Introduction
This document is broken into two main sections, the first reviews the concepts and information that was used to develop the approach to making the interface box for handsfree car kits. The second part describes in detail the particular approach the author took in making an interface box to work with the Nokia handfree carkit for the 51xx and 61xx series cell phones. I reused some photos from my installation into my old 1999.0 2.8, so please ignore any interior discrepancies you see.
Contents
	Part One:
	Disclaimer Overview Tools Parts The optional circuit The Interface Box Additional Steps specific to the Nokia handsfree carkit
	Part Two:
	Construction Summary: The major parts of the project Building the Interface Box Installation
	Appendix
Disclaimer:
	The reader assumes all liability and risk associated with using the information contained in this document. The information is based on sources that were assumed to be accurate at the time this document was written, apply to the specific needs of the author, and are tailored to the particular A4 used in the article. Every effort has been made to describe the intent of each step so that the information may be interpolated for use with other model years, vehicle options, and different handsfree kits; however, the author cannot anticipate every possible permutation that readers may encounter. The reader should research any specific issue that may modify the information in this document.
Overview
	The author's car, used for this installation, is a 2000 A4 1.8t with the following options: Convenience package Sport package Quattro Bose system with CD changer Head restraint system
	I wanted to use the pre-wired features that came with the car. More importantly, the final installation had to look like it was factory installed. This meant that I was NOT going to muck around with the wiring harness and start splicing and cutting just to make the installation work. If there was not a way to use the connectors as is, I was not going to attempt this project. That may make some folks out there breath easier come time to sell or return the leased car. In addition, if the reader would want to change the cell phone in the future, the modular approach taken here, would facilitate the changeover. The car kit installed by the author was the Nokia CARK-91H handsfree carkit for use with a 6120 (51xx and 61xx series). For those with other phones or brands, refer to the appendix for further information. The Audi has the advantage of having integrated support for handsfree operation. This in large part because Audi offers as an option a cellular telephone. Audi has integrated the speaker output, microphone input, and stereo muting during active handsfree operation into the car's systems; the stereo and trip computer both display "PHONE" during use. Most handsfree car kits that require "professional" installation have the same requirements, namely: Unswitched 12 volt positive Switched 12 volt signal Negative ground Speaker output Microphone input Some kits such as the Nokia support stereo muting; this is standardized as a ground sensing circuit. To work, a stereo headunit that supports this mutes when a signal wire is grounded; a handsfree carkit that supports this feature will ground a signal wire when a call is connected. Kits may provide other features, but for the Audi, they are largely unnecessary and therefore unused for the purposes of this document. Because of the limited number of electrical requirements, most any carkit can be made to work. Audi provides a standard DB25 connector in the trunk on the driver's side for the OEM cell phone. The pinout for this connector is as follows: 1. Pin 7 of the RJ-45 in the armrest 2. N/C 3. Ground 4. Unswitched 12 volts 5. Switched 12 volts 6. Pin 8 of the RJ-45 in the armrest 7. N/C 8. Pin 2 of the RJ-45 in the armrest 9. N/C 10. STEREO MUTE 11. Pin 3 of the RJ-45 in the armrest 12. Pin 4 of the RJ-45 in the armrest 13. N/C 14. Microphone (-) 15. Microphone (+) 16. Unswitched 12 volts 17. Ground 18. Pin 5 of the RJ-45 in the armrest 19. Speaker (+) 20. Microphone shield 21. N/C 22. N/C 23. Pin 1 of the RJ-45 in the armrest. 24. Pin 6 of the RJ-45 in the armrest 25. Speaker (-) As you can see, this connector is extremely versatile; the wiring is direct and does not interface with any critical elecronic systems other than than incidental connections to the radio. It should also be fairly obvious how the connector can be utilized to integrate a handsfree kit into the car. One additional item featured in this document is a means of adjusting the Audi microphone output to match the car kit microphone input requirements. The Audi microphone has a built-in preamp that overdrives most kits. The circuit described in this document is optional; the microphone in the headliner can be switched out with the microphone that comes with the carkit, but this limits you from changing phones in the future. The rest is fairly straight forward wiring. Those not interested in a handsfree kit can use this pinout information to redirect signals or power into the connector in the armrest; remember that the wiring to the armrest is direct and does not pass thru any critical systems. To put it in the most basic terms, to make the handsfree kit work, you are marrying the power and signal requirements of the carkit with the power and signals available thru the DB25 connector via an interface box.
Tools
	The following list outlines the tools I used in conjunction with this project: PC Board (optional, see following sections) PC acid etchant PC Resist decals Cordless dremel tool Various size drill bits Dremel cutting wheel Soldering iron Rosin core solder Rosin flux Interface Box Cordless dremel tool Various size drill bits Dremel cutting wheel 3/4" Power drill Wire cutters Wire strippers Car Interior Installation Phillips head screwdriver Rachet driver & extender 8mm socket 10mm socket
Parts
	This section and others to follow are broken into 2 levels of complexity and parts requirements: BASIC - Minimum parts required ADVANCED - Basic plus tunable microphone input, convenience components, and privacy handset in armrest. For manual muting switch, or powered connector in armrest for other gadgets, refer to the appendix.
	Basic parts requirements
	Carkit. The author used a CARK91-H Nokia handsfree carkit that includes: Control module Cradle with antenna and main data connector External speaker Eexternal microphone Otional handset (see below) Power harness Several mounting pads and miscellaneous screws
	DB25 male/female STRAIGHT THRU cable. Length is dependent on placement of interface box; this placement will be determined by wiring length limitations of the handsfree carkit. The author located everything under the rear seat - 6'-0" was sufficient. The picture shows the unfinished interface box attached to the end.
	DB25 male connector. The author used the solder type, but a crimp type is also available.
	Project box. The author used one from Radio Shack measuring 3"x2"x1", but use whatever size fits and you feel comfortable with.
	1 & 2 amp fuses and holders Only if they were NOT included with the carkit.
	Advanced parts requirements
	In addition to the basic requirements, there are author specific approaches that are outlined in this section; the parts listed here are for the optional circuit. Single sided copper coated PC board (optional - circuit and/or the use of a PC board is the discretion of the reader.) One 10uf capacitor. One 1uf capacitor. One 10k ohm resistor. One 50k potentiometer. Optional two 1 amp thermal circuit breakers to replace the 1 amp fuses Optional one mini 2 amp fuse and holder to replace the 2 amp fuse RJ45 8-pin patch cord (for tying in the privacy handset connection to the armrest) The following is complete parts list the author ordered from www.mouser.com
	PART	QTY	DESCRIPTION
	ME442-76-0100	2	MOUSER FUSE CIRC.BRKR 250VAC 1A
	5768-53007	1	LITTELFUSE FUSEHOLDER PCB HORIZONTAL
	5768-97002	1	LITTELFUSE AUTO FUSE MINI FUSE 2 AMPS
	72-T70YP-50k	1	DALE SINGLE-TURN TRIMMER 1/4" SQ V/ADJ 50K
	ME271-10k	1	XICON 1/4W 1% METAL FILM RESISTOR
	571-7479122	1	AMP AMPLIMITE D-SUB 25P PLUG TIN
	140-LLRL50V10	1	XICON LOW-LEAKAGE RADIAL ELECTROLYTIC CAPACITOR
	140-LLRL50V1.0	1	XICON LOW-LEAKAGE RADIAL ELECTROLYTIC CAPACITOR
	571-13809360	1	AMP MINIATURE CONNECTOR
The Optional Circuit
	As stated before, the Audi microphone has an integrated preamp circuit that boosts the level of output. In addition, the Audi microphone expects a line level voltage of 12 volts. The green wire of the nokia kit supplies 12 volts and is meant to operate a motorized antenna; it can source 300 ma. The unique feature of this wire is that it only supplies the voltage when the phone is in the cradle and has been turned on. This wire is protected by the 1 amp fuse or circuit breaker. If you are using a carkit other than the Nokia, you can substitute a connection from pin 5 of the DB25 to supply SWITCHED 12 volts from the car. This voltage, via the 10k ohm resistor, provides the necessary line level to the Audi microphone via pin 15 of the DB25. The DC voltage is decoupled and isolated from the circuit by using a 10uf capacitor with the positive lead connected to pin 15. The negative lead of the capacitor connects to one end of the 50k potentiometer. The other end is connected to qround (pin 3). The Nokia microphone positive input is connected to the middle post of the potentiometer, allowing for variable adjustment of input level. To ensure that all voltage is decoupled from the carkit, an additional 1uf capacitor is used between the potentiometer and Nokia input; the negative lead connected to the potentiometer post. This circuit is optional; you can eliminate it and connect the carkit microphone positive wire directly to pin 15 of the DB25 if you solder the carkit microphone in place of the Audi microphone. You also don't have to use a PC board. A bread board, or even no board need be used if you isolate the components from each other.
The Interface Box
	Basic Approach
	This will accommodate most carkits. Here is a quick summary of the necessary connections made inside the interface box to the male DB25 of the interface box. The color of the following wires refer to the Nokia carkit power harness. Interpet as necessary if you are not using the nokia carkit. Red - Unswitched 12 volts. Connects to pin 16 on the DB25. This provides power to the carkit. Isolate with the 2 amp fuse. Blue - Switched 12 volts. Connects to pin 5 on the DB25. This provides the carkit with a means to determine when the car is turned off. Isolate with a 1 amp fuse. Black - Ground. Connects to pin 3 on the DB25. Self explanatory Speaker positive (red) - Connects to pin 19 on the DB25. Speaker negative (black) - Connects to pin 25 on the DB25. Microphone positive (red) - Connects to pin 15 on the DB25. See section on the optional circuit that I installed between the microphone wire and pin 15. Microphone negative(bare copper) - Connects to pin 14 on the DB25. I used heat shrink tubing to protect the wire. The green wire is used in the optional circuit to provide 12 volt line levels.
	Additional Steps Specific To The Nokia, non Nokia Stereo Muting, and armrest power.
	Privacy Handset/Data connector The privacy handset uses the same connector as the one in the armrest. To this end, the connections are straightforward. An RJ45 patch cord is used with a connector at one end that plugs into the handsfree control module, the other end wired into the interface box as follows: Wire for pin 1 of the Nokia connector - goes to pin 6 of the DB25 Wire for pin 2 of the Nokia connector - goes to pin 1 of the DB25 Wire for pin 3 of the Nokia connector - goes to pin 24 of the DB25 Wire for pin 4 of the Nokia connector - goes to pin 18 of the DB25 Wire for pin 5 of the Nokia connector - goes to pin 12 of the DB25 Wire for pin 6 of the Nokia connector - goes to pin 11 of the DB25 Wire for pin 7 of the Nokia connector - goes to pin 8 of the DB25 Wire for pin 8 of the Nokia connector - goes to pin 23 of the DB25 NOW THIS IS VERY IMPORTANT! Either Audi or Nokia uses an inverted pinout. To compensate, and for the purposes of the connections listed above, I consider the RIGHTMOST pin of the connector ON THE NOKIA CONTROL MODULE to be pin 1. Make sure you properly trace the correct wire that CONTACTS the RIGHTMOST pin of the Nokia connector ON the CONTROL MODULE to pin 6 of the DB25. Failure to properly wire these connections will result in the carkit not functioning correctly.
	Nokia Stereo Muting
	The Nokia carkit supports stereo muting. Again, the color of the following wires refer to the Nokia carkit power harness. Interpet as necessary if you are not using the nokia carkit: Yellow - Stereo mute. Connects to pin 10 on the DB25
	Non Nokia Stereo Muting
	For kits that do not support this feature, it is possible to implement its equivelent in two ways: Use a third party mute device. These work by sensing current drain when the phone is in use, and then provide the grounding necessary to activate the mute feature in the radio. I am aware of one available at autotoys.com Wire a manual push button to activate stereo muting. One possible approach to do this is to take advantage of the connector in the armrest.There is also a connector UNDER the center console that ties the armrest to the car. Either one can be used. Simply wire one RJ45 pin to pin 3 of the DB25 to provide ground, and another RJ45 pin to pin 10 of the DB25 to provide stereo muting. To provide MANUAL stereo muting, wire these two RJ45 pins via cable and connector to an on/off switch to complete the circuit when needed. You can perhaps wire this into a replacement for one of the dummy switches on the center dash. This can only be done if the RJ45 in the armrest is not being used for anything else, such as the privacy handset connection described above. At this point, you can also take the opportunity to use the RJ45 as a power source. DO NOT do this without using a 1 amp fuse, as the gauge of wiring used is very small. Example of mute switch: Connect pin 23 to pin 10 of the DB25. Connect pin 8 to pin 3 or pin 17 of the DB25.
Construction Summary: The major parts of the project
	Carkit The CARK91-H/US is pictured.
	Interface Box and 25-pin STRAIGHT-THRU Cable. I got my cable at CompUSA.
	Antenna: includes: Antenna mast coupler repeater box 10 feet of coax with a mini-uhf connector A glass mount antenna acts as a conduit enhancing the signal flow in and out of the car. Just mount the antenna mast to the outside of the car windshield with the doublestick tape then mount the "repeater box" on the opposite side of the windshield. No hole drilling through the windshield required. Too many options. I am looking into a patch antenna or a bumper antenna for a more stealthy look. i might even cut the connecting post on the cradle so the atnenna won't deactivate when I put the phone into the cradle.
Building the Interface Box
	Start by preparing the wires from the control box. I cut the wires to about 10 inches.The wires and connectors from right to left are: Power harness: 3x2 molex connector includes unswitched 12v (red), switched 12v (blue), ground (black), stereo mute (yellow), 12v aux supply (green). Privacy handset/data port: RJ45 8 pin wiring. I used a CAT5 network patch cord for this. It does NOT come with the handsfree kit. Microphone connector: 2 wire positive and negative. negative is bare copper that must be protected using shrink tube or similar. Speaker connector: 2 wire positive and negative. Red is positive, black is negative.
	The wiring diagram is illustrated on the right. Review the Interface Box overview above for the specific wiring to the DB25.
	The PC board method
	Make an accurate image of the PC board by print or copy. This is then temporarily glued to the fiberglass side of the PC board, NOT the copper side. Next, drill the holes for the components thru the board, so you can locate the positions of the parts. You then use the holes to lay out the copper pattern using the resist decals.
	Once the pattern is laid out, you then remove the unwanted copper in a bath of copper etching solution. Clean the copper trace.
	Make sure the PC board you have just made fits the project box. Your application may vary from the pictures, depending on the box you chose. The image pictured shows the board in the box, copper side DOWN. At this time, you want to make the openings for the wires that need to come into the box, and the hole for the DB25. Thsi way you can remove the board while you cut and not ruin the board in the process.
	The first part to install on the board are the thermal circuitbreakers. These work by tripping when the amperage is exceeded or you short out a connection. They work much like the ones in your breaker box at home, but these reset themselves after a couple of minutes. They may trip as you solder them, as the heat from the soldering iron will trip them. The upper right breaker is how they come. The lower left is how I modify it prior to installing them. I bend the thicker base out flat, then bend the thinner leg back on itself, leaving a gap. This gap is used in the next step, to make a way to solder on a wire to then connect to the board. One tip - lightly sand the legs before you bend them, so that soldering is easier.
	Next, use some epoxy to glue the breakers onto the fiberglass side of the board. Notice that the gap in the bent legs align with the holes in the board. Take some wire - I used the excess legs from the trimmed 10k ohm resister - and make shepards hooks. I then ran it thru the gap into and thru the hole in the circuit board. I then soldered the wire to the legs of the breaker, and the wire to the board. Make sure that the solder doesn't get excessive between the breakers, since the 2 amp fuse fits in that space.
	Install the 10k ohm resistor, the adjustable potentiometer (blue square component in image), and capacitors onto the board and solder them. CRITICAL NOTE: The positive/negative orientation of the capacitors are very important. Both the 10uf and 1 uf capacitors has the NEGATIVE lead going to the potentiometer. The POSITIVE leads go to the wires leading away from the circuit board.
	Next, install the mini fuse holder. Here is a picture of the completed circuit board. The fuseholder has a somewhat large center post. Make suer the hole is large enough for it to pass thru, and don't force it.
	From then on, it's straight forward wiring and soldering. Here's a shot of the box in progress. Tip - start by soldering the wires from the control module first, otherwise you'll have a tough time getting behind to the backside of the board to solder the remaining wires. Use the schematic and wiring information presented earlier in the document.
	A little tight, I know. You will adjust the microphone input by turning the potentiometer to suit your taste. The wiring that I did, has most of the wires going to the board, then from the board to the male DB25 connector. The eception to this is the 8 wires for the armrest. I soldered them directly without first running to the PC board. Remember, you do not need to use a box this small. YOu may want to try out others.
	Here's a picture of the final interface connected to the control module
	Installing into the Car
	I ran the cables from the handsfree cradle under the carpet along the side of the drive tunnel, and pulled it through the hole pictured on the right. I then ran the DB25 cable behind the trunk trim on the drivers side, behind the rear passenger's bolster, into the rear seat area. The bolster easily unclips. I used the existing cable ties to neaten up the wiring.
	For the cradle, I ordered stock Audi trim pieces that fit in the same space as the cupholder did. For onyx interiors the part number is: 8D0 863 301 J 1EJ For opal grey, the part number is: 8D0 863 301 J 3PF As you can see, the last four digits identify the interior color. I then epoxied this piece, which is a bit flimsy, to a stiff material, in my case a pice of steel. The plastic part is covered in carpet fleck, but is too weak to handle a cradle base. Notice the cutout that leaves room for the cable to get by the plate, so it can go under into the center console.
	Removing the center console is easy. Unbolt the armrest. You can see the three bolts when you undo the rear ashtray, and the cupholder. Then unscrew the two bolts that are under plastic covers right by the shift. Finally, undo the emergency brake handle and boot by simply sticking a small flathead screwdriver as pictured, and pulling out the plug. The covers then slide right off. You also need to pull up on the shift boot to losen the front edge of the center console. You also need to undo the small plastic cover at the back of the front seat slide rail. pop off the cover with the small screwdriver, and undo the screw. CAREFUL! It strips easily, so don't force it or overtighten when you place it back.
	With everything undone, it is very easy to lift the carpet to run the wires under it. I could stick my arm in there. One thing to look out for. Make sure the cables from the cradle pass BEHIND the emergency brake connector. If you goofed, you don't need to pull the calbes, just undo the connector, move the cables then reattach the connector again.
	This is what the final installation looks like.
	The antenna. No need to go into detail here.There are too many options to go over, so you need to research this one yourself. Suffice to say, there is room between the headliner and glass to push the coax into it. The TSB for the OEM Audi install says to do just that.
APPENDIX
	This section is done as a mini-FAQ. I hope it covers most of the questions I've seen on Ausiworld over the last year or so...
	Can I use a carkit other than the nokia? Sure can. It may not support the stereo muting feature on it's own, but you can either use a third party device, or make a manual pushbutton that will perform the same function. See the information above.
	I don't have a Bose system. What's the difference in how to implement the handsfree kit? The Bose system has a dedicated speaker, but is still controlled by the radio. This means that if the radio DOES NOT MUTE, the speaker is dead. When the mute is activated, the speaker output can be heard. For the NON-BOSE, the speaker output is routed thru the radio, and is user selectable from the right to left front speakers. This is done by the controls on the radio. Read the manual on how to do this. Other than that, the wiring is IDENTICAL from the perspective of this interface box.
	I can't solder. Can I still do this FAQ? No. Hire a pro, and give them this info. It may cost you though.
	What if I don't want to mount the cradle where you show it. What do I need to know and/or do? The limitation is the length of the cable from the cradle. If you locate it too far, it won't reach into the rear seat area. In this case, get a longer DB25 cable, and place the interface box in the front area. This will give you alot of options for the cradle. You'll just have to run the DB25 cable a longer way, that's all.
	If I wire the armrest for power, and NOT the privacy handset, or I am not doing the carkit, but am interested in the connector, what can I do with the 8-pin connector in there? There is alot you can do with power in the armrest. Yu can supply power to the pins so you can pull power to a G-Tech, radar detector, you name it. It doesn't have to be power. You can wire a video recorder in the trunk, and use the wiring to tie the board camera into the cabin. Plug and play. Personally I like the idea of a power source and a manual mute button. This way I can mute the radio, and unmute it without changing the volume.