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Is PWM of OE pin on 74HC595 okay for LED brightness?

Started by dw September 26, 2004
First off... I'm a PIC and electronics novice.

I am breadboarding a cicuit to control LEDs using IO
expansion.  I am using the serial in - parallel out
double-buffered 74HC595 shift register to control the
LEDs.  The shift register is controlled by a PIC
18F452.  I created a PIC program to control the LED
patterns.  

Now, I want to control the brightness of the LEDs.  I
tried using the PIC PWM on the output enable (OE) pin,
and this seems to work.  However, I am concerned that
the "high-impedence" mode of the shift register will
leave the ground "floating" (through the LEDs).  

Is this a problem?  Is there a better way?  I would
appreciate any help.

Thanks.

dw
On 2004-09-26, dw <duaneleslie@bellsouth.net> wrote:

> Now, I want to control the brightness of the LEDs. I > tried using the PIC PWM on the output enable (OE) pin, > and this seems to work. However, I am concerned that > the "high-impedence" mode of the shift register will > leave the ground "floating" (through the LEDs).
I don't understand what you mean by "leave the ground floating'. How do you have the LEDs connected? Vcc | _|_ \ / V LED --- | | \ / Current limiting resistor \ | 74595 Qn --------+ This would probably be the normal way to do it, and putting Qn into a H-Z state is perfectly OK. Alternatively, you could source current from Qn through the LED and current-limiting resistor to ground. That works fine too.
> Is this a problem? Is there a better way? I would appreciate > any help.
-- Grant Edwards grante Yow! Hello? Enema at Bondage? I'm calling visi.com because I want to be happy, I guess...
On 26 Sep 2004 16:10:32 -0700, the renowned duaneleslie@bellsouth.net
(dw) wrote:

>First off... I'm a PIC and electronics novice. > >I am breadboarding a cicuit to control LEDs using IO >expansion. I am using the serial in - parallel out >double-buffered 74HC595 shift register to control the >LEDs. The shift register is controlled by a PIC >18F452. I created a PIC program to control the LED >patterns. > >Now, I want to control the brightness of the LEDs. I >tried using the PIC PWM on the output enable (OE) pin, >and this seems to work. However, I am concerned that >the "high-impedence" mode of the shift register will >leave the ground "floating" (through the LEDs). > >Is this a problem? Is there a better way? I would >appreciate any help. > >Thanks. > >dw
No, it's not a problem to just tristate the parallel outputs. It's a good way to do it. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com