Headphone Switch

2002-09-13 : One problem I have at work is that when I'm programming in the zone with my headphones on, no-one can get my attention. They usually resort to tapping me on the shoulder or waving their hand in front of my face, which always makes me jump. So, I wanted to create some sort of device to get my attention without causing my heart to skip a beat.

I suppose I could create a doorbell of some kind that sounded on my headphones or popped up a window on my computer, but I don't know how to interface hardware to the computer off the top of my head. Next I got the idea of creating a mute switch for my headphones. My initial idea was to create basically an in-line mute switch so that people could push a button to mute my headphones and get my attention. I figured this would be easy, because all it would require is a switch in the ground wire of the headphone cable.

Diagram 1.

So, I went to Radio shack to see what I could find. They had nice little plastic project enclosures that would work perfectly, and all the jacks and switches I could want. Well, kind of. I couldn't find a push-button "momentary contact normally closed" switch that was really the appropriate size and shape. I settled on a big red "single pole single throw" rocker switch which was big, had a low profile, and stood out against the black plastic box I wanted to mount it on. I bought all the parts and brought them home, but didn't have time to work on it.

As time passed, I got a better idea. The problem with using the rocker switch is that once the sound has been switched off, it stays off. Assuming I put the switch a ways away from where I was working, I would have to get up and walk over to it to turn my heaphones back one. Plus, there was another problem, in that not only would I have to run a cable out to the box, but I would have to run a second cable back to where I was sitting and hook my headphones up to that. It would basically be loose at where I was sitting and could easily get dragged around.

I realized I could use the two-way switch trick. It's the same way that the two-way light switches at each end of a long hallway work. By using a single pole double throw switch at each end, a circuit can be changed from open to closed by toggling either switch.

Diagram 2.

Now I could have one switch far away and one switch close by. Even if my headphones were muted remotely, I could re-enable them locally. I could use one regular stereo headphone cable (which has three conductors) to hook up the remote switch, which would reduce the number of long cables. I would have one enclosure with a switch next to me, which I would plug the audio source and my headphones into. Then there would be a remote switch attached by a single cable. Much better!

I got a chance to go to Radio Shack for more switches and another enclosure. They still didn't really have the switches I wanted, but I again found some big red double pole double throw switches that were overkill but would do. I even bought a closed-circuit headphone jack, because I suspected there must be a way to wire it such that if the remote was unplugged, it would still work. I brought my materials home and sketched up a wiring diagram. Yup, the special jack could be used the way I wanted to.

Diagram 3.

Then I pulled out my Dremel and used it for the very first time! I cut holes in the plastic enclusures to mount the jacks and switches, then wired and soldered everything. I used my Ohm meter to check for continuity and make sure I wired it right. Everything checked out and behaved as predicted. I even colored the silver screw heads black with a Sharpie so they wouldn't stand out against the black plastic enclosure. (It turned out to look pretty good, actually!) Here is what the completed device looks like, with headphones wired up to a CD player.

Picture 1.

All done, I hooked it up to my laptop. I played an MP3 and everything sounded good. I flipped the mute switch ... and did not get the result I expected! I exercised all the switches and it went from working to not working at the appropriate times. I had to study the wiring diagram and think for a while to figure out what happened. Can you figure it out?

Show the hint (what I heard).

Show the answer (why).

The problem with my device is thus not a mistake in the circuit I wired, but a flawed fundamental assumption. Cutting the ground wire is not sufficient to mute a set of stereo headphones! I guess those movies where they have to cut exactly the right wire to disarm the bomb seem more plausible to me now. ;)

So, in some sense I was successful, since a person throwing the switch would certainly get my attention. I'll definitely take it to work and use it. It was a bit of a let down though.

Since I completed it, I've been thinking how I could fix the problem. I'm going to need more conductors going to the remote. I could use a second stereo cable, but I don't really want to run two parallel cables out to the remote. One idea was to use a USB cable. However, that only has five conductors and I think I need six. I may have to go with a serial cable and a DB9 connector.

2002-12-04 : OK, the six conductor solution is pretty obvious. The problem is finding a six conductor cable. Sure, they're out there, but they're usually stiff and have big connectors (e.g. serial, ethernet, s-video) since they usually have more than six conductors. I was wondering if I could do it with fewer than six conductors.

Well, I just figured out a four conductor solution today! Four conductors is a regular telephone cable! Assuming I'm allowed to connect Left-Source to Right-Source, then I can use a "rotary" switch between Left-Source, Left-Headphone, Right-Source, and Right-Headphone. That's only four conductors. Since I can chain two rotary switches together and still get the behavior of a single rotary switch, I can just put one rotary switch at the end of the (four cunductor) tether. Tada! A real mute switch!

Now I just have to decide if I want to build it or not. Probably not, since I've found out that my device doesn't actually prevent me from being startled. The switch makes a loud enough click that I still jump. Pbbbb.

C o m m e n t s :     updated: 2004-01-30 (5042 days ago)
Thanks, Louis! I'm building a similar kind of switch, allow me to listen to audio through headphones continuously but switch an external speaker on and off, to avoid feedback while I'm transmitting. Your experiences helped me! -Randy