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'[EE] LED Challenged'
2000\05\28@200640 by Ed Edmondson

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I guess I am really "LED Challenged" as I can't find a circuit that will dim
an LED used as a backlight.  The LED has a Vf of 3.5 to 4.0 volts @ If of 20
mA.  I want to run it off 5 volts and want to vary the intensity from "Off"
to "On" (less the drop across the Vbe) using a 2N3904?  I figured the LED
limit resistor to be about 45 ohms (5 - .6 - 3.5)/.020) ?  Is this right?

How do I vary the base drive to vary the LED intensity?  A resistive divider
with a pot?  How do I calculate the values for the divider?  The 2N3904 has a
hfe of 100 to 300.  Does anyone care to help me out?

Regards,
Ed

2000\05\28@214626 by Bob Ammerman

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Use PWM (pulse width modulation).

Assuming you are driving this with a PIC. Some PICs have built in pulse
width modulators.

Alternatively, since you don't need a particularly fast output, you could do
it in software, perhaps driven off a timer interrupt.

Bob Ammerman
RAm Systems
(high function, high performance, low level software)

{Original Message removed}

2000\05\28@223031 by Robert A. LaBudde

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<x-flowed>At 08:06 PM 5/28/00 -0400, you wrote:
>I guess I am really "LED Challenged" as I can't find a circuit that will dim
>an LED used as a backlight.  The LED has a Vf of 3.5 to 4.0 volts @ If of 20
>mA.  I want to run it off 5 volts and want to vary the intensity from "Off"
>to "On" (less the drop across the Vbe) using a 2N3904?  I figured the LED
>limit resistor to be about 45 ohms (5 - .6 - 3.5)/.020) ?  Is this right?
>
>How do I vary the base drive to vary the LED intensity?  A resistive divider
>with a pot?  How do I calculate the values for the divider?  The 2N3904 has a
>hfe of 100 to 300.  Does anyone care to help me out?

The emitter voltage is the base voltage V(B) minus the diode drop of 0.7 V.
The voltage across the emitter resistor determines the emitter current. The
emitter current is the collector current I(C) (99+%).

So use a voltage divider to generate a voltage V(B) such that

        I(C) = (V(B)-0.7))/R(E)

The maximum current you will be able to get with 5.0 V is

        I(C)max = (5-0.7-3.5)/R(E) = 0.8/R(E)

For this to equal 20 mA, you'll need R(E) = 40 ohm.

However, I believe you've got a mistaken impression on the LED voltage
drop. They usually start about 1.7-2.0 V. In this case

        I(C)max = (5.0-2-0.7)/R(E) = 2.3/R(E)

and you'd need R(E) = 110 ohm.

Also, modern diodes have the same brightness at 5 mA as the original ones
did at 20 mA. You might do better to get by with 0-10 mA instead of 20 mA.

Assuming a 150 ohm resistor for R(E) (to be conservative), then

        I(LED) ~ I(C) ~ (V(B)-0.7)/R(E)

and you set V(B) with a voltage divider. The r(B) will be about beta*150
ohm, or about 20k. You need voltage divider resistors of about 2k to avoid
feedback from the base current.

In summary, use a 2k pot as a voltage divider, backed by 200 ohm resistors
at its ends (unless you want to go fully to saturation and cutoff), use a
150 ohm emitter resistor, and put the LED across the +5 V and collector.

Once you've determined the brightness you want, replace the pot with fixed
resistors.



================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: spam_OUTralTakeThisOuTspamlcfltd.com
Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
824 Timberlake Drive                            Tel: 757-467-0954
Virginia Beach, VA 23464-3239                   Fax: 757-467-2947

"Vere scire est per causas scire"
================================================================

</x-flowed>

2000\05\28@232516 by Bob Blick

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>However, I believe you've got a mistaken impression on the LED voltage
>drop. They usually start about 1.7-2.0 V. In this case

The original poster probably had the drop right.
Backlit LCD displays I've used have a series+parallel string and you have
the equivalent of 2 series LEDs.

Back to the original topic, is it neccessary to have infinite levels? Why
not just 3 levels controlled by two pins, each driving a transistor and
resistor.

If you decide to so software PWM, make it very regular, or you'll notice
pulsing as the PIC does different tasks(especially if you do a timer based
interrupt and sometimes shut off interrupts!).

-Bob

- Every day, computers are making people easier to use -

2000\05\29@001724 by Spehro Pefhany

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At 08:06 PM 5/28/00 EDT, you wrote:

the base drive to vary the LED intensity?  A resistive divider
>with a pot?  How do I calculate the values for the divider?  The 2N3904 has a
>hfe of 100 to 300.  Does anyone care to help me out?

Assuming you just want to vary the brighness with a pot, not under program
control...

If you intend to use a pot to control the brightness, one easy way would
be to use a 555 to do PWM and also switch the LED.

Have a look at the data sheet for the 555. If you connect the backlight
from +5 , through a series resistor, to pin 3 the 555 will easily drive
20mA (even a CMOS 555 will easily sink the 20mA).

If you use a fixed resistor from +5 to pin 7, perhaps 10K, then use a pot
of, say, 100K with a 1K series resistor from pin 7 to pins 2 & 6,
you can get from 10% to 90% brightness. Use a timing capacitor such as
1n0 that will give a reasonable frequency.

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
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
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2000\05\29@024918 by Steve Smith

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Consider a different approach.
Dont vary the current but vary the on time!
have the led current set to 25mA via the series resistor and then turn it on
and off with a pic pin the longer the led is on the brighter it will be .
Make the frequency of the "flashing" high enough and it wont be percived as
flashing but as varied intentisy.

Cheers Steve.....

2000\05\29@103708 by Alok Dubey

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why would anyone want to use a PWM thru a 555 waveform to change the
intensity of an ordinary LED?.....if u req a large sensitivity use a small
FET , or somethg in series with the LED or somethng.. of course u need a god
amt om resistance calulations and make sure no surge current in LED et al.
Alok


{Original Message removed}

2000\05\29@115459 by Spehro Pefhany

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At 08:02 PM 5/29/00 +0530, you wrote:
>why would anyone want to use a PWM thru a 555 waveform to change the
>intensity of an ordinary LED?.....if u req a large sensitivity use a small
>FET , or somethg in series with the LED or somethng.. of course u need a god
>amt om resistance calulations and make sure no surge current in LED et al.
>Alok

Why would anyone what to reduce the brighness of the backlight?

Maybe to save power and extend battery life?  PWM will do that.

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
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

2000\05\29@120331 by Alok Dubey
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u mean using a 555 with all its cap and res and the sudden volatge spikes u
get out of switching the o/p on pin 3 and hence causing some amt of PS droop
, gnd bounce etc is better than using a sort of mosfet whose current i can
control ?
alok


{Original Message removed}

2000\05\29@120742 by Spehro Pefhany

picon face
At 09:28 PM 5/29/00 +0530, you wrote:
>u mean using a 555 with all its cap and res and the sudden volatge spikes u
>get out of switching the o/p on pin 3 and hence causing some amt of PS droop
>, gnd bounce etc is better than using a sort of mosfet whose current i can
>control ?
>alok

Yes, it could well be.

See http://www.ti.com TLC555 for a modern version of the 555.

Best regards,


=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam.....interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
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2000\05\29@121817 by Alok Dubey

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okay..
try analog dev site for a list of CMOS ratings
alok

-----Original Message-----
From: Spehro Pefhany [EraseMEspeffspam_OUTspamTakeThisOuTINTERLOG.COM]
Sent: Monday, May 29, 2000 5:10 PM
To: PICLISTspamspam_OUTMITVMA.MIT.EDU
Subject: Re: [EE] LED Challenged


At 09:28 PM 5/29/00 +0530, you wrote:
>u mean using a 555 with all its cap and res and the sudden volatge spikes u
>get out of switching the o/p on pin 3 and hence causing some amt of PS
droop
>, gnd bounce etc is better than using a sort of mosfet whose current i can
>control ?
>alok

Yes, it could well be.

See http://www.ti.com TLC555 for a modern version of the 555.

Best regards,


=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
=
Spehro Pefhany --"it's the network..."            "The Journey is the
reward"
@spam@speffKILLspamspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
=

2000\05\30@055934 by Vasile Surducan

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On 29 May 00, at 20:02, Alok Dubey wrote:

> why would anyone want to use a PWM thru a 555 waveform to change the
> intensity of an ordinary LED?...

  If you have a three colour (RGB) LED it's the simplest solution to
achieve any colour you may need from a digital output, else, why
not more complicated as possible...
  Vasile



..if u req a large sensitivity use a small
> FET , or somethg in series with the LED or somethng.. of course u need a god
> amt om resistance calulations and make sure no surge current in LED et al.
> Alok
>
>
> {Original Message removed}

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