Hacking a mobilephone radio transmiter

Discussion in 'The Projects Forum' started by Abysium, Jan 14, 2013.

  1. Abysium

    Thread Starter New Member

    Jan 14, 2013
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    Hello everyone

    First I need to say that althou I have always been interested in electronics, I am not anywhere near beeing an electro-engineer. I know the basics and not really much more on the subject, but Ive always been prone to thinkering.

    I was clearing out some old junk from my closet when i found an old car phoning gadget, that you clip on over your mobile phones speaker and it picks up the sound via a condenser microphone and sends it out on 88,7 MHz and 106,4 MHz, which you can then catch with your car radio.
    Since the rise of bluetooth this device has become rather obsolete, so it was just lying in my storage for the last 10 or so years. Now before i decide on throwing it away, I was thinking on possible alternative uses.

    If i could get it to take the sound from my laptop and feed it to my HiFi via radiowaves it could live on as somthing usefull and I could stop having to deal with the extra cable that i currently use.


    So, the gadget is powered by a CR2032 battery, which meens 3V of power. Now I would like to power it via usb, so I need to drop the usb power line for 2V. I was thinking resistors, since the device probably has a constant drain. Not really sure there.

    The second bit that I need to do is feed it the audio. The easiest way would be to use a single headphone that could be cliped to the gadget, so the microphone would do the work, but i was thinking if there are any alternatives. Ive opened it and saw it uses the xf-18d condenser mic,
    I was kinda hoping i could just solder it off and instead of it solder on a connector that i could plug into my headphones jack.

    So the esencial bit is how to drop the voltage from 5V to 3V, but if anyone knows if(/how) it would be possible to take out the mic and wire sound directly into the device, that would really be the cherry on top.

    Possible?
     
  2. wayneh

    Expert

    Sep 9, 2010
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    I'm thinking that's unlikely, but it is something you can measure. A couple AAs might be a simpler choice.

    You can definitely improve the audio by bypassing the speaker-to-mic link with a direct connection, but somebody here will have to guide on the best way to do that.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    The output level from the microphone is only 10mV but the output level from line level is at least 27 times higher. Speaker level is even higher. So you need to make an attenuator with two series resistors to ground. The electret mic is powered by a resistor that should be removed.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    The unit runs off a CR2032 3V battery. It is possible that it will run off 5V with no problem.
    If you are worried about trying it, you can put three 1N4001 diodes in series with the 5V supply.
     
  5. Abysium

    Thread Starter New Member

    Jan 14, 2013
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    Ok, so for the power supply I can get the 1N4001 diodes easily, thats no problem.

    For the audio link Audioguru mentioned 'an attenuator with two series resistors to ground'
    Is that a 'π-type unbalanced attenuator' ? Im now reading up on those, trying to make some sense of it all.

    If you guys think this would be the best way, Ill be more than happy to try it, but would it be possible to get some help with the numbers? How to make the attenuator, from which resistors?

    If necesary, I can do measurements, but I would need to go get a battery first.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    Here is an attenuator that reduces the signal level 27 times and converts stereo to mono:
     
  7. Abysium

    Thread Starter New Member

    Jan 14, 2013
    5
    0
    Thank you,
    Now that I have all the data I need, I can try this idea out for real.

    Thanks again, you've been a great help


    Edit:
    Actually, a thought just crossed my mind:
    If I remove the microphone and the resistor thats bridging the 2 mic pins, I should by wireing the transmiter input from the attenuator on one of them and ground to the other, yes?
     
    Last edited: Jan 16, 2013
  8. Audioguru

    New Member

    Dec 20, 2007
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    The microphone is probably an electret type that has a resistor in series that powers it, not a resistor bridging it. The resistor that powers it should be removed.

    The 1k resistor in the attenuator connects to the hot mic input and to ground.
     
  9. Abysium

    Thread Starter New Member

    Jan 14, 2013
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    0
    Ill provide pictures as soon as possible
     
  10. Abysium

    Thread Starter New Member

    Jan 14, 2013
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    0
    Ok, it took me some time, but as said: here are the pictures of the circuit
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    The simple circuit does not use a modern IC so it is probably mono, not stereo.
    It probably has horrible frequency response (no high audio frequencies).
     
  12. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    For a stereo broadcast of about the same quality, you may be better off with something like This

    There are better versions as well, but that one is under $10 w/shipping (closer to $5, so don't expect too much). It is powered by USB or a 12V -> USB Power adapter.

    Most every new cheap one will perform at least as well as the one shown in the image, and in stereo as well.
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    I wonder why it is so CHEEEEP.
    If it works properly then they could charge 10 times more and it would still be a good deal.
     
  14. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Yeah, I was amazed at the price. Just trying to buy a retractable USB cable that comes with it generally costs more than the transmitter + shipping.

    Since the OP would need to purchase/scavenge a similar cable in any even to power the transmitter he has, the result is a better solution, IMO. Reviews say it works great in houses, not so good in cars. Since the application is home use, it's worth a shot and cheaper than hacking the existing one.
     
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