Mobile with two microphones?

Q. We have DM4601e radios installed to our ambulance and the units (with attached microphones) sit inside the cab. Whist out EMTs etc use DP4801e handsets, often the ambulances travel outside the transmission range of the handsets hence the mobile radios which have a higher power work perfectly in those situations.

However, we don't have a way to connect another handset (whether regular microphone, or something like HMN4098A etc) to the DM4601e's as we're already using the front accessory port for the front cab microhphone.

Is there a way to connect such a handset to the rear accessory port of the mobile radio (obviously we'd need to build an interface cable), though can it be done and if so, do you have the wiring diagram please?

Alternatively, if there's an official motorola part that will do what we need all the better though google has yet to locate one!

A. There is nothing from Motorola Solutions that does what you describe on MOTOTRBO radios. There are solutions for TETRA and (I think) Astro.

Although this is not tested or approved, you could in theory, connect a microphone to the radio's accessory connector on the back. This connector has all the needed inputs to make this work.

Two (actually four) thoughts/concerns come to mind:

1. You will need to use slielded cable and the cable run will need to be protected and kept short. The inputs of the accessory connector are quite sensitive and any unuszal voltage/spike can damage the radio circuitry. In terms of protection I would suggest using some kind of cable hose and running rhe cable out of harms way. You might need to use a ferrite clamp choke on either/both end(s).
2. I'm not sure how the radio will behave when two microphones are connected to it. For example, if it gets a PTT trigger from the accessory connector, what will the front microphone do? My suggestion would be to test this first.
3. Some microphones need power to work. You can get this from pin 7 (with ground on pin 8) of the accessory connector.
4. This is not tested or approved by Motorola Solutions so I need to remind you of caveat emptor.

In terms of a wiring diagram, I don't have anything but you will need to connect the microphone to pin 11 (hot) and 12 (ground). PTT will need to go between pin 17 (GPIO) and 18 (ground).

Pin 11 needs about 80mV and a microphone impedance of around 600Ω.

If the microphone needs power, you might need to use a voltage dropper. If this is needed, it makes sense to include a fuse, just in case. Switched B+ is on pin 7 and DC groupd is on pin 8. I think pin 11 has a series decoupling capacitor in the radio.

One very rough hand drawn wiring diagram. The bottom (optional) circuit might not be needed!

In the radio codeplug, you will just need to make sure GPIO1 (pin 17) is set to PTT; debounce is ticked and the active level is high.

I'd be keen to hear what you find! Let me know in the comments.

Remembers this is this is not tested or officially approved so caveat emptor.

There may be other, more elegant solutions out there so please check the comments - both here and on LinkedIn.