Olin, what have you been talking about so long?
Was it in few words:
At full load: the PWM duty cycle =3D PWM Period, say 255
At each step when dimming divide PWM duty cycle by some value,=20
say 2. (255,127,63,...) Loop until LED brightness will get less then=20
brightness of an environment (you may use A/D with photosensor=20
to catch the level) .=20
By the way, the divider value may be modified at each step to get=20
desired curve.

You style is really cool:
...
>               1                1
>               2                1.06
>               3                1.12
>               4                1.18
>               5                1.25
...
> Of course you will have to round the output values above.
  =20
Of course we will, and what do you suggest we'll get?

Mike.


SomebodyWhoIsAlmostNeverMentioningWhomHeRepliesTo wrote:

> The preceived brightness is essentially the logarithm of the actual
> brightness, and the actual brightness of LEDs is reasonably linear to
> current.  First you have to decide what dynamic output range you can
> produce, then map that to the non-zero intended brightness values
> logarithmically.  Since the log will never let you get to zero, you
> arbitrarily assign 0 input value to LED completely off.  Note that you
> will lose resolution in the conversion, but how many different
brightness
> levels do you really need?
>=20
> Let's do an example.  You are using 8 bit PWM output, so the linear
> brightness can vary from 0 to 255, with 0 completely off and 255
> completely on.  The dynamic range of the non-zero output values is
255/1.
> Now map this range logarithmically to the non-zero range of input
number.
> If the desired perceived brightness values are 0-100, then 1-100 maps
to
> output values 1-255 lograithmically.  In other words, each single step
up
> in the 1-100 range increases the output value by a fixed *multiple*
(fixed
> increment would be linear).  There are 99 steps from 1-100 to cover
the
> dynamic range ratio of 255.  The step multiple is therefore the 99th
root
> of 255, or
>=20
>   StepRatio =3D DynamicRange ** (1 / NSteps)
>             =3D 255 ** (1 / 99)
>             =3D 1.057568
>=20
> The output value resulting from each non-zero input value is then:
>=20
>   output =3D StepRatio ** (input - 1)
>          =3D 1.057568 ** (input - 1)
>=20
> Since logarithms don't go to zero, you arbitrarily assign input 0 to
> output completely off.  So here are a few sample input and output
values:
>=20
>    INPUT (0-100)   OUTPUT (0-255)
>               0                0
>               1                1
>               2                1.06
>               3                1.12
>               4                1.18
>               5                1.25
>              10                1.65
>              20                2.90
>              30                5.07
>              40                8.87
>              50               15.53
>              60               27.18
>              70               47.56
>              80               83.25
>              90              145.70
>             100              255.00
>=20
> Of course you will have to round the output values above.
>=20

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