Of soldering guns and audio switches

I want to be able to switch easily between my computer speakers and headset.  Apparently, that’s easier said than done.

Some motherboards shut off audio output in back automatically when you plug something into the front.  Alas, my ASUS motherboard doesn’t do that.  Even if it did, that wouldn’t be “easily” — I don’t want to have to get down on the floor every time I want to use my headset, and what’s more, repeatedly plugging and unplugging the headset will wear out the jack.

One of my coworkers suggested plugging a splitter into the audio output jack, plugging both the speakers and headset into the splitter, and muting the speakers when I want to only use the headset.  There’s just one problem with that — my speakers don’t have a mute or volume control.

A third possibility is to use a USB headset instead of one with a 1/8″ jack.  Then, I could use the volume control applet to switch between output devices.  That would work, but I do this often enough that this would be annoying.

Option four is a switch box.  Like the splitter, I would plug both the speakers and headset into the box and the box into the audio output jack.  The difference is that, as the name implies, the box has a switch to connect either the speakers or the headset, but not both at the same time, to the audio output.

The switch box idea seemed perfect to me, so I set about trying to find one to buy.  Neither my local Radio Shack nor Micro Center were any help.  Plantronics used to sell one, but apparently they discontinued it.  I thought I’d hit pay-dirt when I found this site, but when I tried to order a switch box from them, I was informed that they were on vacation and not accepting orders between December 23 and January 4.  That was the last straw!

For almost two decades, I have been privately ashamed of the fact that I, an MIT EECS graduate, had not once in my life wielded a soldering iron for fun or profit.  I didn’t even own one!  “Self?”  I said to myself.  “Self?  This is your chance to erase that blot from your character.  An audio switch box is not rocket science.  Build it yourself, you EE wimp!”

Buoyed by those encouraging words (I give a great pep talk, don’t I?), I browsed over to radioshack.com.  Although their retail outlets have become little more than cell phone / MP3 player kiosks, on their Web site you can still get all the component parts and gizmos they retailed in the good old days.  There, for the low, low price of $38.04 including shipping, I purchased:

• Two DPDT 6VDC slide switches (one extra in case of assembly errors)
• One 3x2x1 plastic “project enclosure” box
• Six panel-mount 1/8″ audio jacks (one extra)
• One coil of rosin-core solder
• One set of soldering / desoldering tools
• One cool-grip 30-watt soldering gun

The more astute among you will note that the money spent on these items is more than it would have cost me to buy an audio switch box from the link above, and that I’d have it by now despite their vacation.  The even more astute among you will realize that by the time I bought these parts from Radio Shack, neither low cost nor low time investment were the point of the exercise.

Today, I assembled my new audio switch box with the help of my two oldest daughters, whom I first gave a crash course in in the basics of how electricity works and how not to burn oneself on a soldering gun.

Perhaps there’s someone else out there who would like to build one of these but can’t figure out for themselves how to do it, so here’s what we did:

1. On one of the long sides of the box, drill three holes big enough for the ring of the audio jacks to fit through, with the middle hole off-center so that you can cut a rectangular hole for the switch next to it.  Note that there are protrusions and such inside the box which limit where the jacks will fit, so you have to take them into account when figuring out where to drill the holes.  An outside caliper is useful for for measuring heights inside the box, so that you can measure the same heights on the outside to help determine where to drill.  Make sure the jacks are low enough that the metal prong sticking out of the top won’t interfere with attaching the box lid on later.
2. On the other long side of the box, drill two holes for audio jacks.
3. Glue into place all the audio jacks and the switch with a hot glue gun.  This is tricky, because once you put the dabs of glue on the corners of each jack’s enclosure around the ring, you’ve got only a few seconds to get it into the hole and pushed flush before the glue hardens.  If  you miss, you’ve got to pull it out, scrape off all the glue with a hobby knife, and try again.  Of course, being an expert with a hot glue gun, you know all this already.  You will probably want to glue in the switch before the jack next to it, especially if you need to slide in the switch through the jack hole as I did.  Make sure the wiring connector right behind each jack ring, which is for Ground, is pointing up.
4. Wire together all of the Ground connectors on the audio jacks using your soldering gun, solder, and segments of wire (If you don’t have a spool of hookup wire lying around, you’ll need to add one to your Radio Shack order).  When you’re done, bend in the connectors and wires a little so that the box lid will fit.  See “Ground” on the diagram below.
5. Wire the other two connectors of the jack next to the switch straight across to the jack across from it.  This is the pass-through for the microphone plug on your headset.  See “Microphone pass-through” on the diagram.
6. Time to test that everything you’ve done so far is correct. Plug your headset or speakers into one of the two fully wired jacks, plug one end of a 1/8″ male-male cable into the other jack, and plug the other end into the output jack of your computer (you’ll need two such cables, one for audio output from your computer to the box and one for microphone input, so if you don’t have them, add them to your Radio Shack order).  If your audio doesn’t work, start looking for bad connections.
7. Wire the jack across from the switch to its middle pair of prongs.  The prongs on the switch are close together, so be careful here and when doing the rest of the wiring not to make any accidental connections.  See “Computer audio out” on the diagram.
8. Wire one of the two remaining jacks to one of the two remaining pairs of prongs.  See “Speakers” on the diagram.
9. Test again.  First, test the jacks you already tested before, to make sure you didn’t inadvertently break them when wiring more.  Now, plug into the two new jacks and make sure the switch is toward the side of the jack you wired.  If your audio doesn’t work, start looking for bad connections.
10. Wire the one remaining jack to the the last pair of prongs, and test it.  See “Headset” on the diagram.
11. Plug in both your headset and speakers and test that you can switch between them.
12. Screw on the lid, and you’re done!

The steps above are exactly how I did things, but there are a couple of things I’d do differently if I were starting over.  You get to learn from my mistakes:

1. Use a bigger switch.  The one I used (Radio Shack #275-007) is just too small, and it was difficult to mount in the box.  I think I’d probably try #275-403 instead, if I were building another box.
2. Don’t put the switch between the jacks.  Once you’ve plugged in cables, that side of the box is quite crowded, and it’s hard to get your fingers onto the switch.  Put the switch on one of the short sides of the box instead.  That makes for slightly more curvy wiring but a much more usable finished product.

It should be obvious that in addition to allowing you to switch between two audio output devices, this exact same box will also allow you to switch between between two audio inputs for one output, i.e., listen to either of two computers with one headset or set of speakers.

Some photos:

Box side with three jacks and switch (click to enlarge)

Box side with two jacks (click to enlarge)

Top view showing wiring (click to enlarge)

The finished product (click to enlarge)