[sdiy] PWM, Vactrols & Opto-Isolators

[Tom Wiltshire]

On 13 Jan 2010, at 16:32, Dustin Withers wrote:
> I'm curious about what the differences are between Opto-Isolators and
> Vactrols. I think they are the same thing, right?

Pretty much, but you might find some opto-isolators are designed  
specifically for digital signals, whereas vactrols are specifically  
designed for analog signals.

> Also I'm interested in converting PWM to a voltage. I know this can be
> done with a low pass rc circuit. I remembered that you could also
> control the brightness of an LED with PWM. Couldn't a
> Vactrol/Optoisolator be used to convert PWM to a voltage? What are the
> pros/cons vs the RC circuit?

Probably pretty much the same. With the simple RC circuit, you've got  
a time constant to consider, which obviously has to take your PWM  
frequency into account. The situation is similar with a vactrol, only  
the time constant is the reaction time of the LDR.
Either way, the smoothing isn't that good (it's basically a single- 
pole filter) so for applications where a bit of leakage is a problem,  
you might be better looking at active filters with more stages.  
Still, if the required output is low frequency or DC, and the PWM  
frequency is high, a simple filter will do.

HTH,
Tom