GPS - Car Stereo Mute

Hi Guys,

Here is a little project that I want to share with you.

Basically this project was born when I was traveling using my GPS and at the same time trying to listen the car's stereo. Well, it was difficult for my little GPS to cover up the music and the highway noise. So, I came up with a solution that would mute the car's stereo and will boost the sound output produced by the GPS. This is how it works: Every time the GPS has something to announce, the car system would be completely muted and all you will hear is the GPS output amplified for a better audition.

The files created for the project, schematics, block schematics and the PCB layouts are attached here. Feel free to ask questions if you want.

http://www.megaupload.com/?d=ZTEHK0BG

PS

Just a couple of additional advices... Maybe too many... :lol:

1. The muting circuit has a variable resistor for setting the threshold (sensitivity). The LED will help in seeing the trigger. When it lights up, you have mute.
2. LM1875 is renowned for its instability due to poor PCB design. The published PCB does NOT oscillate. If you wish to modify anything, please read the data sheet first. It helps a lot.
3. The PC board that I used is called Veroboard Grid 2,54 mm, strips.LMB.
4. I have attached two additional files, listing the components for each board.
5. After you are done with building them here what it should be done.
a) Test them separately.
b) The amplifier does not need any adjustment if built as specified. Its gain is around 20. If you wish to increase/decrease the gain, read the data sheet first and then modify the feedback loop.
c) The muting circuit needs to be adjusted in order to react properly to the audio level. If the output of your GPS is very low, you may want to feed this circuit from the amplifier's output. Take care here, as you may create oscillations or ground loops.
6. As a general advice, try to keep your soldering clean, without any whiskers or too much rosin between the copper strips.
7. You can stash the case that will host the project under your central console, underneath the stereo. You will be amazed how much space is in there...
8. How to find the mute input on your stereo. These days, because of the cell phones development, many car stereos have a MUTE input. Take the stereo out of its place and observe the back connectors. Usually, if there is no other usage for the mute input, there is no connection to it. Using a resistor, valued between 1k and 10k, probe the unconnected pins from the back. Connect the resistor to one of these unused pins while keeping the other end of the resistor connected to the ground (chassis). Obviously the stereo has to work so you will notice that the sound is muted when you will find the mute input. How you will connect the wire to the rest is up to your imagination.

Good luck!

 

Very cool.  Thanks for posting this; it turned up at the top of a search I did for this exact concept.

I noticed that at least two parts were missing from the mute-circuit parts list: U1 and U2.  I don't have anything that'll read the "dsn" files, but by loading that one into a text editor I could see that these are supposed to be NE5532 op amps.  So whatever software generated that list appears to have a bug.

Rather than build an external amp, I'd like to direct the GPS's audio output to the car's amps.  Since most cars these days have head units that are connected to power amps, you could use your mute circuit to trigger some switches that disconnect the head unit from the power amps and connect the GPS's audio to them instead.

Not only would this interrupt the radio, but it would pipe the GPS output through the car speakers and even work with radios that don't have cell-phone mute inputs (I don't know how many don't).

Got any ideas for that one?  

Sounds excellent - please provide updates as they occur!  

Looks like small relays are the best thing to use for audio, since switching ICs can add noise.  I need to switch eight poles (4 channels x two poles) when the GPS starts talking.  Step 1 is getting the parts together...  

Excellent write up and thank you for all your effort on this very popular topic. I am going to try this. Question: could a sound activated fm transmitter with switch connected to the mute input of the stereo accomplish the same