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PICList Thread
'Driving LEDs other than with a PIC'
1999\03\22@185157 by Steve Ridley

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This is more of a general question. What is the common way to drive a LED if
you can't connect it directly to a PIC pin. I am using 4000 series cmos
and/or 74HC chips. I need to switch LEDs on and off under the control of
these chips. For various reasons I can't use PIC pins. I am thinking of
using a single transistor for each LED. Is there a better way ?

Steve Ridley
Blue Ocean Telecommunications
Sydney,  Australia

1999\03\22@213031 by russellh

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Steve Ridley wrote:
>
> This is more of a general question. What is the common way to drive a LED if
> you can't connect it directly to a PIC pin. I am using 4000 series cmos
> and/or 74HC chips.

The 74HC part can drive an LED, generally. Pulling down, anyway. Look it
up, the spec sheets are available on line. Phillips has them, so do
others.

The 4000 CMOS, if you have more than just a couple, then us a 74HC part
as a driver! For just a few LED's, try 2N2222's.

> I need to switch LEDs on and off under the control of
> these chips. For various reasons I can't use PIC pins. I am thinking of
> using a single transistor for each LED. Is there a better way ?
>
> Steve Ridley
> Blue Ocean Telecommunications
> Sydney,  Australia

1999\03\22@222100 by Eric Oliver

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Steve,

I far from experienced and others might have a better idea, but why not use
something like a 74HC595 shift register.  This effectively turns three PIC
pins into eight output pins ( not exactly, but close ). The 595 use a data,
clock, and latch.  The latch line latches the shifted data onto the output
lines ( this keeps your LEDS from rippling as you shift out data ).

HTH,
Eric

-----Original Message-----
From:   Steve Ridley [SMTP:spam_OUTsteverTakeThisOuTspamNSW.UNILINK.OZ.AU]
Sent:   Monday, March 22, 1999 6:46 PM
To:     .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
Subject:        Driving LEDs other than with a PIC

<< File: ATT00003.txt; charset = windows-1250 >>

1999\03\22@222112 by Steve Ridley

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A million thanks Russell. I Checked the specs and a 74HC04 hex inverter will
sink and source 25mA each gate. That is just what I wanted. Thanks again.

Steve

{Original Message removed}

1999\03\22@223632 by Mike Keitz

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Use a high-efficiency LED.  They are decently bright with only 2 mA
drive.  Maybe even the 4000 series chips can handle that.  Old LED's from
your junk box aren't nearly as good as the ones they're making today.

Russell's suggestion to drive the LED with an active low output is good
too.  Most IC outputs, even CMOS, can sink more current than source.

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1999\03\22@233942 by Vincent Deno

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On Tue, 23 Mar 1999, Steve Ridley wrote:

> This is more of a general question. What is the common way to drive a LED if
> you can't connect it directly to a PIC pin. I am using 4000 series cmos
> and/or 74HC chips. I need to switch LEDs on and off under the control of
> these chips. For various reasons I can't use PIC pins. I am thinking of
> using a single transistor for each LED. Is there a better way ?
>

Obviously there a plenty of ways to do this.  For my current application
(many LEDs, few available PIC I/O pins) I simply used a serial to parallel
shift registers and connected the LEDs to the outputs with a current
limiting resistor.  Just one of many options.

> Steve Ridley
> Blue Ocean Telecommunications
> Sydney,  Australia
>

--------------
Vincent Deno
Design Engineer
Theta Digital Corp.
http://www.thetadigital.com
denovjspamKILLspamemail.uc.edu
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1999\03\23@091939 by Stan Ockers

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                     RE: Driving LEDs other than with a PIC       3/23/99

Steve wrote
>using a single transistor for each LED. Is there a better way ?
>
If you are trying to save PIC pins, I think the 4051 1-of-8 switch will allow 25 milliamps to be routed through it, (either direction).

Stan

1999\03\24@063510 by Marc

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> (many LEDs, few available PIC I/O pins) I simply used a serial to parallel
> shift registers and connected the LEDs to the outputs with a current
> limiting resistor.  Just one of many options.

If you want to cut part count, you can use one current limit resistor for
a bunch of LEDs (in the VCC supply). The software must then pulse the LEDs
to avoid them burning, and can pulse another LED while the first is on
pause. This way you can run all 8 LEDs on the HC595 with just one resistor.

... But make sure your PIC doesn't crash :-)

1999\03\24@073036 by Stefan Sczekalla-Waldschmidt

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Hi,

Marc wrote:
>
> > (many LEDs, few available PIC I/O pins) I simply used a serial to parallel
> > shift registers and connected the LEDs to the outputs with a current
> > limiting resistor.  Just one of many options.
>
> If you want to cut part count, you can use one current limit resistor for
> a bunch of LEDs (in the VCC supply). The software must then pulse the LEDs
> to avoid them burning, and can pulse another LED while the first is on
> pause. This way you can run all 8 LEDs on the HC595 with just one resistor.
>
> ... But make sure your PIC doesn't crash :-)


I would suggest to add a monoflop which is triggered to switch the led
on.
If the Software crashes the Monoflop don't get triggered any more -
preventing
the leds from "melting" :-)

Kind regards,

       Stefan

1999\03\24@081924 by paulb

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Stan Ockers wrote:

> If you are trying to save PIC pins, I think the 4051 1-of-8 switch
> will allow 25 milliamps to be routed through it, (either direction).

 You'd be lucky!  In particular, the "either direction" is wrong.  The
chip latches up in one particular direction at this current, as the
specs point out.

 If you insist on using a part *not* designed for the application, at
least use the 74HC4051.
--
 Cheers,
       Paul B.

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