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?

--
*Strip mining prevents forest fires.

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:
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.

In article ,
D Yuniskis wrote:
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?

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

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" wrote in message
...
In article ,
D Yuniskis wrote:
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?

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

Dave Plowman London SW
To e-mail, change noise into sound.


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

On Sep 11, 10:13*am, "Dave Plowman (News)"
wrote:
In article ,
* *D Yuniskis wrote:

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?

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

* * Dave Plowman * * * * * * * * London SW
* * * * * * * * * To e-mail, change noise into sound.

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

G²

On Sep 11, 2:48*am, "Dave Plowman (News)"
wrote:
[my device] 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?

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.

In article ,
Bob Larter wrote:
wrote:
On Sep 11, 10:13 am, "Dave Plowman (News)"
wrote:
In article ,
D Yuniskis wrote:

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?


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

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. ;-)

--
*When you've seen one shopping centre you've seen a mall*

Dave Plowman London SW
To e-mail, change noise into sound.

On Sep 13, 9:56*am, "Dave Plowman (News)"
wrote:
In article ,
* *Bob Larter wrote:





wrote:
On Sep 11, 10:13 am, "Dave Plowman (News)"
wrote:
In article ,
* *D Yuniskis wrote:

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?

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

--
*When you've seen one shopping centre you've seen a mall*

* * Dave Plowman * * * * * * * * London SW
* * * * * * * * * To e-mail, change noise into sound.

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

http://pdfserv.maxim-ic.com/en/ds/MAX530.pdf

http://eshop.engineering.uiowa.edu/N...3/DS008333.pdf

It's WAY easier than you think.

G²

In article ,
Bob Larter wrote:
Sadly rather uncharted territory with me - I'm more an OC71 sort of
guy. ;-)

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.

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

Dave Plowman London SW
To e-mail, change noise into sound.