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Car Key Transponder

Discussion in 'Radio and Wireless' started by Rajinder, Nov 28, 2020.

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  1. Rajinder

    Rajinder

    531
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    Jan 30, 2016
    Hi all,
    I have attached a circuit diagram from a car key transponder emulator that my friend has.
    I believe these work by putting the key emulator in an energised electromagnetic field, the inductor from the field mutually couples to the LC tank circuit (as in the diagram).
    I am not too familiar with the schematic, it seems the PNP is driven to provide 3V to the LC circuit from a Microcontroller pin, this will energise the LC circuit to produce oscillations and interact with the ignition coil, the output is taken from the 10K into the microcontroller.
    Is this a Armstrong Oscillator circuit?
    Does anyone know how this will interact and authenticate with the car?
    Any help would be appreciated
    Thanks in advance.
     

    Attached Files:

  2. dave9

    dave9

    1,126
    307
    Mar 5, 2017
    I can't tell you if your circuit is right, but i can tell you that when the key transponder is powered by the magnetic field, it transmits a code that a computer verifies as valid then allows the ignition coil or other necessary vehicle components to function.

    All you have there is possibly a means to power the active circuit in the key, not the circuit itself nor what the code programmed into the vehicle as valid is, so it by itself doesn't emulate a key. Of course this may also depend on the specific vehicle design, but it is not going to be so simple as produce an output voltage and that's all that is needed.
     
    Rajinder likes this.
  3. Rajinder

    Rajinder

    531
    9
    Jan 30, 2016
    Hi,
    This is not the complete circuit, as there was a microcontroller on board. I was not sure how the authentication process is done i.e. how the code is transmitted to the key ignition and vice versa. Could it be some sort of ASK via the mutual coupling?
    Also I have not seen the LC circuit at the base of a transistor before, so not 100% sure on the operation entirely.
     
  4. dave9

    dave9

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    Mar 5, 2017
    I don't think it is in the best interest of car owners to help. You have no need to do this.
     
    Rajinder likes this.
  5. Rajinder

    Rajinder

    531
    9
    Jan 30, 2016
    I was only curious to see what was inside the key fob.
    Thanks
     
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