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Thread: Converting an Audio Signal to a Voltage

  1. #1
    FLAC ukchris's Avatar
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    Converting an Audio Signal to a Voltage

    I'm sure the title sounds a little obtuse, let me try to explain. My head unit supports a connection for a phone kit, phone mode is activated by applyting 12v (or ground, can't remember which) to a cable, this then takes the audio feed and plays it through the left speaker.

    Now, what I want to do is take the audio output from my iPaq which I use for GPS and hook it in via this kit so that when it gives me directions it will mute the head unit an play through the speaker. To do so I suspect I need some way of detecting the audio signal and using it to cause some for of switching (a relay?) to activate the phone setup.

    Do any of you know if this is viable or how it might be done? My concerns are how fast it would switch and what would happen in dead spots or pauses between words, would it be switching on and off etc.

    Any thoughts welcome..!

    Chris.

  2. #2
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    First of all, on all the mute inputs I've seen, they have all been negative. Consult your install manual on this one.

    As for getting a low voltage AC audio signal converted to a switched ground, I may have an idea.

    If you are comfotable with relays then this could be a snap! I've never done exactly what you are trying to do, but my idea is very similar to what I've done to create a viable amp turn-on signal by using speaker outputs from a head unit.

    Anyways, get a hold of a LVT (low voltage trigger). Try to find one that is adjustable if at all possible. Tag this to the audio output of your nav device. Then the LVT should have a positive output that is 12v. If you find one that has a negative output as well then your set. If not then just use the relay to switch to a negative output witch you can connect to the mute wire! If you need any further help, such as how to wire up the relay or LVT, I can help.

    Let me know how it turns out!

  3. #3
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    Oh, btw...... these parts should not run more that $20, so have a ball!

  4. #4
    FLAC ukchris's Avatar
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    I've been looking at this http://www.aaroncake.net/circuits/vidswitch.htm my concern is when it will reset, it has to stay active through pauses but disengage when the speech is done, would a LVT do that do you think?

    I'd have a go at this circuit if I could translate the parts list in to part numbers at Digikey or Mouser or somewhere!

  5. #5
    MySQL Error scott_fx's Avatar
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    david navone sells a relay that is triggered by an audio signal. should be what you need in a nice neet package
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  6. #6
    MySQL Error scott_fx's Avatar
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    http://www.autosound2000.com/pulsestretchers.htm
    This new device was designed to supply +12 volts to activate aftermarket components with OEM decks that do not have electric antenna control leads. The N-902 works by sensing speaker voltage from terminals of a readily available speaker. When the speaker voltage is present, the circuitry activates and +12 volts DC IS produced on the green wire.
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  7. #7
    FLAC ukchris's Avatar
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    Excellent!! Thank you!

  8. #8
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    Yup, this is a LVT that you speak of. But again, that mute wire is probably negative! Its not that difficult to do with a relay.

  9. #9
    FLAC ukchris's Avatar
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    David Navone just replied to my email, unfortunately the unit as it stands would not disable until there was a 16 second delay in the audio signal, this is a little long for my needs. I've emailed him to see if it could be "tuned" at all, meanwhile I might build the other VOX circuit as a test.

    Chris.

  10. #10
    Constant Bitrate NotchbackVW's Avatar
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    UKChris,

    Have you built this circuit yet? I want to do the same thing to fix an issue with my TomTom Mobile and Nokia handsfree.

    Thanks in advance.

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