? simple way to 'slow ramp' an LED

[petemoore]

 I'd like to be able to make a momentary contact which starts a slow-feed circuit so an LED would steadily rise in lumens, and transition slowly from off to bright, then smoothly ramp back down.
It seems like it might be as simple as large capacitor and input resistors and a drain resistor [or the LED current would drain after the momentary switch cuts power, but it's going into a phaser so I'm not sure if pops when current path opens might be an issue.
Or what values to start with for the 'fill resistor', whether this would really work out well or not.

[Seljer]

If you've got a current limiting resistor + LED you can just slap a big electrolytic in parallel with the LED and it'll slow the tranistion from off -> on. It however won't function with the same ramp in the for on->off, since its in parallel with the LED it'll just discharge much faster as there is no longer the resistor the in path. The capacitor probably has to be in the 10uF to +100uF range for a noticeable ramp.

You can mitigate the issue on the discharge by adding some diodes to route the current through an extra resistor.


What is probably a better solution is something like this: http://img33.imageshack.us/img33/2275/ledfadeinfadeout1.png where the capacitor isn't actually holding the current that'll flow through the led, but instead it's just a voltage thats then in a way buffered by the transistor.


edit: or this, the ramps aren't exactly symmetrical but its a reasonable solution

Code Select Expand



    |
   | |
   | | 2k2
   | |
    |
    +--------+
    |        |
   | |    -------
   | |    ------- 47uF
   | | 1k    |
    |        |
  -----      |
   \ /       |
  -----      |
    | LED    |
    |        |
    |        |
  -----    -----
   ---      ---
    -        -



[petemoore]

 Yupp, the one with the NPN controlling the LED current, the capacitor controlling the base looks like a good one for this.

[GibsonGM]

But the question has to be asked, Jimi:  What is "this"? 

[petemoore]

 This'd be for the Popsickle Stick Vibe.
I'm planning some speed changes via LDR on the LFO speed pot, this would be for a 'speed swell' effect.

[R.G.]

I'd like to be able to make a momentary contact which starts a slow-feed circuit so an LED would steadily rise in lumens, and transition slowly from off to bright, then smoothly ramp back down.
It seems like it might be as simple as large capacitor and input resistors and a drain resistor [or the LED current would drain after the momentary switch cuts power, but it's going into a phaser so I'm not sure if pops when current path opens might be an issue.

How accurate must this be? And how much control of ramp speed do you need?

Simple resistor/capacitor/switch circuits can do something like this, but they will suffer from differences in the up/down speed and not have even response in general. If you're willing to go wild with circuits, you could use something like half of a dual opamp to integrate up and down, and then use the second half to force the LED current to be proportional to the integrator output voltage.

Then there are opamp/current mirror setups which will do this same trick to many LEDs at the same time, but in independent current paths.

[merlinb]

Does it have to be a momentary switch? You want to push the button once and very briefly, and it will initiate a complete, slow cycle from off, to on, to off? The circuits proposed so far don't do this.

If you were happy with a push-and-hold action then you can at least avoid big current pops and humongous capacitors by using the transistor as an integrator:

(Notice this needs only a 1u cap instead of 470u!)