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.