Fading a light source in and out automatically.

[Dave Plowman (News)]

I've got a low voltage PWM unit which is controlled by a DC voltage. Can be used for motor or light control etc.

In this case I want to use it to drive high power LEDs. Which it does just fine.

What I want to do is have it ramp up to max when a switch is made, and ramp down to minimum when it's opened. I've experimented with RC circuits which do sort of work - but don't give a linear fade in and fade out. Any clever ideas? Preferably using just one cheap IC that I'll have in my box...;-)

It would also be nice to arrange things so it takes no current when the switch is off. Although I can get round this.

[NT]

There are fairly simple methods using opamps. Eg set one to deliver a constant current, feed this to a cap on the output. Or set one to deliver a fixed b drop across an output R, which gives constant i, etc.

Or more basically, stay with a CR and just use a percentage of the v scale, making it somewhat more linear.

NT

[fred]

In article , Dave Plowman (News) writes

An integrator is what you're looking for, here's an op-amp based circuit:

(Fixed width font reqd) ___ ___ | | | | | |R1 | | R3 || C1 | | | | ------||--------- | | | | | || | |_| |_| | | | | | |\ | | _______ | | | \ | ----_______|--|-----|-----|- \ | | | | \_____|________ Vcontrol | R2 | | / | |-----------|+ / | | | / | | |/ PB1 | | | R4 | o--| | | --| | | | o--| |_| | | | | _|_ _|_

The ramp time delay is R2*C1 seconds.

If you chose a single supply fet input op amp then R3,R4 can be 1M, R1 roughly R2/10 to avoid skewing the delay.

Off state current drain should be in the uA range.

[fred]

Actually, for your non critical app, this may well do the job, values left as an exercise for the reader ;-)

___ ___ | | | | | |R1 | | | | | | | |_| | | | / TR1 (pnp) | _______ |< ----_______|--|--------| | | |\ | R2 | | \ | | || C1 | | ----||------| | || |___________ Vcontrol PB1 | | | o--| | | --| | | R3 | o--| | | | |_| | | _|_ _|_

[Dave Plowman (News)]

Thanks Fred- I'll give that a try.

[NT]

=A0 \_____|________ Vcontrol

Even simpler is a fet and resistor configured to deliver constant current, plus a capacitor.

NT

[Jon Fairbairn]

The solutions posted give you a linear voltage ramp, but I'd have thought that what you want is something that /looks/ linear given the eye's logarithmic response. But then, an RC circuit would give an exponential decay, so I'm confused.

[Dave Plowman (News)]

I tried something similar using an LM318 in 'soft start' configuration which gives a nice ramp in - but would need some sort of constant current load to give the same ramp out.

[Dave Plowman (News)]

Reasonably linear to the eye is what I want. An RC circuit on discharge gives a steep fade at first followed by a long slow one. As it were.

[NT]

for which an opamp delivering constant current is ideal

NT

[NT]

and the LM324 possibly a good candidate. There's a P version if you need to save power.

NT

[Dave Plowman (News)]

Not quite sure what this is, mathematically speaking. Is the * a symbol which has come out wrong on my browser?

Basically, I'm after a fairly slow fade - perhaps 3 seconds from min to max.

[NT]

time constant is R times C

NT

[fred]

In article , NT writes

Yep, times it is, most of the packages I use denote multiplication with an asterisk so I techno spake.

For 3s you could kick off with R1 = 10k, R3 = R4 = 100k, R2 = 330k, C1 =

10uF non polar or 2 x 22uF polar back to back to create a non polar cap.

[Dave Plowman (News)]

Ah. Right. I've never seen it written like that.

I'll give that a go - the largest non polarized I had was 1µF and space only allowed two, so perhaps not an ideal ratio. Never thought of using electrolytics back to back. Although I do have some non polar ones.