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Car Key Transponder

Rajinder

Jan 30, 2016
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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.
 

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dave9

Mar 5, 2017
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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

Jan 30, 2016
568
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Jan 30, 2016
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568
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.
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.
 

dave9

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