This is my first instructable!!
I am going to show you how to make universal headphones. How universal? well...these headphones:
have volume control
have a mono-stereo selection switch (for some devices that only produce sound in one ear when connected to stereo headphones)
can hook to almost any audio device
can even act as a connector between an output and an input (like a computer to a TV)

tools you will need:
solder iron and solder
heat shrink or electrical tape
glue gun
multimeter (with ohm reading)
wire strippers or scissors

parts you will need:
1) 2.5 mm stereo male Tip Ring Sleeve (TRS) connector
2) 3.5 mm stereo male TRS connector (also sometimes called 1/8'' headphone jack)
3) 1/4″ (6.35 mm) male TRS connector (also sometimes called a phone jack from old manual phone switchboard days)
4) small potentiometer (about 500 ohms)
5) small Single Pole Single Throw (SPST) switch
6) your favorite pair of headphones/ear-buds (or your least favorite, depending on how well you think the project will turn out)
7) some kind of casing
8) mono RCA connector
9) some headphone wire (any small gauge (24+ AWG) insulated wire will work)

Items 1-5 and 7 can all be found at www.digikey.com (for about $40). This is the quickest way to get them but also the most expensive. Part numbers are below as search for 'phone jack' brought up items 1-3.
1) CP-2501SP-ND
2) CP-3502-ND
3) SC1081-ND
4) 3352W-501LF-ND
5) EG4415-ND
7) 377-1691-ND

Alternately items 1-3 and 7 can be found at radio shack for about $20.
NOTE-a quick search got me items 2 and 3 but item 1 only came with a headset attached to it, which is fine because you could use the microphone casing as a enclosure for your project. Model information below.
2) 1/8" Stereo Phone Plug Model: 274-284 | Catalog #: 274-284
3) 1/4" Stereo Audio Plug Model: 274-139 | Catalog #: 274-139
4) 25-Ohm 3-Watt Rheostat Model: 271-265 | Catalog #: 271-265
5) 2-Pack 3-Amp SPDT Sub-Mini Slide Switch Model: 275-409 | Catalog #: 275-409
1) and 7) PointMobl Mono Bud Headset Model: 170-5429 | Catalog #: 17-5429 (use microphone as casing)
8) Phono Plug Model: 274-319 | Catalog #: 274-319

or if you like free:
item 1 is a bit hard to find but old cordless house phones use 2.5 mm connectors, also some cell phone headphones or headsets (it doesn't matter if it is a headset or headphones as long as it has 3 conducting bands on the connector)
item 2 can be gotten from almost any normal headphone pair very easily
item 3 can be from an old pair of old huge headphones or are still used a lot in the audio industry. If you know a church, recording studio or really any place that has an amplifier and speakers chances are they have some bad cables that they will give to you (the end connector is usually perfectly fine)
items 4 and 5 can be gathered from an old mini radio (see picture)
item 6 (headphones) are thrown out by people ALL THE TIME and usually the speakers only need to be soldered back on to make them work.
item 7 can just be replaced by covering the whole thing in hot glue!

**These are not the only ways to get these things, if you collect old electronics like me then chances are you have a lot of this stuff lying around.

Step 1: Getting the Pieces Ready

If you consider yourself good at electronics, you may just want to skip this step and move on the the circuit diagram in step 2.

From now on I will be referring to the tip, ring and sleeve of the connectors if your not familiar with them, look at the picture (refer to HTTP://Pinochet/Home/Tele35s_platitudinous if you want even more information).

Note:These steps depend highly on where you got the your pieces. If you got them from digikey or radio shack, not much work is needed, it is pretty easy to determine which contact corresponds to the tip the ring and the sleeve. If you got them for free it might not be as obvious. In either case if you are not sure, use a ohm meter to figure out which connector leads to which contact on the plug.

Get headphones ready
Cut your headphone wire at a length that you want the volume control knob at.
Remove insulation from internal wires, sometimes this requires putting solder on them. (see picture)
In headphone wires, each side needs 2 connections going to it, the ground and the signal. The ground is common to both sides but the signal goes to each side. It doesn't matter which signal is which (but typically left is green/blue wire and right is red wire)  as long as you don't get it confused with the ground

Figure out the connectors
Put your ohm meter on the tip and find the lead that reads close to 0 ohms (the others should read infinity or at least millions of ohms).
Repeat for the ring and sleeve. Then repeat the whole procedure for the other jacks be sure to remember which lead is which. If you already know which is which you can skip this step.

Figure out the potentiometer
these typically have 3 leads coming off of them. You will only need 2. to find which one, attach your ohm meter to each possible combination and move the dial back and forth. When you find 2 contacts where the resistance changes (from ~0 to ~500), you have found the 2 contacts you want. (see picture)

Figure out the switch
If your switch only has 2 contacts, your all set. It doesn't matter which one you hook up where (see circuit diagram step 2)
Some switches you might find have 3 contacts. If this is the case, again you will only need 2 of them. To find the ones you need, connect up every possible combination as was done with the one and switch the switch back and forth. When it goes from infinite resistance to almost zero, you found your contacts to use.

PS I got some of this information from pinouts.ru . It is the BEST place I have found on the internet to find pin-outs for almost any connector!

Step 2: Wire It Up

Wire it up as shown in the diagram. 
To wire up the jacks, I recommend taking a length of headphone wire and connecting it to each of them and then soldering all of those together on the other end into three big connections for the left right and ground. Then take electrical tape or heat shrink and cover it up.
The potentiometer and the switch belong in your casing (or hot glue). See picture.
As I said before, don't worry too much about which headphone wire is which, you can always add a left and right label on the outside later.

Step 3: Glue It Together

You can also use this for an audio connector from your laptop to the TV, just turn the volume all the way down on the headphones.
When using this with the RCA connector, close the switch so you can get audio in both ears.
Your done!

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