iPod Line In for Clarion radio in Saab NG900
Yes there are a number of other line in mods, I didn’t like them, they were too hack-ish. So I made my own hack.
Disclaimer: I designed this to use exclusively on my own car, but have shared it here for the community. There is no effort taken to ensure safety of longevity of this system other than the fact that my car has been fine so far (about 6 months). Building this system is done at you own risk and so on and so forth. Information herein is provided free for personal use, any form or commercial use is strictly forbidden without prior written consent from myself – Andrew Leech.
On to the fun stuff ->
The audio system in the clarion head unit consists of 3 audio sources – Radio, Tape and CD. Each of these come from their respective modules/connectors, run into separate preamps (with capacitive coupling on the input of the preamps) before feeding the signals into a standard 4000 series analog mux chip. This mux chip is running on an 8V rail which is used for a number of audio line level subsystems. This mux chip also has 4 channels. This means we have one free to use 🙂
The mux is controlled by an internal microcontroller, which has no capability to control the 4th channel (actually it’s channel 0 that’s unused by memory) of the mux, it’s analog pins are shorted to the cd’s ones. So to use it we have to disconnect the audio pins from the pcb (lift them up) and put our own wires on. That’s a nasty hack, but it sounds smooth. The control lines to select that channel are easy however. Both control pins on the mux simply have to be pulled low. And seeing as the micro only ever pulls them low, with a resistive pull-up, it’s safe for us to just pull them low whenever required. This is good. So the basic theory is to have our line in, run it though a similar preamp to the built in ones, feed it into the mux pins and figure out some way to pull the control pins low whenever we want to listen to the line in. I’ve used a micro, which monitors the control lines from the front face, and if you hold down the radio button for a couple of seconds it switches to line in, which works very well.
Mux Chip Closeup:
Building the extra electronics
The micro and the preamp were designed as two separate modules. Now I designed these to my own build skill level, and as this is my professional line of work, they may appear to be above the skill of your average hobbyist to assemble, but really they are fairly simple. It may be composed largely of smd parts but they are pretty easy to work with after only a little practice, so long as you have a decent light and a nice set of tweezers. The wiring of the modules is easy enough, with all pins all being marked appropriately. Any pin labeled with ‘M’ and a number refer to pin number on the mux chip that it needs to be connected to. GND is obvious, and 8V can be tapped in a number of places, one of which will be highlighted below.
Micro Module – bottom and top
Preamp Module – bottom and top
The reason I used a micro for the front panel sensing is that the main clarion micro uses a pulsing signal to scan across all the buttons and detect which ones are pressed. Trying to filter this signal and decode it with basic analog electronics sounded like too much hard work for me, it’s much easier to slap a $5 micro down and use that to do all the heavy lifting. Makes for much simpler electronics. The basic atmel Mega16 chip I used is common and easy to program, either from a simple parrallel port programmer of the isojtagisp project of mine. There’s a parr port programmer linked to from the isojtagisp page if you only want to program this one micro and not bother with any other development of your own.
Schematic (click for pdf)
The pcb template is mirrored, and is intended for use in laser printer style pcb production, although could almost as easily be used in photographic style production.
Installing the microcontroller module.
Also highlighted are the pins on the mux which need to be lifted to run the audio signals into. I suggest heating the pins with a soldering iron while carefully levering them upwards with a pin. Some desoldering braid can assist to remove the solder from them. Then a small piece of masking tape or similar stuck under them will stop them from being shorted to the pads under them ever again.
The upper right hand view shows where the taps into the front panel controls are made. The pads circled in red are for the radio button, moving the two wires to the yellow ring’d pads will watch the cd button instead. Catch then is that the cd unit will always be playing while the line in is being used. It wont affect the sound, but will lose your place on the cd being played. The lowermost wire in this frame isn’t necessarily so obvious, but it’s soldered onto the end of the smd resister that’s there.
Installing the Preamp Module
To be completed.
Feel free to hassle me on andrew (a) alelec dot net if you’re interested and I’ll write more here!