How I added a 3.5mm earphone socket to a Sena SMH10 helmet headset.
I'll answer queries & comments emailed to firstname.lastname@example.org, but don't ask me to do the wiring for you.
Preparation: Remove the headset clamp from the helmet. Remove the headset from the clamp.
Slide a blunt kitchen knife blade into the space between collar and clamp, lever the collar out.
Two screws are under the rubber backing. Lever it up enough to get to them.
A Phillips head screwdriver that fits the screws well is essential.
Gently remove the PCB from the clamp, taking care not to stress the microphone wires.
Mount the PCB so you have two hands free for soldering. A vice is good.
Unsolder the speaker wires.
Solder in three wires, ground, left, and right, commoning the speaker -ve terminals (L-, R-).
I used shielded twin cable, but any wire will do. Use red for R.
Tuck the new cable in place so the clamp screw holes are clear.
Screw the back plate on, keeping the microphone boom placed correctly, mic facing the right way.
Slide the locking collar back on along the boom.
Cut the cable to length. Around 10 to 15cm should do. Bare and tin the ends.
Solder the cable to the input terminals of a switching 3.5mm socket.
Solder the helmet-speaker cable ends to the output terminals of the socket.
Earth is commoned (white from the helmet speakers). Ensure you preserve the stereo pairing.
Whether you use a socket with mounting collar depends on your helmet setup.
I used the collar type on this helmet, no collar on another.
You might like to test the wiring at this point.
Screw the clamp back onto the helmet.
If mounting the socket on a flange as shown, drill a hole for it.
Use double-sided foam tape to stick the socket to the helmet on its non-terminal side, especially if not using a locking collar.
Position the wires so they sit nicely under the helmet padding.
I used a blob of blu-tack to waterproof around where the new cable enters the clamp.
You should now be able to use the helmet speakers normally.
When earphones are plugged in, the speakers are bypassed.
I use Etymotic er6 earphones, for good noise blocking and quality audio (ie music).
I have successfully wired three SMH10 headsets like this.
On mine, I also wired in the microphone from the Sena earphone-clamp, in place of the boom mic.
The mic is buried in the front of my Nolan N84 helmet.