Heaphone/Stero Splitter

Here's a very simple project for a rainy afternoon. You my already have all the parts, as I did, and even if not, the cost is minimal and you'll end up with a useful gadget to add to your box of audio cables and adapters.

Some while ago I built a left/right stereo splitter to allow me to drive two mini-speakers from a smartphone or Bluetooth receiver. It simply consisted of three 3.5mm jack connectors, one male and two female, wired to split the left and right signals.

More recently I needed a headphone splitter to allow me and a friend to listen to a choral rehearsal aid while both following the score.

I could have rewired my stereo splitter to feed left and right to both female sockets, but I certainly wouldn't want to have to rewire it again next time I needed its original function! But by adding a switch and a small box to put it in, I could make it serve both purposes. I had both a suitable switch and a suitable box, so didn't need to purchase any parts.

You wil need the following parts:

1. Stereo 3.5mm male jack plug
2. Stereo 3.5mm female jack socket - 2-off
3. 2 pole 2 way toggle or slide switch (a toggle switch might be more reliable)
4. A length of miniature stereo audio cable (minimum 30cm)
5. A small project box 40 x 25 x 15mm or larger - or you could 3- print one if you have a 3D printer.

Perhaps you already have a headphone splitter, in which case you could canibalise that.

In addition you wil need the following tools:

1. Wire cutters and wire strippers
2. Small pliers
3. Drill, with 4mm and stepped conical drill bits
4. Small adjustable spanner for the nut on the toggle switch
5. Soldering iron
6. Label printer.

I have found that a conical drill bit is much the best way of making holes of various sizes in plastic. Start with a conventional 4mm drill bit, which is small enough that it's unlikely to snag or cause problems. You can then enlarge it 1mm at a time with the conical bit, the small increments, equally, unlikely to cause problems. Some come with a 4mm hex shaft, which means you can fit them to a universal screwdriver handle and enlarge the hole by hand for extra safety. Search online for "conical drill bit" and you'll find them.

Operation is very simple.

The two female sockets (the outputs) are marked Left and Right. For both functions, the Left connection from the plug (the input) always goes to the Left connection of the Left socket, and the Right always to the Right connection.

1. For stereo splitting (the switch in the upper position as shown in the diagram), the Right connection of the Left socket is also connected to the Left input, and Left connection of the Right socket also goes to the Right input.

So both Left and Right on the Left socket receive the Left signal, and both Left and Right on the Right socket receives the Right signal.

1. For headphone sharing (the switch in the lower position), the Right connection of the Left socket goes to the Right input, and the Left connectionof the Left input goes to the Left input.

Hence both left and right sockets receive both left and right signals.

All this can be achieved with a 2 pole 2 way switch, as shown in the circuit diagram.

Start by drilling holes in the box for the input and output cables and the toggle switch, enlarging them as necessry with the conical drill bit.

Next, solder the jack plug and sockets to lengths (around 10cm) of stereo audio cable. The cable will have 2 inner conductors, often one red (the left channel) and the other white (right), plus the braided common return.

A stereo jack plug has a tip (the left channel), a ring (the right) and the sleeve (the common return). It's obvious which is which, but less so on a stereo socket. Insert the plug into a socket and trace through which is connected to which with a multimeter.

Feed the free ends of the cables through holes in the box, ready for soldering to the switch.

The switch will have 3 pairs of terminals.

In the accompanying illustrations I assume your stereo cable will have red and blue conductors for the left and right channels respectively. Yours might be different, but often the red is used for the left channel.

Start by wiring the top left and bottom right switch terminals together, as idicated by the green wires in the first illustration. Short lengths of solid core hookup wire are ideal for this as you can pass them through the holes in the contacts and bend them round to secure them. Don't solder yet as there are other wires to go through the holes.

Next wire the left and right conductors of the input cable (the one with a plug on it) to the top terminals - second illustration. Now you can apply the solder.

Wire the left and right output cables (the ones with the 3.5mm sockets) as shown in the third illustration and solder.

Finally, twist the braided screens of all three cables together and apply solder. Make sure they are angled away from the switch terminals so as to avoid short circuits, or apply a short lemgth of heat-shrink sleeving.

If you're using a toggle switch you can now insert it into its hole in the box and secure the shaft with the nut. A slide switch is normally secured with two screws.

Secure the box lid and apply a label showing the two functions, and you're done!