One of the items low on my list of things to figure out has been finding a way to interface the factory installed visor microphone to a standard aftermarket stereo. I hadn't seen much in searches of this forum, so I figured there wouldn't be that much information or interest in this tiny puzzle until @michaelh and @RayR were nice enough to give some pointers in a thread about the telephone.

First of all, I have a '97 that came without a phone or upgraded stereo. I assume they just installed the mics into all of the cars in case a phone would be installed later, The microphone was mounted in a small black box right next to the visor against the windshield. I believe later models were in a tan bump better integrated into the roof, so I've no idea if the guts were the same.

Opening the black case showed a circuit board full of components and a Motorola logo. Part of the reason I'm interested in this project is to see what the fancy circuit board can do about things like wind and road noise. It's clearly more than a standard microphone.

Simply wiring the mic leads into an aux input does nothing. With active components on the board that's not surprising; it needs to be powered somehow, but how?

I bought a new microphone off eBay to play with in my shop, ordered based on part number so they should be identical. However today I noticed the mic on my bench shows a resistance of 18 ohm across its leads while the one in the car shows 2.58 kohm. Both were measured on the solder pads where the lead attaches to the circuit board.

I'll attach front and back pictures of the two microphones. If anyone has pointers about any of this, I'd love to hear them. I'm hoping it's just a matter of a resister and capacitor connecting the stereo input and a voltage supply to the mic, but especially with those resistance measurements I'm not quite sure which way to do it.

 

I took some measurements last night, as follows. Note these are for the 'tan bump' microphone, although I expect the results will be indicative of the units you have.

The mic. unit draws ~6mA. The PSE outputs* 10V on the yellow signal wire with the mic. disconnected, dropping to just over 4V when connected. Ohm's Law thus gives a value for R of 1K ohm. Use 1.2 or 1.5K if dropping from 12V. For C, 1 - 4.7μF 16V would be OK.



I suspect that the chip is just a preamp - can you read the part # on it? You would likely be better using a more modern noise-cancelling device as suggested for a convertible car.
I'd recommend that you cut off the long tails of the electrolytic on one of your modules.

Curious why you want to connect it to a stereo system - what does it do at the moment?

HTH

*only when dialling/connected. It's around 1.1V with the normal 'ignition on' state.

 

I must confess I can't see the point of trying to get a 22 year old microphone to work with something it wasn't designed for, when you can just cut it off and replace it with the brand new microphone that will come with your new bluetooth kit.

 

Thanks so much @michaelh. That gives me plenty to work with.

I did read the part number on the chip a few weeks ago, and it was just one of the generic numbers for an opamp.

Right now I have a Kenwood KDC-BT762HD HD radio installed, which I'm plenty happy with. When my phone connects to the radio over Bluetooth I can play music from the phone through the car speakers, but I can't do things that involve talking back without a microphone. Many applications won't use the phone's internal microphone when Bluetooth is connected at all.

I've installed a few Bluetooth enabled radios over the years, and when they came with a microphone at all it was a cheap looking, unpowered thing. This one, though, has chips! And a trademarked brand name, Motorola IMPRES(TM)! It must be good!

I'm half joking, but you know, working on this puzzle is also a chance to learn something new about electronics. Maybe I could buy a brand new fancy microphone that would be a dozen times better than the generation old microphone, but first I want to see where this takes me.

 

Ah the penny drops.

I see the Kenwood kit includes a passive (i.e. no pre-amp) microphone, so it may not cope with the higher audio level delivered by the Moto mic. You do have the ability to set 'mic level' so try it and see. There are even echo cancel and noise reduction settings

I got this off Amazon to play MP3 USB sticks via FM, but it also does BT (built in microphone) and has a spare USB slot for a charging lead. Less than $10 and works flawlessly.



Phone calls are taken care of by this piece of 90s bleeding-edge OE techno:

 

Yes, but look how tiny that microphone is! It can't be nearly as spectacular as the Motorola IMPRES(TM) technology microphone hanging from the visor!

I'll report back when I have some time to tinker with the microphones.