Figure I have some free time so I'll add some details.
Note that I still haven't had a chance to use it on the road, so I don't know how on-the-road volume/noise/interference will be, but I'm expecting it to be similar or better compared to the same Ericsson kit installed in my E46, where the speaker and mic are both further from the driver's head.
I used an older Ericsson HCB-30 car BT kit. I have one, so I already know how the wiring works, size of the 'brain,' and that it has 4 buttons like the NSX panel. I found one on eBay for <$10. It's a very basic kit, but that's what I wanted for this. If you want multiple phone pairing, screen, etc. you're going to give up the stock look no matter what.
First step was to look at if this was even feasible. I disassembled both button panels, and quickly discovered that it won't be a simple pinout change. Luckily, the Ericsson panel is VERY simple: a momentary switch for the 4 buttons (seemingly without even debounce), backlight power, status LED signal (more on this later), and a common ground, with each of these getting its own wire coming out to a 8P8C (ethernet) connector. I got some docs for the Honda/Motorola system from an engineer friend at Honda, which told me (if it wasn't obvious from disassembly) that it works totally different. If I were some electronics expert, it may be possible to build some sort of converter circuit to make it work, but I'm not. So I figured the easiest way would be to build a copy of the Ericsson button panel, reconfigured so the 4 NSX buttons (originally rubber dome) would push 4 tact switches, and backlight LEDs to go into the NSX's backlight locations, as well as add the status LED.
Next was interfacing with the factory speaker. It would've been easy to just disconnect it from the stereo and dedicate it to the phone, and probably wouldn't have missed it if I did, but I decided I wanted to do what the factory kit does, which is to use a relay (I opened the original control box to discover this) to switch it between the stereo and phone. I didn't want to build another circuit board with just a tiny relay on it, so I used a big one with QD terminals directly on it. It works fine, but it makes a loud click every time it engages. :redface: It's a DPDT relay, connected to both sides of the 4-pin speaker/phone splice harness, the speaker output of the BT kit brain, with the coil powered by the same switched 12v powering the brain, and switched using a 2nd, higher-voltage radio mute ground that the Ericsson kit provides. I don't think you could use the regular radio mute, since if you're going to use that to mute the radio, you probably don't want 12v running back through it, though I suppose it wouldn't be too hard to find a way to make a single radio mute do both (diode? 3PDT relay?). I built a single harness that connects to all 3 factory connectors; the two 4-pin connectors for speaker in/out and radio mute, and the 13pin DIN that goes to the trunk for power and ground. It also connects to the BT kit's speaker and power plugs, and integrates the aforementioned relay. Sorry, it's the one piece I don't have a pic of.
The easiest piece was the jumper connecting the trunk power connector (the one that originally powered the phone) to the DIN cable that runs back to the bulkhead. I ran all 3 (batt, switched, ground) through this, but you can probably get all of these directly off the bulkhead someplace. I just don't like making permanent changes or cutting factory wires wherever possible.
If you don't care about modding the button box, you can stop here. Plug in and mount the BT kit mic and button panel somewhere, and go. This also means the last 2 sections would be applicable to pretty much any other phone kit. This last section is the ultimate point of this project.
Now to the button box. This has the mic, 4 buttons, backlight, and status light. The mic is completely separate (the kit mic, disassembled, and attached to the mic grille by friction from the original rubber mount ring) so that's easy. I used a pair of tact switches under each button paralleled together, since the dome buttons are large and have 2 carbon contacts, so I wanted to be able to push any part of the button to activate the switch(es). Factory backlight is 4 LEDs (what color are they? I chose 610nm orange to match the dash). The kit supplies 5v for the backlight, so there are 2 sets of a pair of orange LEDs in series with a 51ohm resistor. The status LED is interesting. The kit's uses a dual-package LED with a red and green half, cross-wired, which I replaced with a 2-pin bicolor LED (most bicolor LEDs are 3 pin). The status light signal works together with the backlight power to drive the LED in one direction or the other, depending on whether it wants to show red or green.
The original PCB not only held all the components, but it holds the rubber buttons up, so I couldn't remove it. So I built the new board to sit under it, and made holes in the stock board to pass the LEDs and switches to the underside of the rubber buttons. Lastly, I made a small hole for the status LED.
Parts list:
-NSX phone button box
-Sony Ericsson HCB-30 bluetooth carkit
-4pin Honda relay plug + 13pin male DIN plug (optional; trunk power jumper. If you still have the trunk harness, you can build it out of that)
-Control box to speakers harness, DPDT relay, 13pin female DIN socket, which you can take from the original harness, or use a new one (optional speaker switching)
- 2x2" plated perfboard, 8x 5mm high tact switches, 4x orange T1 LEDs, 2x 51ohm resistors, 1x 2pin bicolor T1 LED, 2ft 8-conductor cable, 1x 8p8c (RJ45) plug.
