Linear or logarithmic or exponential

metron9

Using PWM to fade an LED from off to full on duty cycle.
Led on for 99 counts off for 1 count at x frequency = full on
Led off for 99 counts on for 1 count at x frequency = full off

The question is in between, scaling up from on time of 1 off time 99 to on 99 off 1
What kind of math would I use to make the led fade evenly from off to on and on to off?

From on 50 off 60 is 10 steps but represents a 17% difference
from on 1 off 99 to on 10 off 89 represents a 1000% gain

I think I need to make a table of some sort to change evenly 99 levels of light intensity
but I may need more than a 100 count.

Anyone have a clue what I am talking about?

bobledoux

I think you are trying to replicate the curve found in an "audio taper" potentiometer. This is found in the volume controls on radios. Turning the knob creates a proportional increase in volume. Try to Google that topic.

Tracy Allen

To get a logarithm, there is the NCD function.

log2x=NCD x - 1 ' log2x goes from 0 to 15 in powers of 2 as x goes from 1 to 65535

I'm not sure that is the function you want. A square law might be more appropriate.

Subjective luminosity of a source depends greatly on the background light level. It is one thing to view the LED in a dark room and another outdoors. For that reason, your setup might have an ambient light sensor.

As one counter goes from full off at 0 to full on at 255, say, the PWM values could take on a value that changes by constant proportions, not linearly, something like this...

    rateFactor CON 64      ' larger value for larger step.  64 is x-> x^0.75 or x^1.25
    fadeFactor CON 256 - rateFactor
    growFactor CON 256 + rateFactor
    DO
        DO     ' increase level in steps
           PWM 0,x,100
           x = x */ growFactor
        LOOP UNTIL x>255
       DO       ' decrease level in steps
          PWM 0, x  ,100
          x = x */ fadeFactor
        LOOP UNTIL x=0
     LOOP

The low end would only be perceptable in a dark room, though, hence necessity for ambient light compensation, or simply truncation at the low end.

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Tracy Allen
www.emesystems.com