[sdiy] PWM, Vactrols & Opto-Isolators

[cheater cheater]

Dustin,
a simple RC filter is going to be good enough and much cheaper than an
opto based design. The frequency of the PWM is constant and you can
tune the filter to it. Bear in mind that the concept of recovering the
duty cycle value from a general PWM signal is very different from
making a DAC out of a PWM driver.

An opto coupler won't give you perfect quality as it has lots of
non-linearities. Also remember that a diode's 'brightness' can be
controlled by PWM not because of the nature of the diode, but because
of the nature of our eyes, which are not quick enough to see the
blinking. A nightfly would see the blinking just too well. And your
circuit would 'se' the blinking too.

An opto coupler geared to digital coupling can be used for a crude
form of PWM: full-wave rectify your signal, and use that as input into
the opto. Use the output to turn the original signal on and off. You
change the duty cycle by dc-offset on the opto's input. Mind you, you
could use a diode just as well, but it would sound just a slight
different.


> Most of the things I'm up to are pretty boring compared to the rest of
> what some of you are doing :)

I've been sleeping all day today. There's no way you had a more boring
way than I have :P

D.

On Thu, Jan 14, 2010 at 02:42, Dustin Withers <fadeddata at gmail.com> wrote:
> Jason,
>
> Thanks for that. I'll be sure to check it out.
>
> -dustin
>
> On Wed, Jan 13, 2010 at 7:14 PM, Jason Proctor <jason at redfish.net> wrote:
>> ymmv, but i gave up on the pwm output option and use an SPI DAC instead
>> (TLV5618). straightforward to use and works like a champ.
>>
>> code and circuits etc in the files section of arduino_synth @yahoo.
>>
>>
>>
>>> The application is a mix of my own interest and that I've noticed many
>>> of these inexpensive micro controller boards have many PWM outs. I'd
>>> like to be able to do digital control of analog circuits. It seems my
>>> best bet maybe some kind of DAC though, right?
>>>
>>> dustin
>>>
>>> On Wed, Jan 13, 2010 at 5:37 PM, Tom Wiltshire <tom at electricdruid.net>
>>> wrote:
>>>>
>>>>  Dustin,
>>>>
>>>>  If you're looking to get a DC level out, then I'd say, yes, 30KHz is
>>>> high.
>>>>  You could set the RC filter frequency for 10Hz and the pulses would be
>>>> more
>>>>  than 3 decades above the cutoff, which ought to give a decent filtering
>>>>  action even with such a simple layout.
>>>>
>>>>  Jay's comment's about linearity are very pertinent though. PWM->LED
>>>>  linearity isn't going to be perfect, and LED->LDR linearity will add
>>>> it's
>>>>  own twist too. How accurate do you need it? What's the application?
>>>>
>>>>  Regards,
>>>>  Tom
>>>>
>>>>
>>>>  On 13 Jan 2010, at 19:03, Dustin Withers wrote:
>>>>
>>>>>  Tom,
>>>>>
>>>>>  Is a pulse frequency of 30Khz considered high?
>>>>>
>>>>>  Thanks for the help,
>>>>>  -dustin
>>>>
>>>>
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