Linearizing an LED for 800 pwm steps'?

[Ioannis]

Do you have any freedom to make hardware changes?

PWM controlling the current of the LED will give the best results.

So you could use a PWM output to drive a programmable constant current which in turn will drive the LED.

It sounds complicated but is not that much. Look at the schematic.


PWM output is filtered and drives the transistor that operates as a constant current driver. Current is controlled by R2 and should be selected to satisfy max LED current.

R1 and C1 be 5-10 times the PWM frequency.

Ioannis

[HankMcSpank]

Darrel,

That works a treat...many thanks :-)

A monster 1024 value table .....& as compact as Mr Compact's very compact car ....which in turn has been compacted & stored in a very compact gap between two houses.

DataWord  VAR  WORD
counter1    VAR  WORD
DataTable CON  EXT
'-----[The DATA table]--------------------------------------------------------
GOTO OverData             ; Make sure data doesn't try to execute
ASM
DataTable
    DW 0,0,0,0,1,1,1,1,1,1,1,2,2,2,2,2,2,2,3,3,3,3,3,3,4,4,4,4,4,4,4,5,5,5,5,5,5,5,6,6,6,6,6,6,6,7,7,7,7,7,7,8,8,8,8,8,8,8,9,9,9,9,9,9,10,10,10,10,10,10,10,11,11
    DW 11,11,11,11,12,12,12,12,12,12,13,13,13,13,13,13,13,14,14,14,14,14,14,15,15,15,15,15,15,16,16,16,16,16,16,16,17,17,17,17,17,17,18,18,18,18,18,18,19,19,19,19,19
    DW 19,20,20,20,20,20,20,21,21,21,21,21,21,22,22,22,22,22,22,23,23,23,23,23,23,24,24,24,24,24,24,25,25,25,25,25,25,26,26,26,26,26,27,27,27,27,27,27,28,28,28,28,28
    DW 28,29,29,29,29,29,29,30,30,30,30,30,31,31,31,31,31,31,32,32,32,32,32,33,33,33,33,33,33,34,34,34,34,34,34,35,35,35,35,35,36,36,36,36,36,37,37,37,37,37,37,38,38
    DW 38,38,38,39,39,39,39,39,40,40,40,40,40,40,41,41,41,41,41,42,42,42,42
    DW 42,43,43,43,43,43,44,44,44,44,44,44,45,45,45,45,45,46,46,46,46,46,47,47,47,47,47,48,48,48,48,48,49,49,49,49,49,50,50,50,50,50,51,51,51,51
    DW 51,52,52,52,52,53,53,53,53,53,54,54,54,54,54,55,55,55,55,55,56,56,56,56,56,57,57,57,57,58,58,58,58,58,59,59,59,59,59,60,60,60,60,61,61,61,61
    DW 61,62,62,62,62,63,63,63,63,63,64,64,64,64,65,65,65,65,65,66,66,66,66,67,67,67,67,67,68,68,68,68,69,69,69,69,70,70,70,70,70,71,71,71,71,72,72
    DW 72,72,73,73,73,73,74,74,74,74,74,75,75,75,75,76,76,76,76,77,77,77,77,78,78,78,78,79,79,79,79,80,80,80,80,81,81,81,81,82,82,82,82,83,83,83,83
    DW 84,84,84,84,85,85,85,85,86,86,86,86,87,87,87,87,88,88,88,89,89,89,89,90,90,90,90,91,91,91,91,92,92,92,93,93,93,93,94,94,94,94,95,95,95,96,96
    DW 96,96,97,97,97,97,98,98,98,99,99,99,99,100,100,100,101,101,101,101,102
    DW 102,102,103,103,103,103,104,104,104,105,105,105,105,106,106,106,107,107,107,108,108,108,108,109,109,109,110,110,110,111,111,111,111,112,112
    DW 112,113,113,113,114,114,114,115,115,115,116,116,116,116,117,117,117,118,118,118,119,119,119,120,120,120,121,121,121,122,122,122,123,123,123,124
    DW 124,124,125,125,125,126,126,126,127,127,127,128,128,128,129,129,129,130,130,130,131,131,132,132,132,133,133,133,134,134,134,135,135,135,136,136
    DW 137,137,137,138,138,138,139,139,139,140,140,141,141,141,142,142,142,143,143,144,144,144,145,145,146,146,146,147,147,147,148,148,149,149,149,150
    DW 150,151,151,151,152,152,153,153,153,154,154,155,155,156,156,156,157,157,158,158,158,159,159,160,160,161,161,161,162,162,163,163,164,164,165,165
    DW 165,166,166,167,167,168,168,169,169,169,170,170,171,171,172,172,173,173,174,174,175,175,175,176,176,177,177,178,178,179,179,180,180,181,181,182
    DW 182,183,183,184,184,185,185,186,186,187,187,188,188,189,189,190,190,191,191,192,193,193,194,194,195,195,196,196,197,197,198,198,199,200,200,201, 201,202,202,203
    DW 204,204,205,205,206,206,207,208,208,209,209,210,211,211,212,212,213,214,214,215,215,216,217,217,218,219,219,220,221,221,222,222,223,224,224,225,226,226,227,228
    DW 228,229,230,230,231,232,233,233,234,235,235,236,237,238,238,239,240,240,241,242,243,243,244,245,246,246,247,248,249,250,250,251,252,253,253,254,255,256,257,258
    DW 258,259,260,261,262,263,263,264,265,266,267,268,269,270,270,271,272,273,274,275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,291,292,293,294,295
    DW 296,297,298,299,300,301,303,304,305,306,307,308,309,311,312,313,314,315,317,318,319,320,322,323,324,326,327,328,329,331,332,334,335,336,338,339,341,342,344,345
    DW 347,348,350,351,353,354,356,358,359,361,362,364,366,368,369,371,373,375,377,378,380,382,384,386,388,390,392,394,396,398,401,403,405,407,410,412,414,417,419,422
    DW 424,427,429,432,435,437,440,443,446,449,452,455,458,462,465,468,472,475,479,483,487,490,495,499,503,507,512,517,521,526,532,537,543,548,554,561,567,574,581,589
    DW 597,605,614,624,634,645,657,670,684,699,717,736,759,786,819
    DW 862,922,1023
endasm
Overdata:   ' this is a jump point to make sure the above data table isn't executed
increment_loop:
if counter1 < 1023 then
COUNTER1 =COUNTER1+1
ReadCODE  (DataTable + COUNTER1), DataWord
duty = dataword
gosub change_pwm
pause 1
GOTO increment_loop
endif
decrement_loop:
if counter1 > 0 then
COUNTER1 =COUNTER1-1
ReadCODE  (DataTable + COUNTER1), DataWord
duty = dataword
gosub change_pwm
pause 1
GOTO decrement_loop
endif
GOTO increment_loop
change_pwm:
CCP1CON.4 = Duty.0       'Bit 0
CCP1CON.5 = Duty.1       'Bit 1
CCPR1L = Duty >> 2       'Bit 2-7
return

I've now get a smoothtastic fading up/down LED. (there did appear to be one gotcha...being of the lazy ilk, I wanted each DW line to be 256 values long - nope, max allowed seems to be about 65-70 bytes - I can't be exact but it is in or around that value ....so lots of DW lines needed)

Ioannis ...that sort of what I'm doing - but rather than a tranny I'm using a mosfet (no filtering either...since the end result is visual, and the eye is only good for about 25Hz ....my pwm is about 16khz...so I didn't see the need?)

[Ioannis]

There is no difference (significant at least) whether you use bipolar or mosfet since the transistor will work in linear region. So, since it works linearly, cap will make calculation easier, because you will have to deal with dc voltages in base of the transistor in regard with the ground.


So, after PWM fltering the resulting dc voltage is the sum of Vbe (~0,6) and V(R1). V(R1) will define the current through the LED.

Have not tested in comparisson with the table but seems easier to control.

Ioannis

[Mike, K8LH]

Hi Hank,

Have you tried using fewer discrete brightness level steps spread across the much larger pwm width? For example, I get very smooth fades using between 64 and 100 gamma corrected brightness level steps spread across a much larger 512 pwm steps.

You can also "bend" the duty cycle curve to help match the characteristics of the LEDs.

Regards, Mike

[HankMcSpank]

Hi Mike,

Thnaks for the input...no I've not tried fewer brightness steps - I figured my problem was an insufficient number of brightness steps ......especially at the low end of the brightness scale, where for example the first 20 steps at 256 pwm levels sees quite a swing in the perceived LED brightness.....this is why I whacked it right up to 1024 brightness levels....which is much better from a less 'steppy' LED response perspective , but I'm still not happy with the overall fade curve, so your formula is of high interest! (there's aplethora of curves out there...and it's difficult to decide which one is best for this application!)

I've copied your formula into an excel spreadsheet for LUT of 1024 (well, I have the code space so may as well use it!) & 10 bit PWM.....the curve looks more like what I'm aiming for (if you look at my earlier curve further up the thread, the last part of the curve was almost vertical - this is too extreme)....I see yours is a little more gradual/linear towards the upper/final part of the plot ....and I shall give this a try tomorrow...many thanks.


What's with the 0.3 at the end of the formula ...where did that come from?

[Mike, K8LH]

Hank,

The PWM "step" size makes a significant difference at the low end too. It's amazing how much more light you get from a LED running at 1/1024th duty cycle with 1-usec PWM steps compared to, say, 125-nsec steps. Getting more brightness resolution at the lower duty cycle range seems to pay off with better smooth fade capability.

What's with the 0.3 at the end of the formula ...where did that come from?

Oh, the 0.3 was for rounding. Sorry if it caused any confusion.

BTW, here's a little program I use for building my gamma array tables. It uses the free JustBASIC program. After running it I copy the array from the console into my PIC source file. Hope it may be of some use.

Regards, Mike

 '  Leo Bodnar's 'antilogarithmic' 
 '  gamma correction algorithm (Mike, K8LH)
 '
 '  JustBASIC (free) interpreter
 '
 Input "Gamma array size: "; arraysize
 Input " Total PWM steps: "; width
 Input "Gamma correction: "; gamma       ' try 0.5 to 1.0


 FOR index = 0 to arraysize-1
    dcyval = INT(width^(((index+1)/arraysize)^gamma)+.3)
    if(index = 0) then
      dcyval = 0
      PRINT
    else
      if(index MOD 10 = 0) then
        PRINT ","
      else
        PRINT ",";
      end if
    end if
    if(dcyval < 100) then print " ";
    if(dcyval < 10) then print " ";
    PRINT dcyval;
 NEXT index
 PRINT
 PRINT
 REM CLOSE

And here's what the output looks like when you run the program;

Gamma array size: 64
 Total PWM steps: 256
Gamma correction: .84


  0,  1,  1,  2,  2,  2,  2,  2,  3,  3,
  3,  4,  4,  4,  5,  5,  6,  7,  7,  8,
  9,  9, 10, 11, 12, 13, 14, 16, 17, 19,
 20, 22, 24, 26, 28, 30, 33, 36, 39, 42,
 45, 49, 53, 57, 62, 67, 72, 78, 84, 90,
 98,105,113,122,132,142,153,165,177,191,
205,221,238,256

[Mike, K8LH]

Hank,

You got a 12F683? If so, and if you want to experiment, connect an LED to GP0 (anode) and GP5 (cathode) and try the program below. The circuit doesn't use a current limiting resistor, relying instead on the RDS(on) resistance of the I/O pin output FETs.

The program fades the LED through sixty four gamma corrected brightness levels spanning 256 1-usec BAM duty cycle steps with a 1280-usec frame rate (781-Hz refresh rate).

:020000040000FA:0600000083018501F128D7
:08000800F000030EF10083017A
:100010000A08F2008A010408F3008C1074088500B5
:1000200083167608850077088500162878088500ED
:100030000030FF3E031819281D287908850002307A
:10004000FF3E0318202824287A08850074087407C6
:1000500074191030F41A0130F4000430FF3E031814
:100060002E28322800007B0885002030F418243028
:1000700074192830741A2C30F41A303084000C3083
:10008000FF3E031840287C0885007D08F5007409B0
:10009000F600F700F800F900FA00FB00FC00FD0094
:1000A0000130741820300018F6068018F706001981
:1000B000F8068019F906001AFA06801AFB06001BDA
:1000C000FC06801BFD06840A0230F418203000185C
:1000D000F6068018F7060019F8068019F906001AC6
:1000E000FA06801AFB06001BFC06801BFD06840A2C
:1000F0000430741920300018F6068018F70600192D
:10010000F8068019F906001AFA06801AFB06001B89
:10011000FC06801BFD06840A1030741A203000187B
:10012000F6068018F7060019F8068019F906001A75
:10013000FA06801AFB06001BFC06801BFD06083031
:10014000FF3E0318A0287508850073088400720814
:100150008A00030E8300F00E700E09008A013F39F9
:10016000820700340134023402340234023403348E
:1001700003340334043404340534053406340634BB
:1001800007340834083409340A340B340C340D3481
:100190000E340F3411341234143415341734193426
:1001A0001B341D341F342234243427342A342D3494
:1001B0003134353439343D34413446344B345134A0
:1001C00057345D3464346B3472347B3483348C3410
:1001D0009634A134AC34B734C434D134E034EF3481
:1001E000FF340730990083169F0170308F000F1D78
:1001F000F72883123F30840080018403841AFC288E
:100200002030F4000130AA000230AB000330AC0013
:100210000430AD000530AE000630AF000730B0004E
:100220000830B1000930B2000A30B3008C019101EE
:100230000530920083167F3092008C148312C030F8
:100240008B0010303321B60A361B2C293608AE201D
:10025000A000212910303321B603B61B212936080E
:10026000AE20A0002A29B500740820393406B4064F
:0C027000340503193429350B332908002C
:02400E00D43F9D
:00000001FF

I'll also write something using the PWM module so that we can try 125-nsec PWM steps (with an 8-MHz clock).

Cheerful regards, Mike