Driving LEDs from PIC weak pullup, saving resistors?

Well, it's worth considering. If you can turn on the pullup but not enable the normal logic input path, it won't be an issue.

Can you configure the pin as an output and pwm it? That gives you brightness control!

John

What did I miss? You have it configured as a digital input pin right? I did see a late post about an A/D function, I assumed you had that turned of...... Or should I be apologizing now

On a sunny day (Sun, 06 Jan 2008 12:04:11 -0800) it happened John Larkin wrote in :

There is a PWM output, I have implented it in the soft, but it is intended to drive high power stuff (MOSFET). The issue here is not so much variable brightness, but sparing out X resistors. As Vladimir ... think of all the energy needed to:

1) make the 8 resistors. 2) drive out and buy the 8 resistors. 3) To solder the 8 resistors. 4) To tin the extra tracks, mine the silver, etc. 5) The time spend with a high wattage lamp to see, and soldering iron fixing the 8 tiny SMD thingies. 6) The manufacturing of the money needed to buy the 8 resistors. 7) The extra accounting, extra paper, paperwork, BOM, design time, board space, documentation, extra bits in pictures, added fault probability, all ask for no resistors. So it is a green solution, even with blue or red LEDs.

On a sunny day (Sun, 6 Jan 2008 12:12:09 -0800 (PST)) it happened "

" wrote in :

DO YOUR HOMEWORK AND LOOK AT THE PDF.

Cool, but it still wastes power in the resistor. You can have your cake and eat it too by using an 18F (most seem to have >100R output impedance on the i/o pins) and PWM the LED. No resistor, no wasted power. ;-)

I was thinking of pwm *without* resistors.

John

You mean this typo: "... and that pin (RB1) will be configured as input. =2E.... PIC 16F690. ..."

Maybe that should be 'RA1'

I'm going for wings and a beer ~ have a better one :)

On a sunny day (Sun, 6 Jan 2008 14:21:11 -0600) it happened "Anthony Fremont" wrote in :

PWM is not always nice, the thing I do not like is that you see a zillion LEds when it moves fast. Als to do PWM on each pin takes a lot of resources in a PIC.

On a sunny day (Sun, 06 Jan 2008 12:22:52 -0800) it happened John Larkin wrote in :

OK.

I was thinking if I would win the lottery (have not looked yet).

Dude you post 300 to 700 times a month, what is up with that? Do you work or what? My wings were goog btw.......

I have yet to read the 30 or so articles down the thread as of my late entry into this thread.

However, I do suggest that InGaN LEDs (blue, blue-green, non-yellowish-green and white, with typical forward voltage drop

3.something volts at "characterization current") do very well at low currents. I see this even more true for blue, blue-green and green ones with nominal wavelength 470 to 530 nm and "characterization current" of 20 mA.

I am aware of some red ones being less-bad than the above at losing efficiency once the current is down to the roughly-10-microamp ballpark. However, if current will be in the ballpark of a couple hundred microamps, InGaN blue, blue-green and green LEDs are the champs!

Beware of color shift from such low current: Most green ones become "lime green" to "yellowish green", most blue-green ones get closer to "emerald green", and most blue ones shift at least to "turquoise", sometimes to a hue closer to "cyan".

- Don Klipstein ( snipped-for-privacy@misty.com)