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?
Constant current source into a cap.
The constant current source into the cap is easy enough - but the constant current load?
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
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=A0 London SW
OK you don't like that. How about an up/down counter and a DAC?
G=B2
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.
Well, that's about as linear as you're going to get. ;^)
Sadly rather uncharted territory with me - I'm more an OC71 sort of guy. ;-)
=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.
It's WAY easier than you think.
G=B2
Jeez, I haven't seen an OC71 in a *long* time.
;-) Was meant as a joke. But they fetch good money on Ebay. Dunno why.
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