Headset mic connected to other devices?

[RustyTheDog]

I think I had a plan that may not work and I would appreciate any input.

I have a GPS with bluetooth for my cellphone. It works great and it has a plug for an external mic, so I can talk into any mic I might plug in there for a phone converstion. Well, I don't want to add a second mic when there's already one in the headset. At first I figured I could take the two wires from the GPS mic jack and wire them in to the two wires of the microphone of the headset.

Well, it's probably a good thing I thought more about it before I did this. The bike and the GPS are two different devices with different power supplies. If I wire + from the GPS (a 5 volt device) to the + from the bike (12V) it could create a mess. It's probably a bad idea to tie their grounds together too.

I'm confident that the headset mic, if wired to the GPS only, would work fine, so there must be a way to make it work.

One approach might be to add a switch to the bike which re-routes both mic wires from the bike to the GPS device. It would disconnect the mic entirely from the bike when it's on the GPS, and vice-versa. But I'd rather avoid that... I really don't want to have to flip a switch.

Another approach: I could take the wires from any headset speaker, use those in parallel to drive an "earbud" speaker, taped right against a small microphone... Essentially, this would isolate the two circuits via sound waves. Although I'm sure it would work, it's awkward and it would seem unnecessary.

Anyone got a better approach to solve this problem? Is it really necessary to isolate the two circuits or are there simple electronics to get this done?

[RustyTheDog]

I didn't get any replies, but I've done a bit more research on this. My new understanding is that most microphones are not powered devices which affect resistance over time from sound waves (this is an assumption I had made before). My new understanding is that unless a mic contains its own amplifier, it is essentially self-powered by sound waves, producing AC at the millivolt level. Microphone signals are usually amplified in the circuits of devices which receive their signals, but microphones themselves are just tiny AC generators.

So, if my new understanding is correct, the front headset microphone has two wires: "Common" and "Signal". (Not +12V and "signal/ground".) If that's the case, then I can confirm it by testing that neither pin that leads to the microphone in the headset wire will supply +12V versus battery ground.

I can also do the same test with both wires from the microphone input of the GPS. It should show zero voltage between the two wires.

Assuming I confirm all of this that's correct, (that neither device supplies power to its microphone pins) then indeed I can wire the same microphone to both devices in parallel. A drawback could be that each device (bike and GPS) may receive signals at a diminished amplitude, but this may not be a problem. Only experimentation will reveal the answer.

That's a theory, and I'll experiment. If my tests show a power supply from either device to its mic, then I won't proceed.

[GoneForTheBorder]

Rusty,

In that case, would an isolation transformer wired in parallel with the mic leads allow you to "clone" the mic signal and send it to the GPS? I'm not an electronics tech (just a No-Code Amateur Tech), but it seems to me that I've seen that solution applied in similar circumstances.

Glen

[RustyTheDog]

Hi Glen. It's funny that you start your post with a question, perhaps assuming I have a clue. :lol: I dunno nothin'. Just enough to know what I want to do, how to solder, and that so far, nobody is telling me to stop. (I haven't gotten around to it yet.)

I asked my father about this because he's an electrical engineer and a Navy vet (late 60's/Vietnam era). He had an interesting anecdote: He says in those days and maybe even today, in case of power failure ships had backup intercom systems consisting of pretty much nothing but microphones, speakers, and wire. They had no power supply at all... They were powered by sound waves moving the coils in microphones. In fact, the microphones and the speakers were the same device. (Speakers and mics are both are just coils and magnets with a membrane.) Essentially, those simple systems were like two tin cans and string, but with electrons. Kinda cool I think.

[GoneForTheBorder]

Sorry about that, Rusty!

Ah, yes, sound powered phones... spent many hours with one clamped to my head and the mic resting on my chest. I hated those things, but they ALWAYS worked.

The reason I mentioned isolation transformers is that it's the only way I know to link one mic to multiple outputs without a switch. I think that's how Kennedy Technologies does it. Have you looked at their site to see if one of their units would work for this application?

Even if neither device uses a powered mic, if you link the two devices through the mic circuit, I think you risk a potential ground loop.

Glen