[sdiy] PWM, Vactrols & Opto-Isolators

[Tom Corbitt]

Normally an opto is a led - photo-transistor pairing, a vactol to me
is a light (either incandescent or led) and a cds cell)

Some optos pull out the base pin to allow you to tailor the response
curve (google 4N35), but many don't.

Of course, I'm sure google can find you something different then the
above, but in general I'll stand by the above.

High betas on the photos allow better parity on/off times; (plus you
can add things as described to keep the device out of hard saturation
if desired)  CDS cells have a long off time (compared to the on) as
the electrons trickle out.

 Tom C.

On Wed, Jan 13, 2010 at 1:36 PM, Tom Wiltshire <tom at electricdruid.net> wrote:
>
> 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
>
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