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Auto fade in and out.

I've got a (Vellerman) PWM device which is DC controlled. I'd like to use it such that it fades up to max when a simple DC switch is made, then fades out when it is broken. I can get it to do this using a simple RC circuit - but the 'law' of the fades isn't as linear as I'd like. Any clever way of doing it?

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    Dave Plowman        dave@davenoise.co.uk           London SW
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Dave Plowman (News)
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Constant current source into a cap.

Reply to
D Yuniskis

The constant current source into the cap is easy enough - but the constant current load?

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*I used to have an open mind but my brains kept falling out *

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.
Reply to
Dave Plowman (News)

You might be able to get away with using a simple 555 timer chip. The ramp produced across the timing cap is pretty linear AFAIR

Arfa

Reply to
Arfa Daily

t

=A0 London SW

OK you don't like that. How about an up/down counter and a DAC?

G=B2

Reply to
stratus46

Use an RC circuit, but clamp the range to a fraction of the full sweep with diodes. R goes to [0 , +5] volts at your switch, but the capacitor only goes to [+2, +3] volt range, which is much more linear... or [+2.3, +2.7] volts, if you prefer.

It'll take two op amps for the clamps, and one or two to amplify up the small range to the full voltage swing and polarity you want. LM324 and a couple of diodes, one capacitor, and resistors.

Reply to
whit3rd

Well, that's about as linear as you're going to get. ;^)

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  . | ,. w ,   "Some people are alive only because
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Reply to
Bob Larter

Sadly rather uncharted territory with me - I'm more an OC71 sort of guy. ;-)

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.
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Dave Plowman (News)

=A0 London SW

OK, I looked up some parts for you. Both are available in DIP so you won't have to deal with surface mount.

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It's WAY easier than you think.

G=B2

Reply to
stratus46

Jeez, I haven't seen an OC71 in a *long* time.

--
    W
  . | ,. w ,   "Some people are alive only because
   \\|/  \\|/     it is illegal to kill them."    Perna condita delenda est
---^----^---------------------------------------------------------------
Reply to
Bob Larter

;-) Was meant as a joke. But they fetch good money on Ebay. Dunno why.

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*I don\'t have a solution, but I admire your problem. *

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.
Reply to
Dave Plowman (News)

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