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'Newbie Question about LED's'
1998\02\28@005041 by James & Ili

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face
Hello,
       I'm in the middle of designing a device that is going down into the
deep, dark ocean. It's just a little quadrature encoder that is going
on the end of a robotic arm to measure the revolutions of a torque
tool. The problem I foresee is driving the LED's to really make them
shine. Is there a device like the ULN2803 that I could use to drive
the LED's to something higher than 25ma ?? The LED's are rated for
30ma and I figure I can pulse them at something higher i.e. 50 to
60ma. Of course I could do it with transistors but an IC would be
more sanitary. I've been to all the local electronics shops looking
for the 2803 or something like it. I've pulsed some IR LED's to 2 A
with no adverse effects therefore I assume you can drive regular RED
LED's at something higher than rated current. Any ideas ?? All help
is appreciated.

       Thanks in advance !!
                               James Holbrook

1998\02\28@012806 by kirmse

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James & Ili wrote:

> Hello,
>         I'm in the middle of designing a device that is going down into the
> deep, dark ocean. It's just a little quadrature encoder that is going
> on the end of a robotic arm to measure the revolutions of a torque
> tool. The problem I foresee is driving the LED's to really make them
> shine. Is there a device like the ULN2803 that I could use to drive
> the LED's to something higher than 25ma ?? The LED's are rated for
> 30ma and I figure I can pulse them at something higher i.e. 50 to
> 60ma. Of course I could do it with transistors but an IC would be
> more sanitary. I've been to all the local electronics shops looking
> for the 2803 or something like it. I've pulsed some IR LED's to 2 A
> with no adverse effects therefore I assume you can drive regular RED
> LED's at something higher than rated current. Any ideas ?? All help
> is appreciated.
>
>         Thanks in advance !!
>                                 James Holbrook

Greetings:

Unless you have any other requirements than simply driving an LED with
100+ ma I do not see any reason why you would not use a transister.
A logic level N channel mosfet and a current limiting resistor would be
all of the parts required. Single mosfets with a 0.5 A rating can be
found in packages as small as a SOT-23 for surface mount and TO-92 for
through hole parts. Dual and quad parts are also availiable.

Kevin Kirmse

1998\02\28@021402 by James Henke

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On Sat, 28 Feb 1998 01:12:52 -0500, Kevin Wrote:

{Quote hidden}

What Kevin said about Mosfets. They're much easier to come by these
days: Digikey, Mouser, even Radio Slock.

But don't forget that if you're pulsing the LEDs in a quadrature
encoder, you must sample the output fast enough to satisfy the Nyquist
criteria, in synchrony with the led pulsing.  What's the maximum RPM x
counts per revolution? Sample at LEAST that fast, and preferably much
faster.

Jim

1998\02\28@092224 by Wayne Foletta

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Be sure to check the "half-life" spec of the LEDs versus forward
current for the LEDs you are using. With some brands of LEDs the light
output falls off very quickly at higher currents (over 20% per decade
of time at 2X rated maximum If).

Wayne Foletta
Data Acquisition Engineering
SiliconSoft, Inc.
http://www.siliconsoft.com


You wrote:
>
>Hello,
>        I'm in the middle of designing a device that is going down
into the
{Quote hidden}

1998\02\28@135226 by James & Ili

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face
Hi Wayne,
       How would I obtain this information ? Manufacturer or trial and
error ?
As far as time goes, the counters would only be underwater for 12 to
18 hours at
the most. But a concern would be that the display stopped working
while down below.
If a return trip had to be made because the display was out it could
get very costly in a hurry
at multiple thousands of dollars per hour.
       I just returned from looking at your web page.. I would sure like to
pick your brain about
the next project. A 2 axis inclinometer that will send RS232 up to
the surface. I was thinking of
using the PIC 12C671 with it's built in A/D converters and 2 pins
left for RS232.
       Thanks for taking the time to reply to my post.
       James Holbrook

On Saturday, February 28, 1998 8:21 AM, Wayne Foletta
[SMTP:spam_OUTfolettaTakeThisOuTspamIX.NETCOM.COM] wrote:
> Be sure to check the "half-life" spec of the LEDs versus forward
> current for the LEDs you are using. With some brands of LEDs the
> light output falls off very quickly at higher currents (over 20%
per
> decade of time at 2X rated maximum If).
>
> Wayne Foletta
> Data Acquisition Engineering
> SiliconSoft, Inc.
> http://www.siliconsoft.com

1998\02\28@164357 by Morgan Olsson

picon face
I don«t know the resolution you want.
Maybe it is possible to use a magnet and ferromagnetic "star" on the
rotating part, and read it out by magnetic sensors?

I once used a magnet on the rotor "outside" which was magnetically coupled
to another magnet "inside", on the measuring device.  So I had high
resolution and no possibility of leakage.  Works only for low rpm though.
/Morgan
/  Morgan Olsson, MORGANS REGLERTEKNIK, SE-277 35 KIVIK, Sweden \
\  .....mrtKILLspamspam@spam@iname.com, ph: +46 (0)414 70741; fax +46 (0)414 70331    /


'Newbie Question about LED's'
1998\03\01@032535 by paulb
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James Henke wrote:

> But don't forget that if you're pulsing the LEDs in a quadrature
> encoder, you must sample the output fast enough to satisfy the Nyquist
> criteria, in synchrony with the led pulsing.  What's the maximum RPM x
> counts per revolution? Sample at LEAST that fast, and preferably much
> faster.

 Hey!  Do I gather he's being very clever here and multiplexing the
quadrature encoder by pulsing the LEDs?  Cool idea!

 Cheers,
       Paul B.

1998\03\01@162751 by H.P.d.Vries

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Paul B. Webster VK2BZC wrote:
>
> James Henke wrote:
>
> > But don't forget that if you're pulsing the LEDs in a quadrature
> > encoder, you must sample the output fast enough to satisfy the Nyquist
> > criteria, in synchrony with the led pulsing.  What's the maximum RPM x
> > counts per revolution? Sample at LEAST that fast, and preferably much
> > faster.
>
>   Hey!  Do I gather he's being very clever here and multiplexing the
> quadrature encoder by pulsing the LEDs?  Cool idea!
>
>   Cheers,
>         Paul B.
Better sample twice as fast. As I remember it (too lazy too look it up)
Fs = 2x Fmax
, where Fs = sample frequency, and Fmax is maximum frequency to be
sampled.

Hans

1998\03\02@021008 by Pasi T Mustalahti

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On Fri, 27 Feb 1998, James & Ili wrote:

> Hello,
>         I'm in the middle of designing a device that is going down into the
> deep, dark ocean.
>                                 James Holbrook
PTM: Use High Bright versions. I have used both 3500mcd and 10000mcd
versions. If one is not bright enough, take two ! If you got to see it by
eye, make it blink. Sometimes it is easier to 'see' a blinking light by
electronic devices too. Then you just use a derivating receiver or
resonating.
If you cant get the transmitter powerfull enough, make the receiver more
sensitive.
--------------------------------------------------------------------------
PTM, pasi.mustalahtispamKILLspamutu.fi, .....ptmustaKILLspamspam.....utu.fi, http://www.utu.fi/~ptmusta
Lab.ins. (mikrotuki) ATK-keskus/Mat.Luon.Tdk                    OH1HEK
Lab.engineer (PC support) Computer Center                       OI7234
Mail: Turun Yliopisto / Fysla, Vesilinnantie 5, 20014
Pt 02-3336669, FAX 02-3335632 (Pk 02-2387010, NMT 049-555577)
--------------------------------------------------------------------------

1998\03\02@034350 by kdowsett

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James & Ili wrote:
>
> Hello,
>         I'm in the middle of designing a device that is going down into the
> deep, dark ocean. It's just a little quadrature encoder that is going
> on the end of a robotic arm to measure the revolutions of a torque
> tool. The problem I foresee is driving the LED's to really make them
> shine.

Why do you need to? All a quadrature encoder needs is enough light to be
reliably detected 2mm away by a phototransistor. 10mA should be plenty.

<snip...>
. I've pulsed some IR LED's to 2 A
> with no adverse effects therefore I assume you can drive regular RED
> LED's at something higher than rated current. Any ideas ?? All help
> is appreciated.

The human eye is sensitive to peak brightness, so you can get a better
response from a large pulsed current than a low continuous one. However,
for an encoder you need a constant light source so that the
phototransistor can detect when it is interrupted. A pulsed LED would be
useless.

If you have volts to spare consider including a constant current source
for the LED, and add some filtering if you are sharing the power supply
with other equipment. This should be more reliable than a simple
resistor. Remember that when designing for remote environments
reliability is more important than component cost. Retrieving a
submersible because a 10p (about 15 US cents) LED has failed will make
you rather unpopular.

Just my thoughts,

Keith.

1998\03\02@074824 by Wayne Foletta

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James - You wrote:
>
>Hi Wayne,
>        How would I obtain this information ? Manufacturer or trial and
>error ?
If the manufacturer does not publish a "Percent Change of Light Output vs Time"
you
could estimate it for the generic type (GaAs is better than GaAlAs) and use
other
manufacturer's specs. Otherwise test aging yourself with a linear photodector.
Also
remember temperature is major factor in LED light output (-0.5% per degree
typical).

{Quote hidden}

1998\03\02@084306 by Tom Mariner

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Unless you aren't planning on submerging your probe more than 100 feet, you
may want to rethink the RS-232. Maybe 422 or some other differential
solution would prove more robust. I think the '71 is a good choice if you
need compact size and performance.

Tom

On Monday, March 02, 1998 7:48 AM, Wayne Foletta
[SMTP:folettaspamspam_OUTIX.NETCOM.COM] wrote:
> James - You wrote:
> >
> >Hi Wayne,
> >        How would I obtain this information ? Manufacturer or trial and
> >error ?
> If the manufacturer does not publish a "Percent Change of Light Output vs
Time"
>  you
> could estimate it for the generic type (GaAs is better than GaAlAs) and
use
>  other
> manufacturer's specs. Otherwise test aging yourself with a linear
photodector.
>  Also
> remember temperature is major factor in LED light output (-0.5% per
degree
>  typical).
>
> >As far as time goes, the counters would only be underwater for 12 to
> >18 hours at
> >the most. But a concern would be that the display stopped working
> >while down below.
> >If a return trip had to be made because the display was out it could
> >get very costly in a hurry
> >at multiple thousands of dollars per hour.
> >        I just returned from looking at your web page.. I would sure
like to
{Quote hidden}

1998\03\02@161954 by Philip Martin

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>Hello,
>        I'm in the middle of designing a device that is going down into the
>deep, dark ocean. It's just a little quadrature encoder that is going
>on the end of a robotic arm to measure the revolutions of a torque
>tool. The problem I foresee is driving the LED's to really make them
>shine. Is there a device like the ULN2803 that I could use to drive
>the LED's to something higher than 25ma ?? The LED's are rated for
>30ma and I figure I can pulse them at something higher i.e. 50 to
>60ma. Of course I could do it with transistors but an IC would be
>more sanitary. I've been to all the local electronics shops looking
>for the 2803 or something like it. I've pulsed some IR LED's to 2 A
>with no adverse effects therefore I assume you can drive regular RED
>LED's at something higher than rated current. Any ideas ?? All help
>is appreciated.
>
>        Thanks in advance !!
>                                James Holbrook

Given the previous advice by others, if your still set on using an ic to
dirve these led's, have a look at the ULN2000 darlington arrays. Ive used
these ic for a number of led driving jobs lately, including 5inch high 7
segment displays. Some applications were at collector currents of up to
200ma strobing across 7 channels. Mind you we did stick a heat sink on the
top of it.

BTW, as a scuba diver, just how deep will this machine be going?

Philip Martin.
AOW

1998\03\04@180910 by Larry G. Nelson Sr.

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Take a look at the Allegro Drivers. Many allow you to interface to your
logic and supply the drive with another voltage. I am doing this with a
high side driver running at 12 volts controlled by a pic. The allegro part
I am using is a serial input 20 bit driver #5812F. I am sourcing at 20mA
but they are rated to a max of 40mA. Other parts can run higher. They are
located in Worcester MA and the phone # is 508-853-5000. They have a new
part coming that is designed specifically to drive LEDs but I do not know
its availability.


At 11:52 PM 2/27/98 -0600, you wrote:
{Quote hidden}

Larry G. Nelson Sr.
KILLspamL.NelsonKILLspamspamieee.org
http://www.ultranet.com/~nr

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