How to Make Bluetooth Music-enable Hearing Protection

I've been spending a lot of time in the shop recently and it can get very loud; since I want to preserve my eardrums, hearing protection is important. Spending hours just listening to muffled clangs and grinding isn't that much fun, though, so I decided to make these bluetooth-enabled ear muffs at TechShop so I could listen to music while working.

Parts needed:

Earmuff hearing protection. It's important that the cans are deep enough to hold speakers and the battery; I chose the 3M Peltor 105 (~$20); they provide +30dB attenuation, protecting you in up to 105dB environments.
Over-ear bluetooth headphones. The key thing to look for here is good controls, a speaker that will fit in the cans, and quality. I chose the Kinivio BTH220 based on amazon reviews and price (~$25-30) and am really happy with them.

Tools required:

Screw drivers (Kinivio requires tri-wing)
Soldering iron
Wire
Exacto knife
Wire snips
Drill
Heat-shrink tubing (optional)
Hot glue or plastic welder

Charge and pair the headphones with your phone before start; it will make it much easier to test that things are still working along the way.

Step 1: Disassemble Everything

First things first: pull the foam out of the hearing protection. That was easy!

Next, disassemble the headphones. Mine had small phillips screws holding the band together, and tri-wing screws holding the phones together. Why did this company think that it was important to keep people from opening the headphones they bought, I don't know. A security-head screwdriver bit set like this one or this one is a useful thing to have on hand anyway. (If you don't have a set and are itching to open your headphones, you could just use a rotary tool to grind through the three screw posts.)

Make a note (or photo) of which wires go to what.

Finally, use an exacto knife to cut through the glue that's holding the speakers to the grill, and use a small flat-head screwdriver to pry the speakers off. Go slow!

Step 2: Desolder Things

De-solder everything from the logic board.

On my headphones the buttons and logic board were in the right cup and the battery in the left, with the battery and left speaker wires running through the head band. I could have re-used the left speaker wire, but I opted to replace it with a larger-gauge wire for strength. I did, however, reuse the battery wire so I didn't have to worry about heating up the battery too much while desoldering.

Step 3: Drill Holes and Run Your New Wires

Originally I had planned on using replacement switches for all 5 control buttons, but I was concerned that drilling 5 new holes (plus one for the micro-usb charging port) would hurt the sound dampening of the muffs, so I decided to keep the whole headphone control module intact.

That meant I only had to drill two holes: one in the top of the left can, and one in the outside of the right can. A #36 drill bit was just right for the 22 gauge wire I used for the speakers.

One mistake I made when drilling the right can (which you can see in the picture) is that when I held the can down the drill bit was perpendicular to the ear-side of the can but at an angle to the surface. This made sticking wires through a little tricky since they had a tendency to get caught up on the inner layers of the plastic.

Next (I did this first, but it doesn't matter), cut two speaker wires for the left speaker (give yourself plenty of extra!) and run it through the headband. This is tricky; you'll need a stiff leader wire of the right diameter to make it through the small gap next to the actual headband wire; I used 1mm stainless steel safety/lock wire.

The trick here to make this easier is to flatten the band out as much as possible; having a friend hold it down while you feed the wire through helps a lot.

Finally, make a loop in your leader wire, loop your speaker wire through it, tighten it down as much as possible with some pliers, and pull the speaker wire through. (again, keeping the headband as flat as possible).

Now you can

Step 4: Cut Speaker Grills and Prep All the Small Bits.

Remember the speaker grills you pried off the speakers? You need them to keep the foam from pressing into the speaker cones, but they don't fit into cans as they are. Cutting to the rescue!

Use whatever means are convenient to cut it down to about this size (just bigger than the mounting circle for the speakers); I mounted the grill on a block of wood and used a hole saw, but cutting it off with a pair of snips would likely work almost as well. Stick the speakers back in them when you're done.

Next, make sure all the control buttons work. (I did this post-soldering, but it makes more sense to do it now). I found that, although the logic board is held in with two screws, the two buttons closest to the USB port wouldn't work without the pressure of the exterior housing pressing the board down. I applied a little bit of super glue between the USB port and the plastic and held it down firmly (using ream-and-key technology) while it dried to ensure that the board was firm against its housing.

Take your exacto knife and slit the two foam pads down the middle, vertically, slightly more than half way. There are lots of fancy ways to cut foam cleanly, but this isn't going to be seen so short repetitive strokes worked fine.

Finally, If you're using shrink tubing, stick it on the speaker wires on either side now.

Step 5: Solder Everything Back Up

Run the battery cable, along with two short lengths of right-speaker wire through the right can, and solder everything back up. It should look like this:

Step 6: Affix Controls

The one downside to keeping the original control buttons from the bluetooth headphones is that the bottom is essentially a flat surface, while the hearing protection cans are curved, so there's a gap between them around the outside. We'll fix this by affixing the controls first, then filling in the extra space.

Wiggle the extra wires back through their hold and press the controls against the can, then note where the three screw posts hit the plastic. Snip them each down so that the top and bottom of the control panel just makes contact with the can as well as the posts. Then, apply a little superglue to the tips of the posts, press against the surface, and hold.

Now fill in the gaps with a glue gun (if I had know about the TechShop's plastic welder I would have used that, and the extra plastic from the headphones band, instead).

Place the speakers inside the foam, drop the battery into the right muff, and carefully cram the foam into place; you'll have to put it in sideways and twisted into place.

Use a heat gun to shrink the tubing, if you used it, and feel free to paint over the hot glue.