Picture of LED, with push button start and fade out
This will describe a simple circuit for allowing a 9 v. battery to power an LED, and then fade out once the pushbutton is released. Something very similar was requested in a question on the forums somewhere. I hope this is useful as a prototype, or in inspiring ideas on how to use this set up.

Step 1: Parts

Picture of Parts
Fading LED.bmp
Here is an inventory of the parts I used:

Q1: most any SS (small signal) NPN transistor will do here.
C1: 100-330 uF electrolytic polarized capacitor
D1: I used a Rainbow flashing LED (so this circuit will flash), if a regular LED is used, it will not flash.
R1: a 10k resistor (carbon, 1/4 w is fine).
For R1, I used a 1 K resistor in series with a 10K potentiometer, for adjustment purposes.
PB: a normally open (N/O) push button switch

Misc.: wire, pc board (or prototype board), 9 v battery snap (and battery), desired case design
vwluvrs1 year ago
i would like to do something similar, thqt will light up the key hole on my viatage car, hooked to the interior lights when the door opens,, what do you think?
thank you! U gave me the ideal circuit 4 the parking lights of my bike! :)
Goodhart (author)  chawla_mohali5 years ago
I hope it turns out well for you.
id like to use ur simple design and integrate with my sound reactive leds however i think they are too reactive and finish as soon as the bass sound finishs instead i would like a type of pulsing effect that lasts 1-2 seconds after the sound reaction. So this easy circuit would suit well however i am guessing that i may blow up the transistor with 12 volts and close to 1.5 amps,,,,,, any advice on capable transistors? or is it not the transistor but maybe the capcitor i have no idea
Goodhart (author)  Applecore8075 years ago
Well, any "power" transistor (i.e. 2n3055 or the TIP3055) should be able to handle the voltage and current ( see specs here ) if you don't need a lot of gain, it seems that it would work (but I haven't tried it).
codongolev6 years ago
I need to get a desoldering braid....
Goodhart (author)  codongolev6 years ago
They are very handy (getting some flux helps too (a tiny bit on the braid), if you have a fairly low powered soldering pen).
I sure do. but my dad has some flux, so I'm good on that part.
baadn3wz6 years ago
Hello! Nice instructable indeed, but i was wondering if you could connect an array of 8X3 led's instead of one led.If it is possible the values for the pot,cap and resistor are still the same? Thank you!(I'm also a beginner)
the transistor powers the leds...so long as the transistor was big enough. A small signal transistor is usually good for 50-80ma, so thats 4 or 5 ultra bright leds. Definitely not a huge amount.
The array is made to run from a 9v supply
Goodhart (author)  baadn3wz6 years ago
I have been doing this for quite a few years, but this one was a starter project and wanted others to see how an LED can be made to fade, simply. I am not sure of the rating needed for your particular array, so I can't really answer, sorry.
Well I tried it and it worked but I used a 100k pot accidentally instead of the 10k pot mentioned in the project. It has a fade out time from about ~10sec up to ~20sec.It is very nice.Thank you again!
Goodhart (author)  baadn3wz6 years ago
It might be good to put in a small resistor in series with the pot. so you don't ever give it full power on the zero setting.
Goodhart (author)  baadn3wz6 years ago
DebH576 years ago
The electrical world is somewhat beyond me but I find this facinating Goodhart. Thanks for sharing. 5
Goodhart (author)  DebH576 years ago
Thank you.
Like math or sewing or any skill, it just takes a little time to learn, and as we all know, a little less time if there is an interest :-)

Not to bore you with a lot of details, but it is funny even that I even ever got interested in electronics, since I was always deathly afraid of being shocked (I hate static shocks). :-)
DebH57 Goodhart6 years ago
LOL I hate those too. I know about learning to sew so now I can relate, what a great way to face your fears though.
NachoMahma6 years ago
. Nice job. . The wood grain is really distracting to my eyes. Try using a plain background. . > Since the schematic is REALLY straightforward . I have a pretty good idea what's going on, but a beginner might not.
Goodhart (author)  NachoMahma6 years ago
Ok, I have made some minor adjustments to the wording....Is this more along the lines of what you mean to help out the beginner ?
. That's exactly what I was thinking it needed.
Goodhart (author)  NachoMahma6 years ago
sorry about the table being a distraction, I will try to photograph it in stages with a sheet of paper under it or something next time
Goodhart (author)  NachoMahma6 years ago
Ok, I should add some instruction to that....sorry about the background, my wife has junk everywhere else LOL so it is hard to find a good background.
have you created one that fades up when button is pushed and fade out when button is released???
Goodhart (author)  dociledragons6 years ago
Well, I haven't tried it, yet. Let's see, maybe a voltage adjustable resistor may work. One would have to find one that has it's greatest resistance at the lowest value of input....and as voltage was applied, it would ramp up by lowering it's resistance....but I am not sure that device exists. Many I have dealt with increase resistance per input voltage. Of course, generating a triangle wave at the point of pressing the button, and ignoring the down side, might work, if you can get the signal generator to supply enough voltage. Hmmm, let me think about this for a bit....
lemonie6 years ago
I'm thinking of having a go at this, thanks. L
Goodhart (author)  lemonie6 years ago
I did not do any "figuring" when it comes to resistor values, so you may need to do that if using another LED.
I've got plenty, thanks for the advice. L
fwjs286 years ago
very nice....i've seen many questions on answers on how to do this, and it seems simple enough to me
Goodhart (author)  fwjs286 years ago
The FADE is easy. If I wanted to add blink without the special LED, that would take a bit more doing (more transistors or a dedicated IC like a 555 timer or a LM3909 *try and find THAT one* :-)