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
watarigoro3 years ago
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)  watarigoro3 years ago
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
Thank you for the reply!
I'll try it out and post pix or video about how it works out!
Mahalo (thank you)!
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?
vitya4 years ago
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
lunerfox4 years ago
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)  lunerfox4 years ago
yea its not too picky, it either works or it doesnt
Kirbsome!4 years ago


5K t-pot

33Ω 0.5W
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?
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.
osgeld (author)  Kirbsome!4 years ago
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!4 years ago
bdneeley5 years ago
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.
He does say on the first step that he's using an SPST, doesnt he?
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.
thewizard425 years ago
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
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)  funbob4 years ago
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
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 thewizard425 years ago
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 1up5 years ago
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
funbob5 years ago
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
Pwntalive5 years ago
I cant seem to find the arduino in your schematic. How can i light the PBR holder on my fixie, without a arduino?
Hahaha, Very nice sir. :]
osgeld (author)  Pwntalive5 years ago
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
pcairic5 years ago
barley and minuets?
osgeld (author)  pcairic5 years ago
i r not goods art engrish
crazyg5 years ago
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.
jochem5 years ago
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)  jochem5 years ago
Ah good eye, I will add that to the end
kelseymh5 years ago
Oh my God! I own that album!