How to make FM station from Car FM transmitter?

Discussion in 'Homework Help' started by heroofdikhan, Dec 6, 2011.

  1. heroofdikhan

    Thread Starter New Member

    Dec 6, 2011
    2
    0
    Hello

    i want to make FM radio station/FM spy bug out of car FM transmitor (limited range though)
    can anyone help me in this regard :)
     
  2. Georacer

    Moderator

    Nov 25, 2009
    5,142
    1,266
    FM transmitters are subject to laws and regulations in almost every country, depending on their power output, so you 'd better state the range you want to transmit at.
    I 'm not sure what you mean by FM spy bug. FM frequencies are not encrypted.

    Finally, what do you mean by "car FM transmitter"? Cars usually have FM receivers, to tune into your favorite radio show. They don't transmit anything. Care to explain more?
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I know what the OP is talking about.

    There are small FM transmitters for cars to feed your Walkman or MP3 player into, and play it over the car radio speakers. It is a very common accessory.

    Many kits advertise a small FM transmitter as a bug, a small secret hidden transmitter to spy on people. The device is the same as the car accessory, more or less.

    One of our other users, AudioGuru, has polished and published plans for such a device. AG is an experience retired engineer, he can be a bit gruff at times, but if you ask nice I suspect he will help.

    Why did you put this post in the homework section, is it a school assignment?

    fmtransmitter????poor design references AGs design. Don't worry about the negative comments, another user is being petty. If you have any other questions drop me a line.
     
    heroofdikhan likes this.
  4. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    He probably is referring to the iPod/mp3 accessory plugins that send your audio to the auto FM radio.

    (Bill beats me to it.)
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Silicon Chip magazine in Australia published The Micromitter FM stereo transmitter project about 7 years ago. It uses a Rohm IC like the new products available. To prevent interference and be legal it has an RF attenuator at its antenna output. If the attenuator is bypassed then the range might cover a home and its yard.
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Is there a link anywhere to this?

    I build a stereo kit a long while back for my C64 stereo setup. I'm weird and know it, I liked listening to some of the music out for the C64. It used a specialized chip, not very stable but it worked. I still having it (and my now C128) setup.
     
  7. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    Wendy likes this.
  8. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Looks like what the OP was looking for.

    I've had some reports on this thread, for the record it is legal in the USA to transmit low power signals, under 10mw and with an antenna under 10' long. As it turns out the antenna length is plenty long enough to set up a resonant dipole antenna with a little math. I have built many transmitters in my time, and may do so again, under the above FCC rule set.
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Before Rohm made modern FM stereo digital ICs one which is used in The Micromitter, They made the BA1404 FM stereo analog IC that had instability and many other problems. The new line of digital ICs use a quartz crystal controlled frequency synthesizer for rock solid radio frequency, stereo 19kHz pilot and 38kHz multiplexing.
    They have most of the required filters to keep beat frequencies quiet.
     
  10. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I suspect the one I have is what you are talking about. For my primative needs it was enough.

    With luck the OP will come back and give us some feedback.
     
  11. heroofdikhan

    Thread Starter New Member

    Dec 6, 2011
    2
    0
    guys i want to modify it just for educational purpose dont need some high range transmiting....
    just want to know weather it is possible to remove the USB inpur and connect/soldier the mini microphone
    pic shown below(Car FM modulator used for playing songs on car radio)
     
  12. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your photo shows the insides of something with a USB jack. What is it?

    Another photo is an MP3 player for a car. Does the MP3 player also have an FM stereo transmitter?

    What do you want to connct a mini microphone to? If it doesn't have an input jack for a microphone then you must add one plus add a preamp for it.
     
  13. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I'm guessing the first photo is a USB/FM transmitter. I suspect this is useless for the OP application, since USB is a digital standard (no audio there, though audio can be inside a USB data stream).

    If you look at a close up of the second picture it is a stereo audio to FM transmitter meant to rebroadcast a MP3 player. This allows the car's FM radio to play the MP3 player on the car's speakers, amplified.

    You could modify this device, if it connected to the audio out. That would be pretty straight forward. You would not want or need to open the device up, it has all the jacks you need.

    The site that was shown on post #7 is your best bet. There are other kits out there that are very similar.

    You never have mentioned what this is actually for, nor why you have posted this in the homework forum. The more information you give the better we can help you, right now we are shooting in the dark.

    Having said that I really like the project AG cleaned up.

    AudioGuru, where do you have the master post (or blog) for that FM transmitter? Is it on ElectroTech?
     
  14. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I didn't post my FM transmitter as a project anywhere because it is illegal in many countries because its RF power and range are too high.

    I simply fixed one that was posted and it didn't work. I fixed it in 4 stages:
    1) The mic preamp didn't work when the battery was new and when it was old so I re-biased the preamp and voltage regulated it.
    2) It sounded muffled so I added pre-emphasis (treble boost like FM radio stations) to cancel the de-emphasis (treble cut) by all FM radios.
    3) Its RF frequency changed as the battery voltage ran down so I voltage regulated its RF oscillator. I used a low-dropout regulator so a 9V battery can last a long time.
    4) Its frequency changed when something moved towards or away from its antenna so I added an RF amplifier to isolate its oscillator tuned circuit from its antenna.
     
Loading...