hRobin Rumbolt is a free lance writer and an electronics engineer and an acomplished PPG pilot and instructor. He lives  in Knoxville  - eastern Tennessee.  Robin has been  an amateur radio operator (WA4TEM)
                                                                                                                                                         for over 40 years and a private pilot for about 30 years.
                                                                                                                                Thank you Robin for your multiple contributions to Alex Varv's Inventions Page.

                                                                                                                                                                                                     Alex Varv the

When all the talk of Bluetooth communications started, I had to try it myself.  So I bought a couple of Sena SMH10 units.

 Photo 1   

  I installed one on my NAC-2000 Deluxe Comm helmet by putting the speakers in the earcups along side the existing speakers, and tying the microphone to the existing microphone stem.  Communications were crystal clear, but the installation was crude.

Then I began to search for a way to use the existing speakers and microphone already in my helmet.  What I found was an easy way to connect the SMH10 unit directly to my NAC electronics WITHOUT modifications to the helmet! Since Icaro helmets utilize NAC electronics, this install can be used with Icaro Deluxe Comm helmets as well.

Connections are made via the existing pigtail connector in such a way that the SMH10 may be easily connected or removed.  Installation is not difficult and may be made by anyone with good eyesight who can solder a small connection. The job is in three parts: connecting to the helmet, connecting to the SMH10, and mounting the SMH10.

Only a few tools and supplies are needed / Photo 2

I chose to use stereo connectors to connect the Bluetooth unit to the helmet pigtail connecto) so that the SMH10 unit might be easily disconnected.

Photo 3   


While the SMH10 can play stereo music, the NAC electronics are monaural. Therefore, the left and right audio channels from the SMH10 will be combined and heard as monaural.

The NAC electronics are only powered ON when the PTT button is pressed. Therefore, VOX operation of the SMH10 is not possible with this configuration.

The wire was purchased from Newark Electronics via internet. ( It was difficult to find a good sturdy wire that was small enough to work with the connectors used, but the Carol C1228 worked fine.

The connectors were purchased from Mouser Electronics (

Connecting to the Helmet

Cut a piece of the cable to a length of six inches. Wire one end of the six-inch cable to the inline female stereo connector as shown in Photo 4:

Photo 4   

 The bare ground wire connects to the large tab.  The red wire connects to the tab on the left (with the hole held vertically).  The black wire connects to the tab on the right. Connect the other end of this cable to Pins 6, 7 and 8 of the helmet pigtail connector as shown in Illustration 5:

Illustration  53f

 You may wire this to the helmet pigtail connector OR to the mating connector from the radio cable. I couldn't get a good photo of this because the connector is so small and so crowded.  The real difficulty here is getting the wire through the connector shell.  It's awfully tight in there. It's easier to wire it to the radio side connector. The pin numbers are marked on the connector itself.

Connecting to the SMH10

Remove the SMH10 module from its mounting plate

Photo 6   

 by depressing the tab on the top of the SMH10 module.  You should have two pieces then as shown in Photo 7:

Photo 7   

Peel back the adhesive backed rubber on the lower half of the SMH10 mounting plate.  This exposes two small Philips head screws. (Photo 8)

Photo 8   
 Remove them. Remove also the small rounded cap on the cable exit point. Pry up the plastic cover over the circuit board to allow access to the connections. You should see the connection circuit board as in Photo 9:

Photo 9   

Carefully unsolder the R+, R-, L+, L-, MIC+ and MIC- connections. Then remove all the wires previously connected there. You now have extra speakers and a microphone for another project!

Solder a small piece of wire from L- to R-. Be careful not to let it touch any other connection.

Prepare two 15 ohm 1/4watt resistors by cutting leads of each to 1/4inch.

Solder one end of the first resistor to L+.
Solder one end of the second resistor to R+. Flatten the resistors against the circuit board.  
Solder the unconnected ends of both resistors together.
Solder the black wire of one end of the 13 inch cable to the junction of the two resistors.
Solder the red wire of that same cable to MIC +.
Solder the shield wire from the cable to the L-.
Route the cable around the plastic peg and out the cable exit. See photo 10:

Make sure that the shield wire does not short to any other connection.
Reinstall the plastic cover, the two screws that held it, and the adhesive rubber piece over them.

At the other end of the 13 inch cable install the stereo plug as shown in photo 11:

Photo 11   

Mounting the SMH10

I mounted my SMH10 on the Peltor mount just above the earcup itself. Putting it anywhere else either limited my head movement or got caught in my lines.  This was the best place that I found that was still easily accessible.

To mount it there, first drill a 1/8th inch hole in the center of the SMH10 mounting plate as shown in Photo 12:

Photo 12   

 I used the larger drill bit to ream out the hole to accommodate the head of the mounting screw, but be careful not to drill all the way through.

Remove the small screw in Peltor mount on the right side of the helmet, the pigtail side. Use the #8 self-taping screw to mount the SMH10 plate to the Peltor mount. The screw will pass through this hole and secure the SMH10 mounting plate to the Peltor mount.

Route the 13 inch cable around the existing pigtail cable to the approximate location of the connector and connect it to the inline jack coming from the pigtail connector.

The installation is complete as shown in Photo 13:

Photo 13   


As mentioned earlier, the NAC electronics are monaural only. So you won't hear stereo when playing music from your iDevice.  Those two resistors you installed mix the channels together.  

You CAN listen to a Bluetooth coupled device as well as whatever radio that you have your headset plugged into, but one will not mute the other.

On transmit, your microphone audio will be fed to both the connected radio AND the Bluetooth device. Anyone listening to a paired Bluetooth device will hear your voice.

When you press the PTT button whatever is coming from a paired Bluetooth device WILL be transmitted through the attached radio. So be careful lest your favorite tunes be transmitted to a control tower!  Turn the iDevice off first. Also, any Bluetooth conversations will be retransmitted out the connected radio.  This could be a handy way to let your Bluetooth paired buddies talk to a control tower via YOUR radio. They would be able to hear the tower, but the tower would only be able to hear them ONLY while you're pressing your PTT button.

Robin Rumbolt

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