Introduction: No CPU / MCU Led Pulse-fade

Picture of No CPU / MCU Led Pulse-fade

Hello everyone this instructable will show you a fairly simple circuit to achieve the "Macintosh" style LED fade off effect

It also can be used to fade on and off using either a switch or a square wave, weather that be from a 555 timer, or your hard disk light, or pretty much anything else where you need a pulsing in and out (or throbbing)  LED effect

its not perfect, but heck were looking at a total of nine components for the effect that can be had for little money and construction effort, and its a fun exploration of analog electronics

shall we?

Step 1: Parts

Picture of Parts

You will need the following parts minimum for both variations of this project

1 SPST push button switch
2 10k Ohm resistors
1 1k Ohm resistors
1 1000 uf electrolytic capacitor
2 2222 NPN transistors (pretty much any npn transistor will do)
1 Resistor appropriate for your LED

some notes

1) the switch could be substituted for a square wave if you dont want to manually hit the button

2) all values in my example are calculated for +5 volts DC

3) the LED I am using is a 10mm 20ma blue model from radio shack (I spent 3 bucks for it to get some good images from it and darn it I am going to use it!) so my led resistor is calculated for a led with a forward voltage of 3.2 volts, and a current of 20ma, please use the appropriate values for whatever LED you choose to use in your application

Step 2: Schematic and Brief Explination

Picture of Schematic and Brief Explination

Both modes ( fade on fade off, and instant on and fade off) use the same schematic, the slight difference between the two will be told later on

As drawn, it is setup as fade in and fade out... for the instant on, fade out version you can remove 1 part and you might be okay, but that one part prevents a very short (time wise) short circuit when first triggered,  and is generally a bad idea to omit the extra resistor, especially if your using current sensitive components to trigger the effect

So plan on 9 components no matter what, if you are feeling lucky you could go for 8, but its not advised (even though the short circuit is only very very brief)

How this works is as follows, when you switch +V the cap C1 is slowly filled though R1, if C1 is totally devoid of energy it is a short to ground, as time passes ( depending on the value of R1 vs C1) the capacitor becomes less and less of a short to ground, which is connected to the base of T1, as the current increases T1 passes more and more lighting up the LED

when you let go of the button voltage slowly comes from the storage of the capacitor,  back though the led fading it out, BUT once you hit a certain point the current will only barley pass though the LED (leaks), this presents a problem as the cap can sustain a couple volts for a few minuets, which causes the LED to glow for a long time (in the scale of things)

SO! the second transistor comes in, and is setup in reverse so that when the voltage on the base (provided by the capacitor) stays high  enough it switches the cap to ground, thus providing a path for the cap's current to discharge( otherwise the current is too weak to make a difference while the switch is held ) , the end effect is that the "fill up" time of the cap is roughly the same as its discharge rate, Instead of filling up in a second and never really fully discharging causing the led to glow for a long time

Step 3: Mode 1, Fade in Fade Out

There is one drawback to this mode, if the capacitor is completely empty it will take a little bit extra time for it to fill up enough to not be a short circuit

once its going, it should stabilize out mostly, this is fine if you are sending it a square wave, but for push button applications it may seem a bit delayed at first,  since the second transistor never fully discharges the capacitor unless left for a decent amount of time

see the attached video for the results

Step 4: Mode 2, Instant on and Fade Out

If you want the LED to instantly come on once the button is pushed, then fade out when released, its simple. Just look at the schematic and place R1 in R2's place, and R2 in R1's place

Step 5: Questions!

Picture of Questions!

Since I am adding this step before anyone sees it, this may be updated later on, but for now

1) what if I want to use a higher voltage (ie 12V)?
use an appropriate resistor for your led, and make sure your capacitor is rated for at least the voltage applied + ~10% or more, I am actually using a 25v cap even though I specified 10V+

jochem says:
when running this on more than 5V, you'll need to increase the value of R3, or your cap will be grounded all the time. I ended up using a 47KOhm resistor, instead of the 10KOhm one listed here (other parts the same).

2) what if I want to slow down the fade in time?
use a higher value resistor for R1 or use a  larger farad value capacitor

3) what if I want to slow down the fade out time?
use a higher value resistor for R3 or a larger farad value capacitor

4) what if I want to speed up either?
fade in time is pretty close to instant with R1 at 1k  so your not going to have a lot of play room, fade out time does have a little bit more wiggle room but results will vary,  of course you could also use a smaller cap

Okay, that is all I can think of at this moment, have fun and play around, see what you can do with it and figure out why its happening...

Till next time!

~ Osgeld ~


watarigoro (author)2011-09-23

This is very cool! I vaguely remember any electronics from when I was very small (and that was VERY long time ago).

I want to build something like this to control three LEDs in mask that use the instant on/fade off version of this circuit. How would I have to modify this to control three LEDs running off, say, four AA batteries (6v)? I'd probably using LEDs like the one you used in your video clip.


osgeld (author)watarigoro2011-09-23

I have not tried it but you should be able to put them in series without goofing up the effect

let us know how it works out

watarigoro (author)osgeld2011-09-24

Thank you for the reply!
I'll try it out and post pix or video about how it works out!
Mahalo (thank you)!

brooklynlord (author)2011-03-23

O M G.

This is what i have been looking for SOOO long!
I have few questions:
Can i run this off of a 3v CR1220 or CR1225? They have about 30-50 milliamps of power, so i'm not so sure.
If so, do i need to change the resistor value of any resistors?

I will be using a 555 timer instead of a switch, and the circuit will be instant on and fade off.

Oh yes, the voltage supply can also be 6v as well.

Can the 2N2222s be subsituted with 2N3904s instead?

vitya (author)2010-10-06

Just an idea: if you remove T2 altogether and put a diode between R3 and GND it works OK. See the attached image (I built it and it works with the given values.
my 0.02USD

lunerfox (author)2010-09-25

I was able to successfully implement this Circuit by changing R1 from the proposed 10k ohm to 1k ohm. Just an FYI for anyone attempting this.

osgeld (author)lunerfox2010-09-26

yea its not too picky, it either works or it doesnt

Kirbsome! (author)2010-09-06


5K t-pot

33Ω 0.5W
RavingNinja (author)Kirbsome!2010-09-23

excuse me, I have a looooot of 2222 NPNs, and a LOOOOOT of resistors and an LED, but not a single 1000uf CAP. The closest I have is a couple 220 uf caps. Would ceramic caps work? or no, just elecs?

Kirbsome! (author)RavingNinja2010-09-23

Ceramic caps would work in theory, you'd just need a LOT of them in parallel.
As for the 220's, put a few in parallel.

RavingNinja (author)Kirbsome!2010-09-24


osgeld (author)Kirbsome!2010-09-23

ayep, ceramics are usually pretty small in capacitance so it would take a ton, you could use a single 220uf cap but you would either have a very short effect, or re think the resistance, a few in parallel would do too

osgeld (author)Kirbsome!2010-09-07


bdneeley (author)2010-08-28

Unless I am badly mistaken, transistor T2 has the collector and emitter backward. The way this is drawn, the emitter will ALWAYS be positive with respect to ground (and the collector). In addition, the base-emitter junction will only be forward biased when the switch is depressed. The picture of the breadboard appears to have the T2 emitter wired to ground (the way it should be).

 I hate to be critical, but the schematic shows a SPST (single pole single throw) switch, not a momentary closed pushbutton.

Otherwise, this is an excellent little project.

RavingNinja (author)bdneeley2010-09-23

He does say on the first step that he's using an SPST, doesnt he?

alberfilba (author)bdneeley2010-08-29

Yeah, T2 is reverse biased. There's no way it can be switched to active state if the voltage at Emitter is greater than Colector's. Anyway, that's a clever design.

thewizard42 (author)2010-08-28

Pop a 555 in astable mode in front of it (with an appropriate circuit) and you should have it go automatically if im not mistaken

funbob (author)thewizard422010-09-07

but isnt the idea not to use IC's? I know 555 isnt much of an IC but still i like the idea of just just using the basics.

osgeld (author)funbob2010-09-07

you could rig up another set of transistors and capacitors to make an oscillator of your choice (evil laugh) but its kind of a pita

besides I never really said not using IC's, I mean not using a computer, 555 is a analog circuit based around some basic transistor oscillators VS using a chip that has more computing "power" than I had the first 15 years of my life

thewizard42 (author)funbob2010-09-07

Yea i know. Analog circuits are my absolute favorite. Digital is missing the life that analog electronics have. There is a way to do it with a couple capacitors, two transistors, and four resistors instead of the 555 :D

1up (author)thewizard422010-08-29

Yes, that is correct. You would probably want a transistor in place of the switch. You would just need to get the timing right on the 555 and it would look really nice.

thewizard42 (author)1up2010-08-29

I'm probably gonna do that haha. I go back to school tomorrow, and its a technical school and im in electronics :D good way to amuse my friends

funbob (author)2010-08-29

could you add an emend diagram if this is in error, was fiddling with something like this a while ago this has got me thinking again thank you

Pwntalive (author)2010-08-28

I cant seem to find the arduino in your schematic. How can i light the PBR holder on my fixie, without a arduino?

French t0ast (author)Pwntalive2010-08-28

Hahaha, Very nice sir. :]

osgeld (author)Pwntalive2010-08-28

There is an arduino in the pictures, but all its doing is supplying 5v power, you can use power from any source you want as long as your cap can handle it and you use the right resistor for your led next time ill use a potato battery lol

pcairic (author)2010-08-28

barley and minuets?

osgeld (author)pcairic2010-08-28

i r not goods art engrish

crazyg (author)2010-08-28

thanks for doing this,my leccy theroy depends a fair bit on what i remember from school 15-16 yrs ago(couple of theroy lessons and two weeks of sanding a perspex box) ,i mostly guess stuff (dragging a live wire accross a board untill something happens) which keeping it low tek (with kits n such) gets me through,hopefully ill be able to make some use of a load of caps iv pulled from things,recently tried to make a cd player focus thingy oscilate with caps (no transistor fail!)other diagrams on t'net were a bit to algebra based to be enjoyable,so i did the stick bits together routine and see what happens, Nice diagrams they will be handy references when i forget what im doing,nice led by the way.

jochem (author)2010-08-28

when running this on more than 5V, you'll need to increase the value of R3, or your cap will be grounded all the time. I ended up using a 47KOhm resistor, instead of the 10KOhm one listed here (other parts the same).

osgeld (author)jochem2010-08-28

Ah good eye, I will add that to the end

kelseymh (author)2010-08-27

Oh my God! I own that album!

About This Instructable



More by osgeld:Project Advantage: Install a famiclone into a NES Advantage JoystickSpinner / Jog Wheel Inside of a VCR HeadNo CPU / MCU led pulse-fade
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