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'[EE:] Driving LEDs from a pic pin.'
2004\10\11@030014 by Jason S

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In July I was talking about a project that would involve strobing an array of LEDs as a flash.

I've finally got the PIC code working, and it does exactly what I want with a single LED connected to the IO pin running at 25mA.  As I expected, it's not nearly bright enough.  The LED is on for 50uS and off for a minimum of 4.95mS.  
I'd like to have an array of LEDs connected to the output.  What's the best way to get maximum brightness.  The specs say the maximum continuous current is 30mA and absolute maximum current is 100mA.  Is it worth it to drive the LED at 100mA?

Either way, how do I connect a transistor to drive the LEDs  I don't know how to connect it to drive a load?  With 4-6 LEDs at 100mA each, I'm looking at 400-600mA.  Will this cause a problem for the batteries?  I have a decoupling cap near the MCU, will that be enough to protect it from the half-amp load switching on and off at ~100Hz?

Jason
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2004\10\11@043718 by Russell McMahon

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> In July I was talking about a project that would involve strobing an array
> of LEDs as a flash.

You said than that it was a flash and/or a strobe.
More information on application would allow better answers.

> I'd like to have an array of LEDs connected to the output.
> What's the best way to get maximum brightness.
> The specs say the maximum continuous current is 30mA and absolute
> maximum current is 100mA.  Is it worth it to drive the LED at 100mA?

You are going to have to drive the LEDS as hard as you can.
If they take 100 MA give them 100 mA at up to approaching Icontinuous/Imax
duty cycle.

> Either way, how do I connect a transistor to drive the LEDs
> I don't know how to connect it to drive a load?

Emitter follower mode has some advantages.


NPN driver rated at say 1 amp with Beta of al least 100 at 1 amp
Discuss.

5 volt supply:

Collector to V+
Base to PIC pin with small series R - sat 100R
Emitter to LED Anode.
LED Cathode to Rb to ground.

Rb is in say 2R2 to 4R7 range to getdesired max current.
LEDs in parallel.

12V supply or 5V supply - both with FET.
.
Can't use emitter follower on 12V

N Channel logic FET (fully on with 5V on gate)
Sets of (3 series LEDS) in parallel for12V
LEDS in parallel for 5v.

FET gate via 1k to PIC pin.
FET source to ground.
FET drain to LED cathode.
LED anaode to V+ via Rb

5V:    Rb = (5-Vled-0.1)/Imax
12V    Rb = (12 - 3xVled - 0.1)/Imax.
0.1 is for FET on drop.
Vled is LED forward drop at Imax.

> With 4-6 LEDs at 100mA each, I'm looking at 400-600mA.
> Will this cause a problem for the batteries?

Only if they are wimpy ones :-).

> I have a decoupling cap near the MCU, will that be enough to protect it
> from the half-amp load switching on and off at ~100Hz?

Possibly not but probably. LED current shoukld be drawn from a ciruit that
does not share ground path with PIC power ground. Ideally not paower path
either.


           Russell McMahon

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2004\10\11@051454 by Jinx

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> What's the best way to get maximum brightness

I'm wondering if you can treat the LED as you would a strobe
tube. That is, dump current through it from a capacitor

In the attached gif, the PIC's Vcc and reservoir cap are isolated from
each other. The current will dump through the LED when the transistor
is turned on. Without knowing further details it's not possible to add
actual component values, although I've suggested ZTX transistors.
They are fast, and have low saturation voltage (= more for the LED).
Someone could probably suggest a FET that would work too. The
series R need only be small, perhaps 1 or 2 ohms

If you find that the capacitor is too big to charge between pulses,
add a second identical LED driver and drive them alternately. This
gives each cap twice as long to re-charge. I needed to do this with
a puny PSU that had to fire a dozen solenoids. Driving them all at
once collapsed Vcc and made a terrible noise spike but driving them
in sequence was fine

> The specs say the maximum continuous current is 30mA
> and absolute maximum current is 100mA.  Is it worth it to
> drive the LED at 100mA?

Yes, to get pulse brightness. You'll need to work out the wattage
dissipated, as LED performance degrades with temperature. Note
that IR LEDs can take a few amps in a pulse - would you be able
to view this strobe with a camera ? (most b/w cameras are very
good in IR light). It's likely that your LED's 100mA is not the
absolute max for short pulse duration. You might be able to drive
it at an amp (just speculating) without causing any damage

> With 4-6 LEDs at 100mA each, I'm looking at 400-600mA. Will
> this cause a problem for the batteries?

Not if you're keeping the average current consumption down. For
example, 50us out of 5ms is only 1% duty cycle, or 1mA average
for 100mA pulse current

> I have a decoupling cap near the MCU, will that be enough to
> protect it from the half-amp load switching on and off at ~100Hz?

Maybe. Diode isolation helps too



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part 4 1 bytes

2004\10\11@074336 by Jinx

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> > I'm wondering if you can treat the LED as you would a strobe
> > tube. That is, dump current through it from a capacitor
>
> You can - but it's only useful to keep peak supply currents down

> As shown it doesn't reduce battery pulse load much except to decouple
> the supply close to the LED. BUT if you replace the diodes with resistors
> which are say N times greater than the resistors in series with the LEDs

How about when the batteries are on the way out and internal resistance
is going up ? One major consideration was keeping the crap away from
the PIC's Vcc without impeding the re-charge time

> mA / 10 mA = 10 although it would be better if it was higher. A BC337
> or similar will work there - and they are much cheaper than Zetex parts

Zetex are knee-jerk reaction when I think of pulsing LEDs. If I wasn't
neck deep in something right now I'd try the two for comparison in this
application. Zetex are definitely my preference for IR. BC337 and the
like just don't cut it

> You may wish to consider using a Luxeon(tm)

They've just released a camera flash LED

www.luxeon.com/newsandevents/releases/LuxeonFlashProductRelease_10060
4.pdf

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2004\10\11@085643 by Russell McMahon

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>> As shown it doesn't reduce battery pulse load much except to decouple
>> the supply close to the LED.

> How about when the batteries are on the way out and internal resistance
> is going up ?

As above.
The basic circuit is a useful one though. The constant impedance of the
capacitor allows better design of the pulse current so you can "sail closer
to the wind". If you rely on the battery then you must design not to blow
your LED when the battery is new.

That's an advantage of the emitter follower arrangement I mentioned. The
current limiting resistor and LED are in the emitter and you drive the base
through a small resistor from the PIC pin. The transistor now functions as a
constant current source of sorts and will provide a more comnstant current
as the battery ages compared to the resistor in collector circuit. You can
actually use it with 3 LEDs in setries and 12 volts supply despite what i
said to the contrary by putting all except 1 LED in the collector. (Having a
base drive divider from the PIC allows less wasted headroom.)

> One major consideration was keeping the crap away from
> the PIC's Vcc without impeding the re-charge time

The diode doesn't really do anything except to stop the various LED caps
voltage sharing if one loads down the supply. He specificed a low duty cycle
so you can have longish charge time, which helps keep pulsed noise off the
supply.

> Zetex are knee-jerk reaction when I think of pulsing LEDs.

If you have to have jerky knees then Zetex is one of the better things to
jerk them for. I think they are marvellous. I wonder if Newmarket
transistors were as good before they changed theuir name?

>> You may wish to consider using a Luxeon(tm)
>
> They've just released a camera flash LED
>
> www.luxeon.com/newsandevents/releases/LuxeonFlashProductRelease_10060
> 4.pdf

This is EXACTLY what he wants !!!

80 Lumen
1 amp.
BUT rated to 100,000 flashes so a strobe would kill it rather quickly :-(
Depends on what's being strobed.

Note can run 168 hours at DC 350 mA. That's only in the 1 to 2 watt range
still!




       RM

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2004\10\11@094927 by Bob Axtell

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I like having the PIC drive a logic-level MOSFET. They are below $1 USD
in u8 or SOIC packages, and they work with minimal loss and generate minimal
heat (very very low sat voltage). If the duty cycle is low enough, you
can easily
drive at 1A without damage to the LED, which will emit a LOT of light as
far as
the human eye is concerned.The key to PWM of LEDs (or anything else) is the
keeping the junction temperature of the LED in specs range.

You'll need caps capable of very low ESR; tantalums are ideal.

--Bob

Jinx wrote:

{Quote hidden}

--
Note: Attachments must be sent to
spam_OUTattachTakeThisOuTspamengineer.cotse.net, and
MAY delay replies to this message.
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2004\10\11@095324 by olin_piclist

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Jason S wrote:
> In July I was talking about a project that would involve strobing an
> array of LEDs as a flash.
>
> I've finally got the PIC code working, and it does exactly what I
> want with a single LED connected to the IO pin running at 25mA.  As I
> expected, it's not nearly bright enough.  The LED is on for 50uS and
> off for a minimum of 4.95mS.
>
> I'd like to have an array of LEDs connected to the output.  What's
> the best way to get maximum brightness.  The specs say the maximum
> continuous current is 30mA and absolute maximum current is 100mA.  Is
> it worth it to drive the LED at 100mA?

Only if 3 times the brightness is worth it.

> Either way, how do I connect a transistor to drive the LEDs

NPN transistor with emitter to ground, base to PIC pin via 1kohm resistor,
collector to the cathode of all the LEDs.  The anode of each LED gets a
separate resistor to the power supply.  You might want to put an inductor in
series with the power lead from the battery to the common side of the LED
resistors, and a cap to ground from there.  This will smooth out the load
for the battery a bit.


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2004\10\11@100437 by olin_piclist

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Jinx wrote:
> In the attached gif, the PIC's Vcc and reservoir cap are isolated
> from each other.

I don't see any point in the isolation diode going to the LEDs.  The other
one makes sense because you are worrying about the sudden LED current
glitching the supply low for a short time.  However, we assume the circuit
on the left is just a PIC, so the same logic doesn't apply to the LED side.
When would the diode on the LED side ever be reverse biased?


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2004\10\11@154857 by Jason S

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From: "Russell McMahon" <.....apptechKILLspamspam@spam@paradise.net.nz>
Sent: Monday, October 11, 2004 1:37 AM


> You said than that it was a flash and/or a strobe.
> More information on application would allow better answers.

It's a stroboscope to measure high speed cyclic motion by a stop-motion
effect.  It's also good for seeing what the object looks like while moving
by slowing it down by having the frequency off a little bit.

> You are going to have to drive the LEDS as hard as you can.
> If they take 100 MA give them 100 mA at up to approaching Icontinuous/Imax
> duty cycle.

In the earlier thread about this, someone mentioned that LED efficiency
dropped as the current went up.

If that's true, it might not be worth the added complexity.  Also, I'm
concerned that if I drive the LEDs as hard as I can, I'll damage them over
time and decrease the brightness too slowly to notice the change.

{Quote hidden}

Rb would have to be a 2W resistor, right?  (If Vcc = 5V and the LEDs drop
3.3V, that leaves 1.7V across the resistor with up to 600mA through it).

With a single transistor and the LEDs in parallel, if a single LED fails
open, each other LED will have 20% more than its Imax.  Is that a valid
concern?

>> With 4-6 LEDs at 100mA each, I'm looking at 400-600mA.
>> Will this cause a problem for the batteries?

> Only if they are wimpy ones :-).

They're AA alkaline ones (so yes, wimpy ones).  My digital camera draws ~1A
and AA alkalines have a lot of trouble powering it.

> Possibly not but probably. LED current shoukld be drawn from a ciruit that
> does not share ground path with PIC power ground. Ideally not paower path
> either.

At what point is it a separate path?  With a separatre wire going back to
the batteries, or with diodes as in Jinx's diagram?


-----------------

From: "Jinx" <joecolquittspamKILLspamclear.net.nz>
Sent: Monday, October 11, 2004 2:13 AM


> I'm wondering if you can treat the LED as you would a strobe
> tube. That is, dump current through it from a capacitor

I would think that circuit would work.  Concepually, it's the same as a
decoupling capacitor for the LED part of the circuit.

> Yes, to get pulse brightness. You'll need to work out the wattage
> dissipated, as LED performance degrades with temperature. Note
> that IR LEDs can take a few amps in a pulse - would you be able
> to view this strobe with a camera ? (most b/w cameras are very
> good in IR light). It's likely that your LED's 100mA is not the
> absolute max for short pulse duration. You might be able to drive
> it at an amp (just speculating) without causing any damage

To clarify, these are white LEDs and the project is a stroboscope, not a
remote control.

> Maybe. Diode isolation helps too

I'm not clear on how diode isolation works in this application.  Both parts
of the circuit will only be drawing the current forward through the diode.
There shouldn't ever be a current reversal when the current switch off.  I
don't have any transformers or inductors in the circuit.

Jason

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2004\10\11@161635 by Jinx

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> I don't see any point in the isolation diode going to the LEDs
> ......
> When would the diode on the LED side ever be reverse biased?

That's a snippet adapted from the solenoid driver mentioned. Due
to the inductive nature of those I found the diode necessary but, as
Russell points out, resistors would do instead. Now that I reconsider,
there is of course no collapsing field with an LED as with a coil

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2004\10\11@162125 by Jason S

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From: "Russell McMahon" <.....apptechKILLspamspam.....paradise.net.nz>
Sent: Monday, October 11, 2004 3:19 AM


> As shown it doesn't reduce battery pulse load much except to decouple the
> supply close to the LED. BUT if you replace the diodes with resistors
which
> are say N times greater than the resistors in series with the LEDs you get
> the desired affect. (Jinx knows this - he just had a brief moment of
> inattention ;-) ). N can be about 1/3 of the duty cycle eg if you have a
> 100:1 duty cycle then the feed resistor can be 100/3 ~- 30 times the
series
> LED resistor.

Are the resistors to limit how much current can go to the decoupling
capacitors on both sides of the circuit to keep the battery voltage stable?

> You could try running the LEDs at > 100 mA. This is outside spec of course
> and YMMV but you MAY find that it works well for you. Lifetime will
probably
> shorten - possibly drastically (as in dead). Absolute current limit used
to
> be limited by bond wires fusing but 100 mA is very low for this. Modern
LEDs
> may have some other death limit. Traditionally you could use 500 mA to an
> amp. Whether this works for you depends on your application and LEDs used.

At the 1% duty cycle, 500mA to an amp is getting close to the same total
power if I were driving it continuously.  I was actually thinking of going
closer to 80mA/LED to have some room below the limit.

> You may wish to consider using a Luxeon(tm) or similar LED module. 1 watt
> modules are now about $US8 and you can get 3 watt modules - maybe even
more
> now. These seem to have about as good a mCd/watt as anything around and
will
> probably be as cheap as combining LEDs yourself.

I looked at the Luxeon site and they seem to be light on information how is
it different from using discrete LEDs except that they're in a nice
convenient package?

> Depending on application a LASER module MAY work for you.

That's an interesting idea.  With how cheap laser pointers go for on eBay
now, it would be cheaper than the LEDs.  Is it as simple as connect the
power to the laser diode and it will respond fast enough?  The last time I
looked at laser diodes was when Radio Electronics had an article on building
a Laser pointer around a laser diode, and it seemed like there was a lot
more to it than powering an LED.

> Marginal maybe:  LCD panel / display used to shutter a halogen lamp.
> Probably too slow (depends on speed needed).

I would have thought that would be too slow.  In any case, it probably
wouldn't be pocket sized or battery powered if I used that.

Thanks for your suggestions,
 Jason

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2004\10\11@163142 by Jason S

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From: "Jinx" <EraseMEjoecolquittspam_OUTspamTakeThisOuTclear.net.nz>
Sent: Monday, October 11, 2004 4:40 AM


> They've just released a camera flash LED
>
>
www.luxeon.com/newsandevents/releases/LuxeonFlashProductRelease_10060
> 4.pdf

That sounds perfect until I read the datasheet.  It's 40 lumens and they say
a conventional LED is 6-7 lumens, so it's only a few times brighter.

Then they achieve this by overpowering the LED.  It's good for 168 hours of
continous use as a flashlight?  My 6V lantern over-powers a 4.8V
incandescent flashlight bulb for maximum brighness and still lasts longer
than 168 hours.  I thought LEDs were supposed to be good for 100,000 hours
unless you burn them out faster.

100k flashes is also pretty useless in this application.  The strobe goes up
to 200Hz, which means the entire life of the LED is under 9 minutes.  Even
at a more typical 50-100Hz I'll get less than half an hour of life out of
the LED.

Those numbers really give me the impression it's an ordinary LED that
they're abusing to death.

Jason

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2004\10\11@165046 by Jinx

face picon face
> now, it would be cheaper than the LEDs.  Is it as simple as connect
> the power to the laser diode and it will respond fast enough?

The diode will, but the slow part could be its constant current supply.
You might get a couple of kHz (which is slower than the 50us pulse
you're looking for). I have such a laser + CC supply and modulation
is done via a 4093 into the the base of the driving transistor. This allows
up to several hundred kHz switching AFAIR

Just out of interest

http://scitoys.com/

http://scitoys.com/scitoys/scitoys/light/light.html#laser_communicator

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2004\10\11@165326 by Jinx

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> Those numbers really give me the impression it's an ordinary LED that
> they're abusing to death.

Then again, 100,000 digital camera flashes would probably do most
people (ol' trigger happy Russell being the exception of course) for
a long time

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2004\10\11@165520 by Dave VanHorn

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>
>100k flashes is also pretty useless in this application.  The strobe goes up
>to 200Hz, which means the entire life of the LED is under 9 minutes.  Even
>at a more typical 50-100Hz I'll get less than half an hour of life out of
>the LED.

You're going to have some interesting thermal issues there, at 200 Hz, the lead frame may not be able to get rid of the heat fast enough.

You might be able to use multiple sections, where each section flashes 1/Nth of the time, and can better get rid of the heat.

The other thing that makes all this fun, is the bizarre units of measurement that we have for light..  

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2004\10\11@192538 by Josh Koffman

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Actually, the big deal with it is that it doesn't include the same
package as their Luxeon emitters. There aren't the same optics and
lead configurations on the flash models resulting in a smaller
package. They are really designed for cell phone cameras. The cynic in
me says that since the picture quality will be so crappy anyways, no
one will notice a sub-par flash.

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
       -Douglas Adams

On Tue, 12 Oct 2004 09:52:50 +1300, Jinx <joecolquittspamspam_OUTclear.net.nz> wrote:
> > Those numbers really give me the impression it's an ordinary LED that
> > they're abusing to death.
>
> Then again, 100,000 digital camera flashes would probably do most
> people (ol' trigger happy Russell being the exception of course) for
> a long time
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2004\10\11@195756 by William Chops Westfield

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On Oct 11, 2004, at 1:55 PM, Dave VanHorn wrote:

>>
>> 100k flashes is also pretty useless in this application.

> You're going to have some interesting thermal issues there, at 200 Hz,
> the lead frame may not be able to get rid of the heat fast enough.
>
It looks to me like the "strobe led" is a pretty vanilla "luxeon" high
power LED, put in a tiny package without good heat dissipation, and
characterized for its behavior under those conditions.  Neat idea, but
as you say, probably not very useful for a ~200Hz stoboscope
application.
(a lot of the advances in high power LEDs are more packaging and power
dissipation advances, than changes in the LED technology itself.)

I don't know if I've seen any information on using ordinary luxeons
in stoboscope-type applications; They're available in 1W, 3W, and 5W
(continuous) configurations, and ought to go somewhat higher for
low duty-cycle applications.

BillW

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2004\10\11@195850 by Robert Monsen

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Here is another strobe circuit you can use. Drive it with 1% duty cycle
positive pulses from the PIC. It'll deliver equal current pulses through
each of the LEDs. As shown, it drives each of them with about 100mA.

You can add as many LEDs/PNP sections as your battery can drive. They
are constant current sections, so no resistors are needed, although
adding a tiny resistor (10 ohm, for example) between the emitter and
rail of the driver transistors might afford some safety from
overdriving. You can adjust the R2 pot for brightness. Note that you can
easily destroy the LEDs with a bad setting.. Using another resistor in
series with R2 (maybe a 2.2k resistor) along with a 3.3k pot would be a
reasonable choice, and would keep it from frying too easily. The current
mirror is set up to do a 1 to 100 mapping of current through the NPN
transistor.

As far as your batteries go, AA batteries can deliver about 6A or so
without a problem. If you are thinking 3 AA batteries, that means
driving 1 amp pulses shouldn't be too much of a problem. The internal
resistance is something around .25 ohm per battery for new batteries, so
a .75 ohm resistance will drop output by about .75V. The 1000uF cap
across the batteries keeps the Vcc rail from drooping too much during
the pulses (simulation shows it dropping about 50mV for 1A total draw at
1% duty cycle.). If you still find your PIC is crashing, put a 1k
resistor from the + terminal to the PIC Vcc, and put a 100uF
electrolytic bypass cap from Vcc to the GND pin. That will smooth out
the power.

I have not built and tested this circuit, only simulated it, so there
could be various unforseen errors.

Have fun,
Bob Monsen



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2004\10\12@042432 by hael Rigby-Jones

picon face


>-----Original Message-----
>From: @spam@piclist-bouncesKILLspamspammit.edu [KILLspampiclist-bouncesKILLspamspammit.edu]
>On Behalf Of Josh Koffman
>Sent: 12 October 2004 00:26
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE:] Driving LEDs from a pic pin.
>
>
>Actually, the big deal with it is that it doesn't include the
>same package as their Luxeon emitters. There aren't the same
>optics and lead configurations on the flash models resulting
>in a smaller package. They are really designed for cell phone
>cameras. The cynic in me says that since the picture quality
>will be so crappy anyways, no one will notice a sub-par flash.
>
>Josh

Absolutely, the other point is that typicaly cell phone cameras are only
used for close up shots of people etc. where a low powered flash is not
going to be such a problem.

Mike

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2004\10\12@135703 by Peter L. Peres

picon face

On Mon, 11 Oct 2004, Dave VanHorn wrote:

> The other thing that makes all this fun, is the bizarre units of
> measurement that we have for light..

Why bizarre ?


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2004\10\12@143142 by Dave VanHorn

flavicon
face
At 11:41 AM 10/12/2004, Peter L. Peres wrote:


>On Mon, 11 Oct 2004, Dave VanHorn wrote:
>
>>The other thing that makes all this fun, is the bizarre units of
>>measurement that we have for light..
>
>Why bizarre ?

Well, Candles, Candelas, Stearadians, Lux, Phot, Lamberts.
One is derived from a specific sort of whale oil candle.

The whole thing looks like a Hogshead of footlamberts per fortnight to me. :)

This is probably the best explanation I've seen so far.
http://www.intl-light.com/handbook/ch07.html

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'[EE:] Driving LEDs from a pic pin.'
2004\11\10@025125 by Jason S
flavicon
face
From: "Olin Lathrop" <RemoveMEolin_piclistTakeThisOuTspamembedinc.com>
Sent: Monday, October 11, 2004 5:53 AM

> NPN transistor with emitter to ground, base to PIC pin via 1kohm resistor,
> collector to the cathode of all the LEDs.  The anode of each LED gets a
> separate resistor to the power supply.  You might want to put an inductor
in
> series with the power lead from the battery to the common side of the LED
> resistors, and a cap to ground from there.  This will smooth out the load
> for the battery a bit.

This is working very well.

How can I extend it to a situation where there are 16 series strings of LEDs
in a 4x4 matrix.  The LEDs are a sealed box, all I have access to are the 8
wires coming out of the matrix.

Thanks,
 Jason

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