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

Question

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

Grant Edwards · Accepted Answer

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.

Spehro Pefhany · Answer

No, it's not a problem to just tristate the parallel outputs. It's agood way to do it. Best regards, Spehro Pefhany-- "it's the network..."                          "The Journey is the reward"             Info for manufacturers:  software/hardware/analog  Info for designers:

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

Join the Discussion

Didn't find your answer?