Step 5: Preparing the circuit

Since I have no experience in electronics whatsoever, this was the toughest part for me to find out.
Other instructables had no good explaining on this subject.

So for everyone who's new to this electronics stuff and have no idea what they're doing, here is some explaining for the circuit.

1. How many LED's to use?
The average LED needs 3v to work. If you are going to put the LED's in series (like I did) you need to calculate how many LED's you can use with your adapter. The formula you can use is adapter output voltage / LED voltage = Total LED's

So if you use a 12v adapter with the 3v LED's its: 12 / 3 = 4 LED's

You can also put LED's parallel in the circuit. But I'm not getting into this, just so that this instructable stays simple and easy to understand for everyone. Let's just focus now on the LED's in series.
If you want to experience with more LED's, you can always calculate your LED's and resistors here.

2. The real voltage on a adapter.
Before I went building this circuit, I thought it would be useful to measure the real voltage on the adapter. The sticker on the adapter (photo 1) says the output 12v. But once I hooked it up on my multimeter, it shows that the actual output is around the 18v (photo 2).

So that means I can calculate the LED's again: 18 / 3 = 6 LED's.

Since I'm going to make the LED's in series I can use 6 LED's in my circuit. 

3. The 3.5 audio jack plug
Which wire is what? That's what I was asking and trying to find out.
As you can see on photo 3, the plug itself has 3 metal parts, and 3 wires in the black protective layer.
On the photo I explained which wire is what.

Now with this information you can go to the next step, building the circuit.

Remove these adsRemove these ads by Signing Up
KruZeey2 years ago
I cut my apple earphones and it had a red really thin wire, a really thin goldish wire and two blue really thin wires what do i use
yaly3 years ago
the adaptor says 12v 200ma that means if it is loaded with 200ma it gives out 12v and unloaded is about 19v so see how much current per led for 3v and calculate using this formula:(N)=(unloaded)/(load per led 3v).
I think it is right can anyone correct me if I'm wrong.
Kruegem4 years ago
Are Skull Candy ear phones 3.5 jacks?
Moofish Kruegem3 years ago
I think that any ear/headphones that fit into an iPod or an mp3 player are 3.5 jacks.
gabrielguy3 years ago
so 20 leds would be 60 volts!?D:
in one line yes in bridge no :D
Master T3 years ago
So lets say i wanted to make a box for left and right... i would just split the ground wire from the audio jack plug and do everything exactly the same just using the left wire for one box and the right wire for the other box right?
roofy994 years ago
 When I stripped my headphone cable it didn't look anything like the ones in your photo. I used an old pair of apple ipod headphones. They had 3-4 seperate strands but they weren't like wires like yours are. Am I using the wrong type of headphones, or what should i do to fix it?

Thank You
same thing! have u figured it out?
vocaxil4 years ago
my 3.5 audio jack plug juz hv 2 wire..which should i solder....on each wire it have blue and red
Salsa7664 years ago
I have a 12 AC wall adaptor but I only have the jack and not the thing that goes on top of it. Is it ok to just solder wires to the positive and negative terminals like how you hooked it up to the multimeter?
squirrelMLM4 years ago
When I stripped open my sony headphones, there were two copper wires (like your ground wire) and a green wire and a red wire. What do I have to do differently? Also, another question. If I plug this into my iPod Touch, it won't play the music while the LED Light Box is glowing, will it? Great instructable BTW!
Love this project. I did this as a light organ. Some of the "elders" might know what  I am saying when I use that word, from back in the '70's.. lol.. But I used x-mas lights, 3 different strings, and 3 scr's, and a  terminal for the speaker wire, a switch, and a small transformer to step down the voltage. After I got the one done I had to build a second one for the right channel... lol.. Loved it! Now I am going to have to do this again, but with LEDs. Great paper trail, details, keep up the great work!
when you have 18v coming out of the power supply, does this mean you would have to use six or could you use less than six. i tried something and shorted everything out. and, if you cant use less than six, how could you put a ressistor in?
Jerb4 years ago
 13r1an has done an excellent and well thought out explanation of what's going on here. this page needs to be updated to reflect the use of a current limiting resistor or other means of controlling the current to our LED's so readers dont burn out their LED's or burn themselves on hot wiring.

cliff notes:

a current limiting resistor should always be used to dissapate heat and stop the circuit from shorting out. 
1/8 watt resistors will be fine for most string of leds 3-4 long

the power supply in this case is cheap so there will be voltage drift. that is to say that as you add a load to this power supply it will become more efficient and the V will drop. if you put your multimeter leads across it it may read higher than when you attach a circuit to it and measure the V again

13r1an4 years ago
A typical LED has a 2.1V drop across it.  Applying more voltage may burn it out.  Another important note is that an LED is still just a diode, which means current flows freely in one direction.  Therefore, doing a string of them with no resistor is basically a short circuit.  It's good practice to *always* use a resistor in series, to dissipate the current in heat form.  In this case, that means you'll need to break it into 2 or 3 strings of 2 or 3 LEDs per string with appropriate resistor.

By using 6 LEDs in series though, you kinda over corrected yourself so the circuit isn't dangerous, per se, but it isn't exactly something that would please the electrical gurus out there, if you know what I mean.