LED Gradual ON/OFF

[brampton]

I would like a circuit that slowly brightens a led, (over 3-5 seconds approx) when logic is applied, and slowly dims the led (again, approximately 3-5 seconds) when logic is removed.
I would prefer a analogue solution, as Im not quite up to microprocessors yet.
Thanks in advance.

 

[carbonzit]

Making it gradually fade out when power is removed is easy:

**broken link removed**

Getting it to slowly brighten on power-up is a bit harder. [scratches head] I'm sure someone else here who knows more about electronics can come up with something.

 

[Sceadwian]

Determine the bulk resistance of an LED and it's current limiting resistor and then use an R/C calculator to determine a capacitor that will give you a time constant of about 4 seconds. The capacitor is going to be fairly large. When power is applied the capacitor will take time to charge, when it's removed it will take time to discharge, though you may need a diode at the very begging to prevent the capacitor from discharging through the power supply circuit this is the absolute simplest analog solution to your problem. Any questions feel free to ask. The capacitor charge/discharge curve will give a very satisfying 'natural' feel to the brightening and dimming effects.

 

[Sceadwian]

carbonzit, that circuit will work both ways if the power supply circuit doesn't ground when it's off, it does then nothing more than an additional diode feeding the cap is all that's needed, that's exactly what I was describing.

 

[carbonzit]

I wasn't sure about that; it seemed to me (I could be wrong) that on power-on, the LED would still light even while the cap is charging, depending on how much current the supply is capable of delivering. Is this the case?

So I guess the principle here is that the (uncharged) cap looks almost like a short circuit to the power supply?

 

[Sceadwian]

I did warn the cap would be large =\

 

[carbonzit]

I did warn the cap would be large =\

Wow.

But I guess you have to deal with the time constants what you gots, right?

Any other way to do this that doesn't require humongous capacitors?

 

[ronv]

slow on off

Little smaller capacitor, but more parts.

 

[brampton]

Thanks

Thanks man, I am going to try both the capacitive, and the op amp circuits

 

[colin55]

**broken link removed**
These two circuits make a LED fade on and off. The first circuit charges a 100u and the transistor amplifies the current entering the 100u and delivers 100 times this value to the LED via the collector-emitter pins. The circuit needs 9v for operation since pin 2 of the 555 detects 2/3Vcc before changing the state of the output so we only have a maximum of 5.5v via a 220R resistor to illuminate the LED. The second circuit requires a very high value electrolytic to produce the same effect.