[sdiy] Alternatives to Faders, level display

[Tom Wiltshire]

On 10 Mar 2007, at 12:04, Ingo Debus wrote:

> Am 07.03.2007 um 16:08 schrieb Edward King:
>
>> The only issue Ive found is that the level display is inadequate for 
>> fine control.
>
> I'm wondering if a "moving dot" display with two LEDs with different 
> brightnesses on at a time would be useful for such a thing. Here's a 
> table showing the brigtnesses for three LEDs and nine level steps 
> using four different brightnesses (view with fixed font):
>
>
> level  0    1    2    3    4    5    6    7    8
> ------------------------------------------------
> led3   0    0    0    0    0    0.25 0.5 0.75  1
> led2   0    0.25 0.5  0.75 1    0.75 0.5 0.25  0
> led1   1    0.75 0.5  0.25 0    0    0    0    0
>
> Get the idea? If a LED could only be on or off, three LEDs could only 
> display three level steps.
> Here the brightness sum is always 1, if a sum nonequal to 1 were 
> allowed even more steps would be possible. Controlling the brightness 
> of LEDs is probably not difficult if they are multiplexed anyway.
>
> Ingo

I like it, Ingo. You're right to point out that just regarding LEDs as 
on or off isn't using them to best effect. Using 4 separate LED 
brightnesses is giving you four times as many levels represented - 
you've squeezed another two bits of accuracy out of it!

The best way I've found to control the brightness of an LED is using a 
PWM output. This provides a nice linear range of brightness (which 
tweaking the current doesn't always) and can be done by either analogue 
(triangle or ramp wave to comparator with brightness CV to other input) 
or digital circuitry.

I'm currently playing with some of these ideas using a PIC and a rotary 
encoder. What I'd like is a standalone 'virtual programmable pot' that 
would look something like the ones on the Nord Lead, but for far less 
cash! The idea is to have a small PCB with a rotary encoder in the 
centre, a ring of LEDs around the outside and a cheap PIC acting as a 
sophisticated display driver and providing a programming interface - 
probably SPI or I2C.

That's the guts of it, but the important stuff that makes the 
difference between 'acceptable but not great' and 'fantastic' is the 
sort of details that have been discussed here in the last few days - 
what kind of encoder to use, encoder resolution and rates, how to get 
better resolution from the LED level display, etc etc. It's amazing how 
long you can spend tweaking one tiny part of a system to get it just 
right - but I guess you all know that already...;)

Tom



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