On Saturday, October 01, 2011 07:20:28 AM andy pugh did opine:
> On 1 October 2011 05:57, Kirk Wallace <[email protected]>
wrote:
> > Thanks for all of the replies. I think I solved it by tying the input
> > to the control common with a resistor and capacitor as shown in the
> > attached file.
>
> A "leaky bucket" filter, I think.
> The usual input circuit is a capacitor as shown with the resistor
> between the output of the PWM and the top of the cap.
> However that doesn't work with an opto, as 0V doesn't discharge the
> cap, it just doesn't charge it.
> I have a circuit very much like yours (with the addition of a
> constant-current regulator) but it turns out to be horribly non-linear
> except at just the right frequency. My solution (and yours too, I
> suspect) is to use just the right frequency.
>
> I have an idea for a way to use a second opto to discharge the cap
> through the same resistor during the low portion of the PWM, which
> should be linear at all frequencies, but have never bothered trying
> it.
That should work just fine Andy. And wouldn't need a 2nd dedicated i/o
pin. Just drive one opto from Vbb, and the other from Vgg. If one is on,
the other is off & vice versa. The buffer would need to be able to drive
the opto in pull up mode as well as pull down of course. The C1G card's
buffers can do that.
Cheers, Gene
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
<liw> damn, the autonomous mouse movement starts usually after I use a
mouse button
<wichert> don't use a mouse button then :)
<liw> yeah, right :)
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