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'[EE]: Pushing LEDs to the limit'
2002\12\27@041159 by Ashley Roll

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Hi Everyone,

How far can you push an LED when you pulse it with a low duty cycle?

I want to get the maximum perceived light (brightness) from a 7000mcd White
LED. (actually a number of them in a series/parallel arrangement)

Lets say that the LED I'm playing with is "rated" at 20mA constant, and I
pulse it at 10:1 duty cycle at 200mA.. Is this a good idea? and where would
a "reasonable" trade off be between duty cycle and perceived brightness be?

I guess what I'm trying to ask is what is the maximum useful duty cycle you
can use on an LED before the law of diminishing returns kicks in.. Has
anyone got a good "feel" for this?

Is it worth going to 10:1 over say 5:1 or even 2:1?

The goal is to reduce the number of LEDs I need to give an equivalent amount
of light.. Assuming of course that the pulse rate is fast enough not to be
seen by the human eye.

Thanks for your thoughts.
Cheers,
Ash.

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Mobile: +61 (0)417 705 718

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2002\12\27@045325 by Thomas C. Sefranek

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> -----Original Message-----
> From: pic microcontroller discussion list
> [.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU]On Behalf Of Ashley Roll
> Sent: Friday, December 27, 2002 4:11 AM
> To: PICLISTspamKILLspamMITVMA.MIT.EDU
> Subject: [EE]: Pushing LEDs to the limit
>
>
> Hi Everyone,
>
> How far can you push an LED when you pulse it with a low duty cycle?

Bonding wires, semiconductor heating are the limiters I can think of.
>
> I want to get the maximum perceived light (brightness) from a
> 7000mcd White
> LED. (actually a number of them in a series/parallel arrangement)

Then DON'T pulse them!
>
> Lets say that the LED I'm playing with is "rated" at 20mA constant, and I
> pulse it at 10:1 duty cycle at 200mA.. Is this a good idea? and
> where would
> a "reasonable" trade off be between duty cycle and perceived
> brightness be?

You will get LESS perceived brightness.
{Quote hidden}

Equivalent to what?

{Quote hidden}

 |  __O    Thomas C. Sefranek   EraseMEWA1RHPspam_OUTspamTakeThisOuTARRL.NET
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2002\12\27@080848 by Spehro Pefhany

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At 07:10 PM 12/27/02 +1000, you wrote:
>Hi Everyone,
>
>How far can you push an LED when you pulse it with a low duty cycle?
>
>I want to get the maximum perceived light (brightness) from a 7000mcd White
>LED. (actually a number of them in a series/parallel arrangement)

In general, where the flicker rate is not perceptible, the highest
apparent brightness will be with DC drive, give or take a bit depending
on the semiconductor material and absolute drive levels. It will *not*
make a big difference  except negatively at the extreme high end.

It's because of the way the human eye works- it is an "average" reading
device, not a peak reading device.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
@spam@speffKILLspamspaminterlog.com             Info for manufacturers: http://www.trexon.com
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2002\12\27@083137 by Mike Harrison

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I suspect it might be  different with white LEDs, as the phosphor may have some time-dependent
characteristics. The only sure way is to try it and see - compare the percieved intensity of a DC driven one with one
driven with an adjustable mark/space ratio.
Pulsed drive is often used in LED traffic lights and roadworks lanterns - this would imply there is
some advantage in pulsed drive.  

On Fri, 27 Dec 2002 08:08:06 -0500, you wrote:

{Quote hidden}

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2002\12\27@084433 by Peter L. Peres

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I don't know where the law of diminishing returns kicks in for your LEDs,
I'd suggest you look into led forward voltage vs. current. Knowing that
only Vj goes into light the rest becomes heat in Ri and Rbulk. Thus you
can set yourself a goal, f.ex. 80% electrical efficiency (meaning 20% of
the input power turns into heat on Ri and Rbulk etc), this will give you a
peak pulse current you can use for the efficiency you are looking for.
There should be some reference on Ri or Rs or something and a Vj/Ij curve
in the data sheet. You can estimate Ri from that curve (take Vj to be Vf
at nominal current and use the slope of the curve from there towards
higher current to find Rs - this is wrong from the academic pov. and close
enough from the real life one. grin.).

Otherwise IR LED's rated 25mA continuous are often pulsed at 1Amp and more
in remote control transmitters and they do not seem to mind (there are
billions of those around). That would be 40x overrating (but the duty
cycle is very low over time). The only thing I have noticed about them is,
that the *may* have a larger cathode electrode (better heatsink ?)
molded in the plastic.

So maybe they would be brighter (or you could force them more) if they
would blink, also with a low duty cycle, like 1:10, 3Hz, in addition to
the high frequency low duty cycle drive. I do not know what this would do
thermal stress-wise. In any case I would plan for some sort of safe
fail-over scheme, it would be a pity if a single led failure would take
out the whole device.

Peter

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2002\12\27@084642 by Peter L. Peres

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On Fri, 27 Dec 2002, Thomas C. Sefranek wrote:

*>Then DON'T pulse them!

Are you sure ? I have a different experience, especially when the pulsing
frequency is low (close to the limit of perceived flicker). Also there is
some information about pulsed lights appearing brighter than non-pulsed
(not true when viewed by direct comparison).

Peter

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2002\12\27@094704 by Spehro Pefhany

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part 1 1144 bytes content-type:text/plain; charset="us-ascii"; format=flowedAt 01:29 PM 12/27/02 +0000, you wrote:
>I suspect it might be  different with white LEDs, as the phosphor may have
>some time-dependent
>characteristics.

It's actually worse because white (blue) LEDs have relatively high internal
resistance, so the heating increases greatly at high peak currents
(Vf is not constant)

See the attached graphs of luminosity vs. current and max current vs.
duty cycle for a high-quality white LED. At 20mA luminosity is 1.0
(normalized),
at 100mA it is only 3.5, not 5 or greater.

Lower perceived light output at the same average current, and lower
maximum permissible current due to the heating (10% maximum at 100mA
compared to 100% maximum at 30mA (Ta = 25'C)- it's *clearly* a losing game.
Not even close to break even.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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part 2 5979 bytes content-type:image/gif; name="nichia.gif"; (decode)


part 3 6688 bytes content-type:image/gif; name="nichiaa.gif"; (decode)


part 4 2 bytes
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2002\12\27@103106 by llile

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>In general, where the flicker rate is not perceptible, the highest
apparent brightness will be with DC drive, give or take a bit depending
on the semiconductor material and absolute drive levels. It will *not*
make a big difference  except negatively at the extreme high end.

>It's because of the way the human eye works- it is an "average" reading
device, not a peak reading device.


I'd have to politely disagree in one case, Sphero.  Flashing light for a
bicycle (which I built into my bike helmet for that ultra-nerdy look). The
goal is to make the most annoying and perceptible light with the least
amount of power.  I run a high brightness LED at about 5 or 6 times it's
rated power for 100 mS, and repeat these flashes every 2 S.  That's only
5% duty cycle, but the 100mS pulse is long  enough that these flashes are
visible for blocks.  I can see the reflection of the flash off a stopsign
a full block away on a dark night.

So far I have run this light on the same set of 4AA batteries for over a
year.  My night riding usually occurs in the wintertime, don't really have
much data on how many hours I have used this thing but it is quite a lot.
Runs on a 12C508 with a small power transistor as the pass element and 4
blinking LEDS, right-left-front and back.

I guess this applies at low frequencies (< 1 hz) but at higher frequencies
(> 10 hz) the eye will begin to average the pulses like you said.

--Lawrence










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12/27/02 07:08 AM
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       Subject:        Re: [EE]: Pushing LEDs to the limit


At 07:10 PM 12/27/02 +1000, you wrote:
>Hi Everyone,
>
>How far can you push an LED when you pulse it with a low duty cycle?
>
>I want to get the maximum perceived light (brightness) from a 7000mcd
White
>LED. (actually a number of them in a series/parallel arrangement)

In general, where the flicker rate is not perceptible, the highest
apparent brightness will be with DC drive, give or take a bit depending
on the semiconductor material and absolute drive levels. It will *not*
make a big difference  except negatively at the extreme high end.

It's because of the way the human eye works- it is an "average" reading
device, not a peak reading device.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the
reward"
EraseMEspeffspamspamspamBeGoneinterlog.com             Info for manufacturers: http://www.trexon.com
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2002\12\27@105832 by Rich

picon face
Nevertheless, I must agree with the averaging eye concept because it is
demonstrable fact.  The eye will integrate, and if the flashing light yields
a higher average it will be integrated by the eye and seem brighter.
Generally, as you probably already know, the light output of a pulsed LED
will be a function of the area under the curve.  HP has excellent APP notes
on this subject.  Try there site or do a search.  Good luck.

{Original Message removed}

2002\12\27@113151 by Spehro Pefhany

picon face
At 09:30 AM 12/27/02 -0600, you wrote:
> >In general, where the flicker rate is not perceptible, the highest
>apparent brightness will be with DC drive, give or take a bit depending
>on the semiconductor material and absolute drive levels. It will *not*
>make a big difference  except negatively at the extreme high end.
>
> >It's because of the way the human eye works- it is an "average" reading
>device, not a peak reading device.
>
>
>I'd have to politely disagree in one case, Sphero.  Flashing light for a
>bicycle (which I built into my bike helmet for that ultra-nerdy look). The
>goal is to make the most annoying and perceptible light with the least
>amount of power.  I run a high brightness LED at about 5 or 6 times it's
>rated power for 100 mS, and repeat these flashes every 2 S.  That's only
>5% duty cycle, but the 100mS pulse is long  enough that these flashes are
>visible for blocks.  I can see the reflection of the flash off a stopsign
>a full block away on a dark night.

I did qualify it above "where the flicker rate is not perceptible",
meaning over 50-100Hz. There's a transition range from 10Hz up to 100Hz
where the flicker is very perceptible to somewhat perceptible (especially
if there is vibration or other movement) but I don't think the apparent
brightness is much better at the top end of it. At the bottom end,
you're obviously getting a much better visible brightness.

So, I don't think we have any disagreement, polite or otherwise. ;-)

What is the off-time between 100ms flashes? 100ms?
It's a white LED?


        1       2           5-6                          1

        --      --          --                          --
       |  |    |  |   \    |  |              \         |  |     |
  -----    ----    -- | ---    --------------| --------    -----
           <-->        \                      \
            ?ms
       <->
        100ms
       |<------------------------ 2s ----------------->|



>So far I have run this light on the same set of 4AA batteries for over a
>year.  My night riding usually occurs in the wintertime, don't really have
>much data on how many hours I have used this thing but it is quite a lot.
>Runs on a 12C508 with a small power transistor as the pass element and 4
>blinking LEDS, right-left-front and back.

That *does* sound nerdy-looking. ;-)

>I guess this applies at low frequencies (< 1 hz) but at higher frequencies
>(> 10 hz) the eye will begin to average the pulses like you said.

There's a factor with the peripheral vision being "faster" response time
that might be an important factor with a bicycle lamp- our eyes are
designed to detect movement of predators or prey off at the sides of our
high-resolution vision, so light changes, fast ones, get our attention.

Just what you want from the guy driving the 18-wheeler at night, when that
mountain of chrome fills up your bicycle mirror.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2002\12\27@170926 by Brent Brown

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This is an interesting subject.

Some time ago I made a multiplexed LED display of 8 x 10 LED bargraph
modules controlled by a micro. The objective was just to have 8
bargraphs on the front panel of an instrument, each displaying it's
own "level" (0-10 LED's on), with no flickering desired.

Each 10 LED bar displays it's data for 1/8 of the time, that is
12.5%. By observation the repetition rate needed to be something like
1.5 or 2kHz to make sure there was no noticeable flicker. The LED on
current was 56mA, giving an average of 7mA. When operating, the
displays did appear to be brighter than the same display running on
7mA DC.

There could be several reasons for this. Most likely (IMHO) these
particular LED's may have been more efficient at 56mA than 7mA,
giving proportionally more light output. Notice that 7mA average
means the LED is well withing it's operating range and it runs
relatively cool. DC 56mA would undoubtably heat the LED too much and
efficiency and life span would drop.

Then there is the idea that the human eye may be somehow responding
to the peak light and giving a higer perceived intensity.

For very low frequencies like the bicycle flasher example, yes,
obviously the eye is fast enough to respond to the true intensity of
a 100ms pulse. The annoyance/distraction/noticability factor of such
a pulse makes it a whole lot more effective than steady light.

What happens at higher frequencies is harder to know.

Also bear in mind our logarithmic rendering of intensity, such that a
light source with twice the light output does not look twice as
bright.

If you want the highest light output possible, with no flickering,
you need to run your LED's at the highest current they can manage. At
this top end of their range of operation further increases in LED
current cause reduced efficiency due to current density limitations
and heating effects. So there is no way high peak pulses can provide
an average light output in excess of the equivalent DC current.

Some designs do make use of high frequency PWM in driving LEDs. This
allows adjustable intensity with "switching regulator" type power
efficiency.

I did have a look in the Agilent Application Brief I-024 on Pulsed
Operating Ranges for AlInGaP LEDs. Despite being interesting and
useful reading it doesn't mention anything special about perceived
intensity.

Regards, Brent.

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Mobile/txt: 025 334 069
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2002\12\27@181818 by Olin Lathrop

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> Otherwise IR LED's rated 25mA continuous are often pulsed at 1Amp and more
> in remote control transmitters and they do not seem to mind (there are
> billions of those around). That would be 40x overrating (but the duty
> cycle is very low over time). The only thing I have noticed about them is,
> that the *may* have a larger cathode electrode (better heatsink ?)
> molded in the plastic.

But IR LEDs are usually used for communication, certainly not general
human-visible illumination.  The human eye will average the total light,
assuming its pulsing fast enough.  However, an IR communications receiver is
much faster and will detect the instantaneous light level.  You can run the
emitter at higher power for shorter bursts to get longer range.

For example, suppose a 1 is on for 100uS and a 0 is on for 50uS, and that
the LED can just take on 100, off 50 continuously.  Now suppose you changed
a 1 to on for 10uS and a 0 to on for 5uS.  You could increase the pulse
current by 10 which would give you about 3 times more range.  Note however
that you still can only send 1 bits every 150uS without overloading the LED.
This is why IR LEDs are usually rated with maximum short term pulse current,
whereas this is not useful for general illumination.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2002\12\27@182204 by Olin Lathrop

face picon face
> Are you sure ? I have a different experience, especially when the pulsing
> frequency is low (close to the limit of perceived flicker). Also there is
> some information about pulsed lights appearing brighter than non-pulsed
> (not true when viewed by direct comparison).

Again, you need to check the context.  Lights on police cars, for example,
are deliberately pulsed to get your attention.  The human visual system keys
on motion and changes in the image.  However, this doesn't apply if your aim
is to provide general illumination.


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(978) 742-9014, http://www.embedinc.com

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2002\12\27@182605 by William Chops Westfield

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This debate (basicly, on whether the human eye is peak or average sensitive)
has happened here several times now - see the archives...

I don't recall whether it has ever been mentioned whether there's a
difference between viewing the light SOURCE, vs trying to illuminate a
'scene' with the output from the LEDs.  I'd suspect that in that latter
case, peak output would become less significant compared to average output.

BillW

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2002\12\28@164737 by Wagner Lipnharski

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llile@SALTONUSA.COM wrote:
{Quote hidden}

There is a big difference in concepts here.

First, the "eye catch" concept, and it is nothing to do with as the eye
works, it is related to the brain, to perceive moving preys and enemies.
Something that changes in time in the field of view creates an alert in the
brain that focus attention to that spot (into the brain, not the eye).  So,
ANY kind of flashing light is much more "eye catch" than a steady light.

Second that "light intensity" is only accounted as "candles / second",
period.   2 thousand candles (Candela?) pulsed at 50% rate, will end up as
just 1 thousand candles average to the eye.  Unfortunately there is no
magic.

The use of pulsed light at the street construction lights, is only used to
save battery, since following the "eye catch" paragraph above, does not
make any sense to keep a lamp lighted 100% of the time, if pulsing at 20%
once every second will catch 500% more attention.  If above the perceptible
scintillation, that it is just saving battery to produce lower candles - in
middle of the night, you don't need to generate 5W of light to alert
someone, 400mW or light can do the same job.

The traffic lights fast rate blinking have the same "eye catch" reason.  In
today's environment, when you have hundreds of different lights all around
your field of view, the traffic light is just hidden in that bright
picture.  When you move your head the fast blinking rate will be catch by
the brain and you WILL notice the traffic light in middle of the light
jungle.

Wagner Lipnharski - email:  TakeThisOuTwagnerKILLspamspamspamustr.net
UST Research Inc. - Development Director
http://www.ustr.net - Orlando Florida 32837
Licensed Consultant Atmel AVR _/_/_/_/_/_/

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2002\12\30@103103 by llile

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Sphero Sez, with his Typical Erudite Astuteness:





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>I did qualify it above "where the flicker rate is not perceptible",
meaning over 50-100Hz.

Aha, always read the fine print, I say.

>So, I don't think we have any disagreement, polite or otherwise. ;-)

Except about brands of beer maybe.

>What is the off-time between 100ms flashes? 100ms?

Let's see, from memory it is a sequence like .. Oh heck I will just look
up the code..

I have four lamps, each consisting of an array of 3 ultrabright red LED
lamps.  There is an array pointing forward, back, right and left.

I energized the arrays in order twice (Front, right, rear, left front
right rear left) for 50mS each time, then let it rest for 900 mS.  Pulsing
them twice gives an extra-annoying double flash.  I determined by
experiment that 50mS was about the minimum time I could energize the LED
array and percieve full brightness (using my oh-so-calibrated Iggle
Eyeball)

>That *does* sound nerdy-looking. ;-)

Definitely high fashion if you are going to MIT, which I am not.  My
teenage daughter says "Dad! Why can't you be normal?" whenever I wear it,
and also any other time too.


An interesting problem occurred when I was designing it.  I am somewhat
prone to migraines.  The day I was first working on it, I had it set up
with 50mS continuous pulses, which would light each LED at about 5 Hz with
no rest between pulses.  The LEDs are extremely bright up close, they hurt
the eyes with a fresh battery.  I found the effect of the continuous
hammering of the 5 Hz light pulses mesmerizing, and I found myself staring
at it blankly, even hypnotically.  Later that day I got a humongous
migraine. The next day, shaken from a night of pain, I turned on the
headlamp again and realized it was stimulating another head-pounding
headache.   I changed to the 900mS between pulses after that, and it never
had the same effect on my aching head again.  Maybe I have discovered the
elusive nonlethal weapon?

--Lawrence Lile

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2002\12\30@112119 by Spehro Pefhany

picon face
At 09:29 AM 12/30/02 -0600, you wrote:

Hi, Lawrence:

>I energized the arrays in order twice (Front, right, rear, left front
>right rear left) for 50mS each time, then let it rest for 900 mS.  Pulsing
>them twice gives an extra-annoying double flash.  I determined by
>experiment that 50mS was about the minimum time I could energize the LED
>array and percieve full brightness (using my oh-so-calibrated Iggle
>Eyeball)

Thanks.

> >That *does* sound nerdy-looking. ;-)
>
>Definitely high fashion if you are going to MIT, which I am not.  My
>teenage daughter says "Dad! Why can't you be normal?" whenever I wear it,
>and also any other time too.

;-)

{Quote hidden}

There have been numerous cases of epileptic seizures triggered in
children by flashes of light from video games. Some individuals are more
photosensitive than others, and the overall lighting affects it.

See, for example:-

http://www.epilepsytoronto.org/people/eaupdate/vol9-3.html

(doesn't look like they tested duty cycle variations) There was an
article in _Scientific American_ many years ago- IIRC, some people
were set off by the flashes of light from driving under tree-lined
roads with bright sunlight shining through the branches.

It might be possible to *treat* migraines with certain frequencies of
flashing light. We're already seeing photo-sensitive drugs applied.

Best regards,

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

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2002\12\30@115821 by Olin Lathrop

face picon face
> An interesting problem occurred when I was designing it.  I am somewhat
> prone to migraines.  The day I was first working on it, I had it set up
> with 50mS continuous pulses, which would light each LED at about 5 Hz
with
> no rest between pulses.  The LEDs are extremely bright up close, they
hurt
> the eyes with a fresh battery.  I found the effect of the continuous
> hammering of the 5 Hz light pulses mesmerizing, and I found myself
staring
> at it blankly, even hypnotically.  Later that day I got a humongous
> migraine. The next day, shaken from a night of pain, I turned on the
> headlamp again and realized it was stimulating another head-pounding
> headache.   I changed to the 900mS between pulses after that, and it
never
> had the same effect on my aching head again.  Maybe I have discovered
the
> elusive nonlethal weapon?

Frequencies in that range are well know to trigger epilepsy attacks in
susceptable people.  Staying out of that range is probably a good idea to
avoid a lawsuit.


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