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
Step 1: Parts
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
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
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
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
Step 5: Questions!
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+
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 ~