LED's are those nifty little lights that never die and are what everything seems to be made out of these days, especially head lamps and good flashlights. They run on DC, which is great if your using a battery, but horrible if you want to plug them into a wall (AC). I'm going to start at the top and give an overview of everything. If you know it, know better, or disagree, skip to the DESIGN section.

AC vs. DC

Really quite simple. Remember, electricity is the MOVEMENT of electrons, not turning them into something else. So as an electron passes through your lightbulb, light is created. No electron movement, no light or sound, or motors turning, etc, etc. There are 2 common ways of moving electrons (not *quite* true, but we can ignore all others for today), DC and AC.

Lets start with DC because it's easy. DC stands for Direct Current and is a conga line for electrons, or prehaps a roller coaster. You start somewhere, you have no choice to follow the guy in front of you, who in turn is following the guy in front of him. The leader of this line is following a set track. It may take you up and down, left or right, even upside-down but your always going forward, always stuck on the track, and eventually you arrive back at the start. You never go in reverse, you never veer off the path.

AC is also quite simple. DC stands for Alternating Current, instead of a roller coaster, picture the ocean motion. Your in a set track, but you swing back and forth, zooming forward, peaking, zooming backwards, peaking, repeat ad infinum. Now remember, electricity is created by movement of electrons, everytime you zoom by the bottom center your going quite fast, ergo you have quite a bit of energy.

The ocean motion is not actually a perfect example, because there is no real room for lights, speakers, etc. How about those inertia balls that swing back and forth passing their momentum across the middle ones. The middle ones can be your lights and stuff. *click* *click* *click* *click* *click* *click*

Now, as you can imagine, everyone must be doing the same thing or disater strikes...A roller coaster that suddenly reversed would not be cool. This is why AC and DC don't get along very well, but a sneaky person can do some fancy tricks to make things work.

Switches, Resisters, and Things

Alright, let's ignore AC, and concentrate on DC, since LED's are DC devices. Imagine a roller coaster with no ups or downs or anything fun, just goes around in a circle forever, Blah! Since you have nothing to stop you either is blah times infinity! This is what you get if you hook a wire to either end of a battery with nothing in the middle (and a lot of heat). Let's fix that.

The first and most obvious thing is your wire. It's the roller coaster tracks.

The second most obvious thing is a switch, on/off, start/stop, etc. It's a door. Open it and you go, close it you stop.

How about a resistor? Hmm, good question, picture them like brakes slowing you before a corner.

LED's are more like a one-way valve/sensor. Go forward through it and a light turns on, try and go backwards and it stops you. But they are very weak doors, so really you can just shove backwards and smash through...kinda permanently destroys the LED though, let's not ever do that.

Capacitors are the most difficult thing we will be dealing with, basically it's a resevoir for electrons, like a waiting room. When too many electrons go flying by the excess electrons fill up the capacitor, when there are not enough going by the cap empties out to keep the flow constant and smooth. We will actually be using them in a different way because it will be in the AC section of the circuit. Basically we will be using it as 2 waiting rooms, as all the elctrons sway forward (remember AC) they will fill up the waiting room on 1 side. Then when everything sways in reverse, they will empty back out. The same is happening on the other side.

Diagrams and Such

For fairly obvious reasons, we draw diagrams of our circuits instead of just taking pictures. It's way eaier to read, and we can specify all sorts of things a picture can't. Also, resistors and everything else come in 700 million different shapes and sizes, so a single diagram is 700 million times easier to recognise :)

Component	Symbol	Picture	Notes
Wire			A solid line, fairly obvious eh?
Connection			A dot. If there is no dot, they don't connect.
Switch			Push it, it completes the circuit. There are all sorts of things to add onto a switch, but I am ignoring them today.
Resistor			zig-zags. Fairly obvious that they slow things down eh?
LED			Look! a one-way arrow! and some arrows shooting out representing light.
Capacitor			You can picture the 2 waiting rooms now, can't you?

Design

Ok, so now that we are all nicely ed-u-ma-cated, lets build us this circuit.

First of all, why?? Well why not? But for a good answer, I built it for my bathroom. At night I flick the switch and 14 red LED's turn on so I can see, but don't lose my night vision! Beat that!

We know that LED's are one-way devices, so they burn out instantly in AC circuits. So how do we turn AC into DC without a bulky converter? Well if you read the above you would have all the clues.

• We need something that stops and stores the swishing of electrons...like maybe a capacitor?
• We need to turn the back and forth motion on one side of the cap into something more like a one-way loop.
• We need to make sure the current is limited on our loop...Maybe a resistor?

Put them together and you get?

Pretty sneaky eh? Now before we deal with numbers, let's see a picture of how it works.

Now that's cool. As you can see, the electrons swish in and out, not burning out the LED's cause there is an easier path. They actually do flicker, but at 60Hz (60x per second), so you can't even begin to see it. If you wave your hands in front you might get some nifty strobe effects...Just a sec, lemme go try...

Sweet! you do!

Numbers

Let's get down to business and crunch some numbers. Or watch Scott crunch numbers. The original circuit (http://www.bowdenshobbycircuits.info/page10.htm#lineled.gif) quotes the following:

• Resistor = 1 K ohm, 1/2W
• Capacitor = 0.47 uF, 200V

I personally would double the wattage of the resistor. Now from this we can derive a standard equation based on the number of LED's. First of all, our math based on this design.

• C = 0.47uF
• f = 60Hz
• Z = 1 / (2*pi*C*f) = 1 / (2*pi*0.47*60) = 5644ohms
• n = # of LED's along 1 leg = 1
• v = voltage of LED's = 1.7
• V = 120 - nv = 120 - (1*1.7) = 118.3V
• I = V / Z = 21mA full wave

Now that we have Z an I, let's combine and rearrange for C:

• I = (120-nv) / Z
• I = (120-nv) / 1 / (2*pi*C*f)
• I = (120-nv) * (2*pi*C*f)
• 1 = ((120-nv)*2*pi*C*f) / I
• 1/C = ((120-nv)*2*pi*f) / I
• C = I / (( 120-nv )*2*pi*f )

C = I / ((120 - n*v) * 2 * pi * f)

Great. So lets simplify this into a table. If 'I' get's much above 25mA you'll cook the LED's, and under 20mA they will be really dim, so keep within those values.

Final Thoughts

• Always use a 1 K ohm resistor, with at least a 1 watt rating.
• Never use more than 20 LED's per leg, or 40 in total.
• ALWAYS have equal numbers of LED's on either leg or the imbalance with fry everything.
• Make sure the capacitor is rated over 200V, ideally 250V so you have a 100% buffer.
• If you start dealing with more than 7 or 8 LED's per leg, insert a quick acting fuse before the resistor.
• Since the LED's are flickering (super fast) they only put out about 1/2 the light.
• You can have parallel loops, just remember not to try and draw more than 120V.
• When taking pictures in the dark, use a stand...