[sdiy] LED current source needed for 'digital' outputs?

[Neil Johnson]

cheater00 wrote:
> To best control the brightness of an LED, typical advice is to use PWM
> rather than vary the voltage. Not only does it make the response more
> linear but also more predictable (even with good LEDs, I hear that
> noticeable differences arise). How does one achieve good control of
> LED brightness without injecting HF trash into their audio output?

PWM is used because it is low power, the only losses being in the
switch during the transition.  It is noisy as hell, but in most
applications where it is used that isn't a problem (e.g., mobile phone
backlight control).  In those situations absolute intensity isn't that
important, as long as the user can perceive "bright" and "dim" :)

LED output is quite linear with current, any differences due to
component tolerance will affect both linear drive and PWM drive.  If
you want absolute brightness control you need to intensity match the
LEDs and then calibrate the brightness with a light meter.

Neil
--
http://www.njohnson.co.uk