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Author Topic: Arduino powered aux control for car radio  (Read 1986 times)
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So I just purchased a 2001 Miata and my first project is to wire in an aux input to the car's stock radio.  Although an aftermarket radio would be easier, I'd much prefer to spend less, learn some things about the Arduino platform, and not give anybody a reason to break into my car.

So my car's main head unit has a CD player and an AM/FM radio tuner.  A dealer option for my car was a cassette deck which connects to the main head unit via a proprietary cable.  This cable includes power, L/R audio signals, and some control signals for the main head unit and cassette player to talk to each other.  The important lines are listed below:

PinDescription
1Right Input
2Signal Ground
3Left Input
4Combi Control - Pulled high when radio/CD is playing.  Pulled low when line input (cassette) is active.  If line input is active when the head unit turns off, a high/low pulse is sent from the main head unit to the cassette deck to turn the cassette deck off.
5Aux Control Output - The main head unit remembers if the line input (cassette) was active when it was last shut off.  When the main head unit is powered on, it will send out a pulse to turn the line input on.
6Aux Control Input - Cassette deck sets this high so the main head unit switches its source to cassette (and turn on if it was turned off).  If pulled low, the main head unit shuts off (unless pin 4 is high).
9Power Supply ACC - Outputs 12V which goes on and off with the ignition switch.  Cassette deck draws 320uA from this pin at all times when the ignition is on.  This pin only allows a very small current without dropping voltage because of a 1K resistor built into the main head unit.
10Ground
11Power +B - Supplies a constant 12V no matter whether the ignition is on or off.  The cassette deck draws 0.2A from this pin when playing.

So it doesn't look like a big deal.  I just need the Arduino to read/control 3 DIOs for Pins 4-6 and another DIO for an external button to turn the line input on/off.  The problem is power.  Ideally, I'd like to use pin 9 for power since it will shut off once the car turns off.  Can I just hook up an LM7805 voltage regulator to convert 12V down to 5V to power an Arduino Board Pro Mini: http://arduino.cc/en/Main/ArduinoBoardProMini?  According to the specs, active power consumption for this board is ~250uA which is less than the 320uA the cassette deck uses.  Will the power regulator work with such a low draw board?  Will the regulator draw any additional current which would put it over the 320uA max?  I don't have much experience with circuits outside of basic I/O.

Am I missing any other problems with this design?

Thanks

Sources:
Similar Radio Project: http://forum.miata.net/vb/showthread.php?t=170495
Cassette Connector Pinout: http://miata.fosketts.net/index.php?title=CN200
LM7805: http://www.fairchildsemi.com/ds/LM%2FLM7805.pdf
Arduino Pro Mini: http://arduino.cc/en/Main/ArduinoBoardProMini
« Last Edit: May 18, 2011, 08:00:14 pm by meatloafsurprise » Logged

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Cool project.

The Mini has a built-in regulator that works from 12V.  Now "Car 12V" can be up to 15V, and have spikes etc.  So maybe better to use a separate 12V to 5V regulator like you suggest. Or at least put some fused protect circuit with a 14V or so Zener diode, some largish capacitors (say 1000+ Uf in parallel with a .1uF) and a small series resistance (you could probably afford 10 ohms or so here).. 

Unless you are really trying to keep the weight down, why not a regular Arduino?? Or a Nano?

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Regards, Terry King  ..On the Red Sea at KAUST.edu.sa
terry@yourduino.com  LEARN! DO! (Arduino Boards, Sensors, Parts @ http://yourduino.com

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