Momentary Sw to make LED pulse smoothly ?

[petemoore]

  I'm trying to figure out a circuit which will make an LED light 'slowly' [about 1 second or so between dark and light], stay lit while the momentary switch is held, then ramp back down in brightness over a fairly short period of time.
 The idea is to somehow get an LDR to control what'd be like a 'typical' volume pedal swell for lead breaks.
 The obvious easy choice has been what a BFC does as power filter, the LED rises in brightness quickly [.5 or less seconds I guess], and fades out over a period of about 5 seconds. Slower ramp up [about 1 second] and quicker ramp down [about 2 or 3 seconds] could be made to make the LDR/related circuit do what I want I think.
 Do you think I can use resistors and capacitors to get LED fade ins described, or is there some other approach which might be better suited to the task?
  Perhaps I can find a very slow lighting incandescent bulb ?

[frank_p]

A capacitor ?  R*C time constant stuff...

[petemoore]

  YEa, a BFC, LED, limiting resistor I suppose, perhaps a BFC drain resistor even? 
  I was thinkin' there may be another way to slow the voltage ramp up into the BFC, maybe that can be made to be slow enough for R&R.

[Processaurus]

Have you seen the DOD FX15's schematic? It is a triggered volume swell pedal.

Maybe use a SPDT momentary switch, if available, and run the NO contact to V+, the common to your RC combo, but the NC contact to ground through a trimpot set up as a variable resistor.  The trimpot will decide how fast the cap discharges .  I would use a transistor or opamp buffer between the RC combo's ramp voltage and the LED, so that the ramp is electronically isolated from the LED's load.  You could even use a constant current source instead of just V+ to feed the RC combo, and get a more linear ramp up.

[scratch]

it took me a bit to figure what the heck a BFC was ... had to think back to the days of playing Doom and getting the BFG ...

[kvb]

On a Tremulous project I haven't yet posted about I am using an inverter flip flop (from RG's site) and some fets to control whether the LED is connected to the LFO or to a constant voltage.

I am only using the RC thing to get the change to happen gradually. It works nicely.

I even figured out how to use a diode to get the change to happen quickly in one direction but slowly in the other.


Incidentally, I'm pretty sure that even without using a diode the charge-up time for the cap is not the same as the discharge time.

[petemoore]

  See that's what I wasn't talking about thinking.
  Perhaps 2 switches, 2 states.
  one switch fills a cap at some certain rate, hopefully can be controlled by resistors, as you say, isolate this so the ramp can be controlled and use it as a control voltage for a transistor to power the LED.
  The other switch provides a cap-drain path through X value resistor, slowly letting the voltage in the control cap sag down.
  I thought about some diodes too, but couldn't really figure how to work those in there.
  It already just got more complicated than 1 momentary Sw, LED, BFC and a couple other things.
 

[PRR]