Smooth LED fading via PWM

[Charlie]

1. Bright LEDs light even with nA current. I had a 5 meter LED stripe that some of the LEDs were ON while not connected to any power source!!! (Yeah, from static charge).

Static charge is by nature static and won't power LEDs. Do you have a wireless router? There is a known issue with early LED Christmas lights illuminating by acting as rectifying antennae. I have a set - turn off my wireless router and they extinguish. Of course, you might live next door to a ham operator as well... but walking across the carpet won't do it.

[Charlie]

Counting from 0 to 255, with the LED on from 0 to 1 then off from 1 to 255 should work nicely, and extending to 512 should make it dimmer yet, I agree. I'd use resistors, but what you are doing "should" work too. The only part of what you are doing that is different from what I've seen work in the past is having the fundamental of the cycle at 32 KHz. It might work, but I've had issues in the past trying to switch LEDs too fast - you might want to drop that down to a couple hundred Hz. But like most of the suggestions, this is only a guess.

[HankMcSpank]

Counting from 0 to 255, with the LED on from 0 to 1 then off from 1 to 255 should work nicely, and extending to 512 should make it dimmer yet, I agree. I'd use resistors, but what you are doing "should" work too. The only part of what you are doing that is different from what I've seen work in the past is having the fundamental of the cycle at 32 KHz. It might work, but I've had issues in the past trying to switch LEDs too fast - you might want to drop that down to a couple hundred Hz. But like most of the suggestions, this is only a guess.

The only reason I run the frequency so high, is because the hpwm carrier *always* manages to permeate its way into the guitar signal (remember this is small signal stuff, that gets amplified by the guitar amp!)...so you always end up with an audible whine in the background.....with the hpwm at 31khz, the problem is still there, but only dogs can hear it!

It does all work at the high frequency...I will revisit this value of '1' (out of 256 vlaues) 'being a bit too bright' issue....but not until my next breadboard session.

[HankMcSpank]

so I'm running out of program memory, I'm presently using the subroutine below to lookup a counter value (count1 below) to get a preset hpwm value (duty)....

Smooth_fade:    
Lookup count1, [ _
  0,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, _
  1,1,1,1,1,1,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,3, _
  3,3,3,3,3,3,3,3,3,3,3,3,4,4,4,4,4,4,4,4,4,4,4,5,5,5, _
  5,5,5,5,5,6,6,6,6,6,6,6,7,7,7,7,7,7,8,8,8,8,8,8,9,9, _
  9,9,10,10,10,10,10,11,11,11,11,12,12,12,13,13,13,13, _
  14,14,14,15,15,15,16,16,16,17,17,18,18,18,19,19,20,  _
  20,20,21,21,22,22,23,23,24,24,25,26,26,27,27,28,29,  _
  29,30,31,31,32,33,33,34,35,36,36,37,38,39,40,41,42,  _
  42,43,44,45,46,47,48,50,51,52,53,54,55,57,58,59,60,  _
  62,63,64,66,67,69,70,72,74,75,77,79,80,82,84,86,88,  _
  90,91,94,96,98,100,102,104,107,109,111,114,116,119,  _
  122,124,127,130,133,136,139,142,145,148,151,155,158, _
  161,165,169,172,176,180,184,188,192,196,201,205,210, _
  214,219,224,229,234,239,244,250,255], duty
  return

,....now since I've not much program space left , is there any way I can win some room back by for example stuffing the above 256 bytes into EEPROM and then look up the values similar to above? (EEPROM is a bit of a new area to me...I've never had to bother with it before!)

If so....would you be so helpful to put me on the right track? (I'm figuring the 256 bytes will need writing into the PIC at the time of programming it & from thereon afterwards it can be read from EEPROM while the program is running?)

[HenrikOlsson]

Hi Hank,
You use DATA to store the constants to EEPROM at program time. Then you use READ to retreive them at runtime. Something like:

Data 0,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, _
  1,1,1,1,1,1,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,3, _
  3,3,3,3,3,3,3,3,3,3,3,3,4,4,4,4,4,4,4,4,4,4,4,5,5,5, _
  5,5,5,5,5,6,6,6,6,6,6,6,7,7,7,7,7,7,8,8,8,8,8,8,9,9, _
  9,9,10,10,10,10,10,11,11,11,11,12,12,12,13,13,13,13, _
  14,14,14,15,15,15,16,16,16,17,17,18,18,18,19,19,20,  _
  20,20,21,21,22,22,23,23,24,24,25,26,26,27,27,28,29,  _
  29,30,31,31,32,33,33,34,35,36,36,37,38,39,40,41,42,  _
  42,43,44,45,46,47,48,50,51,52,53,54,55,57,58,59,60,  _
  62,63,64,66,67,69,70,72,74,75,77,79,80,82,84,86,88,  _
  90,91,94,96,98,100,102,104,107,109,111,114,116,119,  _
  122,124,127,130,133,136,139,142,145,148,151,155,158, _
  161,165,169,172,176,180,184,188,192,196,201,205,210, _
  214,219,224,229,234,239,244,250,255
 
Smooth_fade:
  Read Count1, duty

/Henrik.

[HankMcSpank]

Hi Henrik,

Great, many thanks ....simple enough & it works a treat!

Weirdly even though I imported all those values to Excel to check there are 256 entries (there are) - I got an error when I came to compile (stack overflow)....I took one entry out & then it compiled ok. Hey ho....but I just got rid of one of the multitude of 1s in there.

Many thanks....I've a bit more room to "breathe" in my bloaty program again

[HankMcSpank]

For this specifically LED it might work and as long as the voltage is absolutely stable.

But I insist on using a current source. You will be able to control the brightness for a really 0 to 100%.

Even for a test it is interesting.

I have a client that for more than a year is refering to his power (chinese) LEDs as 24Volts... And I reply with the current of your LEDs is....

Ioannis

If my calculations are right, when sinking current, my 16f1828 is working out at about 52 Ohms of series resistance per pin - now whether the resistance comes by way of a resistor, or the resistive properties of the silicon inside the PIC IO driving circuitry...resistance is resistance?

V Supply = 3.268V
V drop measured across blue LED = 2.931V
remianing V drop across PIC pin = 3.268V-2.931V = 0.337V
Current through LED (inferred by measuring current into whole circuit - 11 LEDS - then taking a little off (3mA for the PIC) & dividing by 11 = 6.4mA

Resistance of PIC pin R = V/I =....or 0.337V/0.0064A = 52.65 Ohms.

So why not simply treat the PIC's pin as a series 52 ohm resistor for the purposes of LED current calculations? (ie what am I missing?)

[SteveB]

How about something like this:

Data 1,1,1,1,1,1,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,3, _
  3,3,3,3,3,3,3,3,3,3,3,3,4,4,4,4,4,4,4,4,4,4,4,5,5,5, _
  5,5,5,5,5,6,6,6,6,6,6,6,7,7,7,7,7,7,8,8,8,8,8,8,9,9, _
  9,9,10,10,10,10,10,11,11,11,11,12,12,12,13,13,13,13, _
  14,14,14,15,15,15,16,16,16,17,17,18,18,18,19,19,20,  _
  20,20,21,21,22,22,23,23,24,24,25,26,26,27,27,28,29,  _
  29,30,31,31,32,33,33,34,35,36,36,37,38,39,40,41,42,  _
  42,43,44,45,46,47,48,50,51,52,53,54,55,57,58,59,60,  _
  62,63,64,66,67,69,70,72,74,75,77,79,80,82,84,86,88,  _
  90,91,94,96,98,100,102,104,107,109,111,114,116,119,  _
  122,124,127,130,133,136,139,142,145,148,151,155,158, _
  161,165,169,172,176,180,184,188,192,196,201,205,210, _
  214,219,224,229,234,239,244,250,255
 
Smooth_fade:
    IF Count1 = 0 THEN 
         duty = 0
    ELSE
         IF Count1 < 26 THEN 
             duty = 1
         ELSE
             Read Count1-25, duty
         ENDIF
    ENDIF
    RETURN

WriteUserByte:
    WRITE 255, UserByte
    RETURN

ReadUserByte:
    READ 255, UserByte
    RETURN

Now you have the upper 25 EEPROM addresses to use. More space could be saved by using similar logic. But this hopefully gets the point across.

Good luck,
Steve

[SteveB]

My math was a little off, the line for the reading of the EEPROM should be

READ count1-26, duty

As a side note, you could make it really simple just by leaving the original table as-is and using this:

Smooth_fade: 
     IF Count1 = 0 THEN 
          duty = 0 
     ELSE 
          Read Count1, duty 
     ENDIF
RETURN

WriteUserByte: 
     WRITE 0, UserByte 
     RETURN

ReadUserByte: 
     READ 0, UserByte 
     RETURN