Using a RF transmitter with a phone

Discussion in 'Wireless & RF Design' started by teknoob, Jun 7, 2012.

  1. teknoob

    Thread Starter New Member

    Jun 7, 2012
    RF bugs connect their positive and negative leads to the phone line or phone's circuitry. They convert the electric pulses from voice on the phone line into radio waves that can be received by any device listening to the same frequency

    I got this information from a website. I want to know how to build this kind of RF transmitter. I want to use it with a mobile phone, so instead of the loudspeaker i will connect the positive and negative leads to the circuitry. I have a small earpiece that can receive radio waves . Does any one know if it is possible to make a FM transmitter like this one: and instead of the mic, i solder 2 wires and connect them with the phone ??
  2. absf

    Senior Member

    Dec 29, 2010
    The telephone is just a simple analog equipment working on low DC voltage (48V for some). The speech is superimposed on the DC transmission lines. Though it only uses 2 wires, both the send and receive signals are there. Your phone circuit would separate the send and receive signals (speech) using a hybrid circuit (sometimes hybrid transformer) so the received speech goes to your speaker and your sending speech from mic goes out of the hybrid circuit so the 2 signals would not interfere with each other.:)

    Your FM bug is sitting on the line where both the sending and receiving speeches are there. So the transmitting of speech to your FM radio is simple and straight forward.

    Transferring the same formula to a cellphone is not that simple. You have no access to the point where the TX and RX are combined and in audio form. The signals coming in as UHF has to go through lots of process to convert the digital signals into analog before you can hear it. And the point where you wanted to tap might be inside the LSI chip.

    Besides these, the chips are so microscopic and a small mistake might render your cellphone to becoming a piece of junk. :D

    Before the digital systems were introduced. We were using analogue system working on 450MHz. At that time people were able to design scanners where they can monitor the speeches as well as the trapping the IDs stored in the cellphone and duplicated them so they can make free calls from the stolen IDs. I am not sure if they can still do this with the digital systems where the speech and control signals are in different channels and are encoded.:rolleyes:

    I dont mean to discourage you in any way. But if you want to continue with your project, you should at least understand how a handphone works. Try to strengthen your electronic knowledge. There are schematics on the web and trouble shooting tips. I have seen the Nokia 3310 schematics and it is not that complicated compared to the latter models so perhaps you can start with that.:)


  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
    It should not be difficult to acess the microphone and earpiece signal of your cellphone (that's just audio). Your major obstacle will be space. Usually there is no room left inside a cellphone. Which means your circuit has to be really small, i.e. all SMD (the smaller packages).

    The circuit shown has probably not a very good frequency stability over a longer time period / temperature drift which means it's gonna be annoying to listen to it and constantly readjusting your receiver.
  4. vk6zgo

    Active Member

    Jul 21, 2012
    The upside of all this,is that when you are in prison,you will have plenty of time to learn about Electronics.
    The downside is that a prison record may affect your employment prospects on release!:D

    The Licencing Authorities & the Phone companies are more than a little "touchy" about the general public doing this sort of stuff,& if you get caught,you may either pay a huge fine,or as I suggested above,spend some time in prison,as "Bubba's" new special friend.

    If you were successful with bugging someone's cell phone,you may also be charged with "Stalking"----more fun time with Bubba!

    You may consider this a bit unfair,but if it comes to prosecuting you,or a
    Multi-national Media group,who do you think would be the easiest target?
    JMW likes this.