No CPU / MCU Led Pulse-fade





Introduction: 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

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

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!

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 ~



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    We have a be nice policy.
    Please be positive and constructive.




    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.


    2 replies

    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.

    2 replies

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

    Can the 2N2222s be subsituted with 2N3904s instead?

    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


    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.

    1 reply

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


    5K t-pot

    33Ω 0.5W
    5 replies

    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.

    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

    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.

    2 replies

    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.

    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

    1 reply

    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.