[sdiy] Re: [AH] totally insane!! system 700 sequencer

[GRAHAM ATKINS]

What does sequencer indicator lamps have to do with vactrols ?

Graham
On Jul 11, 2008, at 21:47, ASSI wrote:

> On Freitag 11 Juli 2008, harrybissell at wowway.com wrote:
>> I'd suggest that you use a suitable color LED, with a capacitor
>> across the LED. The current limiting resistor and the cap will
>> form a LPF that will make the turn-on slow, the turn-off slow,
>> and the LEDS to get dimmer as the sequencer runs faster...
>
> This does not even begin to mimic what a Vactrol does.  For
> starters, total brightness of the filament is related to the
> fourth power of the filament temperature (Stefan-Boltzmann law).
> Then the LDR has a spectral response that needs to be folded with
> the with the temperature dependent spectral density of the
> emission, so you need to go into Planck's radiation law.  The
> resistance and hence energy dissipation of the filament is also
> highly dependent on temperature, which adds another nonlinearity
> that also depends on the characteristics of the drive circuit to
> boot.
>
> The emission from an LED is proportional to drive current to first
> order and it is in a very narrow spectral band so that most of it
> will be captured by the LDR, even with the slight shift of the
> spectral emission peak with current.  No amount of linear
> filtering can change that.
>
>
> Achim.
> -- 
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