on (at some "dimness level") or all *off*?
*Not* 1 on, 2 on, 26 on, etc.?

(which begs the question, why not use "some number on" to give you
your variable intensity?  unless you need illumination over a
wider area, etc.)

On 5/22/2011 4:31 PM, linnix wrote:
> On May 22, 4:22 pm, Rich Webb<bbew...@mapson.nozirev.ten>  wrote:
>> On Sun, 22 May 2011 15:34:58 -0700 (PDT), Gene<profii...@gmail.com>
>> wrote:
>>
>>> On Sunday, May 22, 2011 4:58:42 PM UTC-4, D Yuniskis wrote:
>>>> [grrr... how about learning to break your lines at something like
>>>> 70 characters??  I've taken the liberty to *chop* off everything that
>>>> extends beyond the width of my window... no fun, eh?  :>]
>>
>>> Sorry; my netiquette has gotten rusty it seems :(
>>
>>>> Is this a 1x63 array?  3x21?  How many lamps do you intend to
>>>> have lit concurrently?  how many "intensities" do you seek?
>>
>>> The array configuration is one of the things that I'm uncertain on.
>>> I would think that it'd be configured based on the end method of
>>> driving said array. I'd need all on at the same time, with at least
>>> a few intensity levels (5 or so would be plenty) for all LEDs.
>>
>> Well, let's back of the envelope this. Figure a nominal 2 V drop per LED
>> and you've got 5 V to work with so run two in series, 32 times. You want
>> to PWM so figure a BJT switch with, say, 0.5 Vce at saturation, so a
>> swag is 4.5 V per pair, and 32 pairs. (If you really want 63 and not 64,
>> put some electrical tape over the 64th, or throw some regular diodes
>> there in its place.)
>>
>> For 500 mA max at the USB port, assuming a proper dialog with the port
>> to get permission to draw more than 200 mA, you can push about 15 mA
>> through each pair. Less, really, since the processor will want some and
>> some will be lost in other ways. So, say you look for LEDs that are
>> adequately bright at no more than 10 mA. A 47 ohm current limiting
>> resistor for each pair might be a starting point.
>
> PIC32 (min 64 pins) has only 5 PWM outputs.  So, you need transistors
> to buffer more than 18mA (max) per port.  PIC24 has 9 PWM, BTW.

If the OP is only interested in 5 light levels, you don't
need a "real" PWM channel (do it "old school")

Gene wrote:

>...using a driver
>via i2c/spi or similar, perhaps.

I2C:  Check TI's TLC59116, NXP's PCA9635, etc.
They control 16 LEDs, could be coerced to control 32 by multiplexing
the common side of the LEDs with a couple of transistors.
--
Roberto Waltman

[ Please reply to the group,
  return address is invalid ]

On May 22, 5:10=A0pm, Gene <profii...@gmail.com> wrote:
> On Sunday, May 22, 2011 7:20:02 PM UTC-4, linnix wrote:
> > BTW, why PIC32? =A0There must be other reasons than just driving LEDs.
>
> It will also have a key matrix and a 16 bit lcd attached. There won't
> be a whole lot of pins left, ergo my thought about using a driver
> via i2c/spi or similar, perhaps.

Actually, you can probably drive all of them with a single pin and
power transistor.  I2C/SPI would need more than one pin.

On May 22, 5:17=A0pm, D Yuniskis <not.going.to...@seen.com> wrote:
> Hi Gene,
>
> On 5/22/2011 3:34 PM, Gene wrote:
>
> >> Is this a 1x63 array? =A03x21? =A0How many lamps do you intend to
> >> have lit concurrently? =A0how many "intensities" do you seek?
>
> > The array configuration is one of the things that I'm uncertain on.
> > I would think that it'd be configured based on the end method of
> > driving said array. I'd need all on at the same time, with at least
> > a few intensity levels (5 or so would be plenty) for all LEDs.
>
> Wait -- you want to treat them all AS IF they were a single lamp?
> I.e., two states: =A0all on (at some "dimness level") or all *off*?
> *Not* 1 on, 2 on, 26 on, etc.?
>
> (which begs the question, why not use "some number on" to give you
> your variable intensity? =A0unless you need illumination over a
> wider area, etc.)

Sound like LCD backlite.  It he said so originally, we wouldn't have
to go through all those guessings.

On May 22, 5:10=A0pm, Gene <profii...@gmail.com> wrote:
> On Sunday, May 22, 2011 7:20:02 PM UTC-4, linnix wrote:
> > BTW, why PIC32? =A0There must be other reasons than just driving LEDs.
>
> It will also have a key matrix and a 16 bit lcd attached. There won't
> be a whole lot of pins left, ergo my thought about using a driver
> via i2c/spi or similar, perhaps.

Next time, try something like this:

Question:

What is the best way to drive 63 backlite LEDs for the LCD panel of a
PIC32,  with upto 5V 500mA USB bus power.

Answer:

PWM the base of a 2N2222, with 32 pairs of LEDs tied between collector
and a 10 ohms resistor from 5V VBUS.

On 5/22/2011 6:19 PM, D Yuniskis wrote:
> On 5/22/2011 4:31 PM, linnix wrote:
>> On May 22, 4:22 pm, Rich Webb<bbew...@mapson.nozirev.ten> wrote:
>>> On Sun, 22 May 2011 15:34:58 -0700 (PDT), Gene<profii...@gmail.com>
>>> wrote:
>>>
>>>> On Sunday, May 22, 2011 4:58:42 PM UTC-4, D Yuniskis wrote:
>>>>> [grrr... how about learning to break your lines at something like
>>>>> 70 characters?? I've taken the liberty to *chop* off everything that
>>>>> extends beyond the width of my window... no fun, eh? :>]
>>>
>>>> Sorry; my netiquette has gotten rusty it seems :(
>>>
>>>>> Is this a 1x63 array? 3x21? How many lamps do you intend to
>>>>> have lit concurrently? how many "intensities" do you seek?
>>>
>>>> The array configuration is one of the things that I'm uncertain on.
>>>> I would think that it'd be configured based on the end method of
>>>> driving said array. I'd need all on at the same time, with at least
>>>> a few intensity levels (5 or so would be plenty) for all LEDs.
>>>
>>> Well, let's back of the envelope this. Figure a nominal 2 V drop per LED
>>> and you've got 5 V to work with so run two in series, 32 times. You want
>>> to PWM so figure a BJT switch with, say, 0.5 Vce at saturation, so a
>>> swag is 4.5 V per pair, and 32 pairs. (If you really want 63 and not 64,
>>> put some electrical tape over the 64th, or throw some regular diodes
>>> there in its place.)
>>>
>>> For 500 mA max at the USB port, assuming a proper dialog with the port
>>> to get permission to draw more than 200 mA, you can push about 15 mA
>>> through each pair. Less, really, since the processor will want some and
>>> some will be lost in other ways. So, say you look for LEDs that are
>>> adequately bright at no more than 10 mA. A 47 ohm current limiting
>>> resistor for each pair might be a starting point.
>>
>> PIC32 (min 64 pins) has only 5 PWM outputs. So, you need transistors
>> to buffer more than 18mA (max) per port. PIC24 has 9 PWM, BTW.
>
> If the OP is only interested in 5 light levels, you don't
> need a "real" PWM channel (do it "old school")

I too was wondering why so many PWM channels were required for a 
non-linear output.
Even a multiplexed array of diodes can get every LED a different level 
of intensity.
Just limit the number of levels, as D Yuniskis stated.

hamilton

On Sun, 22 May 2011 17:19:34 -0700, D Yuniskis
<not.going.to.be@seen.com> wrote:

>On 5/22/2011 4:31 PM, linnix wrote:
>> On May 22, 4:22 pm, Rich Webb<bbew...@mapson.nozirev.ten>  wrote:
>>> On Sun, 22 May 2011 15:34:58 -0700 (PDT), Gene<profii...@gmail.com>
>>> wrote:
>>>
>>>> On Sunday, May 22, 2011 4:58:42 PM UTC-4, D Yuniskis wrote:
>>>>> [grrr... how about learning to break your lines at something like
>>>>> 70 characters??  I've taken the liberty to *chop* off everything that
>>>>> extends beyond the width of my window... no fun, eh?  :>]
>>>
>>>> Sorry; my netiquette has gotten rusty it seems :(
>>>
>>>>> Is this a 1x63 array?  3x21?  How many lamps do you intend to
>>>>> have lit concurrently?  how many "intensities" do you seek?
>>>
>>>> The array configuration is one of the things that I'm uncertain on.
>>>> I would think that it'd be configured based on the end method of
>>>> driving said array. I'd need all on at the same time, with at least
>>>> a few intensity levels (5 or so would be plenty) for all LEDs.
>>>
>>> Well, let's back of the envelope this. Figure a nominal 2 V drop per LED
>>> and you've got 5 V to work with so run two in series, 32 times. You want
>>> to PWM so figure a BJT switch with, say, 0.5 Vce at saturation, so a
>>> swag is 4.5 V per pair, and 32 pairs. (If you really want 63 and not 64,
>>> put some electrical tape over the 64th, or throw some regular diodes
>>> there in its place.)
>>>
>>> For 500 mA max at the USB port, assuming a proper dialog with the port
>>> to get permission to draw more than 200 mA, you can push about 15 mA
>>> through each pair. Less, really, since the processor will want some and
>>> some will be lost in other ways. So, say you look for LEDs that are
>>> adequately bright at no more than 10 mA. A 47 ohm current limiting
>>> resistor for each pair might be a starting point.
>>
>> PIC32 (min 64 pins) has only 5 PWM outputs.  So, you need transistors
>> to buffer more than 18mA (max) per port.  PIC24 has 9 PWM, BTW.
>
>If the OP is only interested in 5 light levels, you don't
>need a "real" PWM channel (do it "old school")

[Crazy usenet. I can see posts from linnix timestamped before and after
that one but... not that one. %!#$!  ::sigh:: ]

Yes, I was imagining one PWM channel driving one transistor that then
tickled the bases of all of the 32 LED string transistors in parallel.
Rather a brute force approach, of course. Since they're operating as
saturation switches, minor differences in the transistor characteristics
should be mostly a wash.

-- 
Rich Webb     Norfolk, VA

On May 22, 6:14=A0pm, Rich Webb <bbew...@mapson.nozirev.ten> wrote:
> On Sun, 22 May 2011 17:19:34 -0700, D Yuniskis
>
>
>
>
>
>
>
>
>
> <not.going.to...@seen.com> wrote:
> >On 5/22/2011 4:31 PM, linnix wrote:
> >> On May 22, 4:22 pm, Rich Webb<bbew...@mapson.nozirev.ten> =A0wrote:
> >>> On Sun, 22 May 2011 15:34:58 -0700 (PDT), Gene<profii...@gmail.com>
> >>> wrote:
>
> >>>> On Sunday, May 22, 2011 4:58:42 PM UTC-4, D Yuniskis wrote:
> >>>>> [grrr... how about learning to break your lines at something like
> >>>>> 70 characters?? =A0I've taken the liberty to *chop* off everything =
that
> >>>>> extends beyond the width of my window... no fun, eh? =A0:>]
>
> >>>> Sorry; my netiquette has gotten rusty it seems :(
>
> >>>>> Is this a 1x63 array? =A03x21? =A0How many lamps do you intend to
> >>>>> have lit concurrently? =A0how many "intensities" do you seek?
>
> >>>> The array configuration is one of the things that I'm uncertain on.
> >>>> I would think that it'd be configured based on the end method of
> >>>> driving said array. I'd need all on at the same time, with at least
> >>>> a few intensity levels (5 or so would be plenty) for all LEDs.
>
> >>> Well, let's back of the envelope this. Figure a nominal 2 V drop per =
LED
> >>> and you've got 5 V to work with so run two in series, 32 times. You w=
ant
> >>> to PWM so figure a BJT switch with, say, 0.5 Vce at saturation, so a
> >>> swag is 4.5 V per pair, and 32 pairs. (If you really want 63 and not =
64,
> >>> put some electrical tape over the 64th, or throw some regular diodes
> >>> there in its place.)
>
> >>> For 500 mA max at the USB port, assuming a proper dialog with the por=
t
> >>> to get permission to draw more than 200 mA, you can push about 15 mA
> >>> through each pair. Less, really, since the processor will want some a=
nd
> >>> some will be lost in other ways. So, say you look for LEDs that are
> >>> adequately bright at no more than 10 mA. A 47 ohm current limiting
> >>> resistor for each pair might be a starting point.
>
> >> PIC32 (min 64 pins) has only 5 PWM outputs. =A0So, you need transistor=
s
> >> to buffer more than 18mA (max) per port. =A0PIC24 has 9 PWM, BTW.
>
> >If the OP is only interested in 5 light levels, you don't
> >need a "real" PWM channel (do it "old school")

Just one channel.  The issues were unclear before.

>
> [Crazy usenet. I can see posts from linnix timestamped before and after
> that one but... not that one. %!#$! =A0::sigh:: ]
>
> Yes, I was imagining one PWM channel driving one transistor that then
> tickled the bases of all of the 32 LED string transistors in parallel.
> Rather a brute force approach, of course. Since they're operating as
> saturation switches, minor differences in the transistor characteristics
> should be mostly a wash.

I was thinking just one tiny metal can (2N2222) that can handle 500mA
collector current.  One of my first transistor and still as good as
any.