Workshop Wireless Noise Attenuating Headset System

Russ Erb

Originally published December 1997

If you're like me (a scary thought) then the TV or radio is an integral part of the workshop. However, there's a problem--it is very difficult, nee, even impossible, to hear anything on such shows as Wings (Discovery Channel) or Victory At Sea (History Channel) when you're drilling the 562^nd hole in your spar or bandsawing that spar splice plate. This becomes even harder because you believe in protecting your hearing (Whaaat? Say again?) by wearing appropriate ear protectors or ear plugs while doing noisy things in your shop. (If you're not, then I would say either 1) you've already lost your hearing, or 2) you're being incredibly stupid, because once you lose your hearing, you can never get it back. You probably already wear ear protection in airplanes, so why not in the shop?)

So now we have two conflicting requirements. We want to hear the TV or radio, but we need to wear hearing protection. So I figured why not use those incredibly expensive ear protectors I bought several years ago? You know, the ones made by David Clark with the wire and two plugs hanging off of them. After all, since I don't have my own airplane right now, I'm not using my headsets to fly and they're just sitting in the closet (with Doug). The headset seemed like the perfect solution, since it was designed to allow you to hear stuff while attenuating all of the noise around you. After all, they quiet a drill just as well as engine noise…

The Perfect Solution, EXCEPT…

There are two problems with trying to use an aircraft headset with typical home electronics. Typical stereo headphones like you might pick up at Radio Shack^® have an impedance of 8 ohms. Aircraft headsets have an impedance of 300 ohms or 150 ohms, depending on how they are wired. Now, as an airplane guy, I don't claim to understand what impedance is, other than it's some sort of "inductive resistance." The one thing I do know is that 300 ą 8. The result is that even if you buy the right size adapter at Radio Shack^® to physically match the plugs, you still won't hear much of anything. I know--I tried it.

The other problem is that nasty cord. While you could try a long extension cord to reach anywhere in the shop, it really gets in the way and can even become a safety hazard. I found that clecos were made with a material that is especially attractive to audio cords, as the cord was constantly getting caught on clecos as I tried to move around the shop. Any acceptable solution would have to do away with that cord. Of course, it would be nice if it didn't cost as much as the headsets, much less the airplane.

Adapting the Headset

If we first look at trying to adapt the headset to a radio, we can solve the first part of the problem. As far as the radio receiver goes, the good folks across the pond at Sony solved that problem about 15 years ago. I picked up a Sony AM/FM/Cassette Walkman at Best Buy for 16.99 plus tax (on sale). (This doesn't address the problem that radio reception of my favorite stations in my workshop borders on non-existent, but that's another issue)

That's great, but there's still that impedance issue. To solve that, I turned to my brother Robert Erb (currently located at EAA Chapter 1000 Det 5, Arlington, TX). He provided me with the simple adapter circuit shown in Figure 1. Again, I don't claim to understand how it works, but it does. Some of you may notice that a 1/4" stereo jack is specified, even though the headset plug is a mono plug, and you'd be right. My only defense is that it works. Modify at your own risk.

Figure 1. Headset Adapter Schematic

I mounted the Walkman and adapter on a belt I had laying around, and secured them in place with a high quality unidirectional bonding strip (i.e. duct tape). I strap it around my waist with the equipment in the back. The microphone jack of the headset is wire-tied to the earphone jack. With this, I can move freely around the workshop, hearing what I want to hear and not hearing (much of) the noise that I don't want to here.

But What About The TV?

So now I have a hands-off, noise-attenuating headset equipped FM radio (also AM and cassette), but I still can't hear the Modern Marvels show about the B-25 crashing into the Empire State Building while I'm drilling my spar. Again, I turned to my brother, tasking him to design an FM transmitter that would plug into the earphone jack on my workshop TV. In response, he created the circuit shown in Figure 2. The portion inside the dotted outline is most of a kit available at Radio Shack^®. I built it, plugged it into the TV, and it worked great. Not only does it work in the garage, but I can receive the signal over about 70% of the yard when it's time to mow (no change in reception when it's not time to mow).

Figure 2. FM Transmitter Schematic

So if you think this system is way-cool and you've just got to have one, then just follow this procedure. First, get thee to your local Radio Shack^® ho, and procure the following items…

Transmitter Materials List

FM Wireless Mike Module Kit	28-4030	$11.99
AA Plastic Battery holder	270-401	$0.79
Resistor 10 ohms 2 watts	271-080	2/$0.99
0.1 mF 50 wvdc PC mount capacitor	272-1069	2/$0.79
Resistors 100 ohms 1/4 watt	271-1311	5/$0.49
1:1 Audio Isolation Transformer	273-1374	$3.99
Submini SPST Switch (optional)	275-612	$2.89
6 ft patch cord	as required	~$2.49
Plastic box (optional)	as required	~$2.59

Building the Transmitter

Start off by building the FM wireless mike module kit per the directions that came with it, with the following exceptions:

Do not install the microphone element. You won't be using it.
Do not install resistor R2 per the instructions with the kit. You will use it in a slightly different location as per Figure 2.

Build the remainder of the transmitter according to the schematic in Figure 2. When you are done soldering to the circuit board, clean the bottom (solder side) with an old toothbrush and isopropyl alchohol.

Adjust the frequency of the transmitter to an empty frequency by turning the core of the coil. DO NOT use a metal screwdriver as suggested in the kit instructions. Use the end of a flat toothpick to turn it (you may need to trim the toothpick).

Locate the transmitter at least 6 feet from the TV or other input source to avoid possible interference.

That's all there is too it! Give it a try, and you'll never have to miss Home Improvement just because you're making noise in the shop!

EAA Chapter 1000 Home Page

E-Mail: Web Site Director Russ Erb at erbman@pobox.com

URL: http://www.eaa1000.av.org/related/headset/headset.htm
Contents of The Leading Edge and these web pages are the viewpoints of the authors. No claim is made and no liability is assumed, expressed or implied as to the technical accuracy or safety of the material presented. The viewpoints expressed are not necessarily those of Chapter 1000 or the Experimental Aircraft Association.
Revised -- 29 July 1998