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Newb Needs Some Guidance

Discussion in 'General Electronics Discussion' started by PhilB, Oct 29, 2014.

  1. PhilB

    PhilB

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    Oct 29, 2014
    Hello,

    I'm new to this and trying to learn about how best to approach what I think is a relatively simple project. Wondering if anyone here can help provide some pointers to get me started.

    I have a 315Mhz transmitter, it sends a standard (non changing) 4 digit code + a rolling sequence number.

    I need to build something that can receive the transmission from this device and display back to me the codes that are being transmitted.

    I've been searching the web for some help and found various hardware devices that seem to be geared mostly towards residential garage door uses, to essentially setup repeaters and similar configurations. But nothing yet where code readout and display is involved.

    Can anyone help point me in a direction to get started? Once I start I'll probably have more questions.

    Thank you,
    Phil
     
  2. hevans1944

    hevans1944 Hop - AC8NS

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    Why do you want to see the codes that are being transmitted? As the datasheet explains, these codes change for each exchange of data from transmitter (encoder) to a paired receiver (decoder). The encoder in the transmitter is initially paired with the decoder in the receiver by the manufacturer or dealer. Certain measures are taken to prevent a second receiver from intercepting a code sequence sent from a first transmitter and re-transmitting that code to operate the first (paired) receiver. If you must re-pair a receiver with a transmitter, and there are only a few situations where this is necessary, the datasheet provides instructions on how to do that.
     
    Gryd3 likes this.
  3. PhilB

    PhilB

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    Oct 29, 2014
    No data sheet available. This is an automotive application, I can buy the code I need from the manufacturer - for A LOT of money - so I'm trying to find a way to obtain it from the transmitter.

    Thanks.
     
  4. hevans1944

    hevans1944 Hop - AC8NS

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    I provided a link to the Texas Instruments TRC1300 and TRC1315 MARCSTAR I E/D remote control encoder/decoders datasheet. Are you not using this particular IC in your key fob transmitter?

    TI realized that it is sometimes necessary to pair these devices in the field after they are paired by the automobile manufacturer, so information is provided in the datasheet on how to do this. The procedure is complicated, which may help to explain why replacement key fobs are pricey. I doubt that the "code" you would receive by intercepting the transmitter output would be useful since TI specifically designed the IC to defeat such "over the air" attempts. If you still want to try, a 315 MHz receiver and a digital logic analyzer should allow you to record the data stream to a laptop computer.
     
  5. PhilB

    PhilB

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    Oct 29, 2014
    Apologies, I didn't realize you included a link in your initial post, I thought you were asking me if I had access to my transmitter's datasheet. You (and the datasheet) are correct, the code "rolls" with each press of the fob button, to avoid someone watching/listening and coming back later on to take the car with their own transmitter. The digital logic analyzer link you provided looks interesting, but given the rolling code I wonder if I should stop considering the transmitter as a possible source of the code, and consider the car itself.

    The car has an ECU that disables the immobilizer once it receives the signal from the transmitter, OR, when it receives the signal from the ignition key when the key is used in such a way as to "enter" the code directly into the car. The way this is done is that the key is flipped on/off for each number in the code, with "on" being "Accessory Mode" in the car (radio, wipers, etc, but motor is off). For example, if the code is 1-2-3, insert key, turn on/off once, wait 3 seconds, turn the key on/off twice, wait 3 seconds, etc.

    The ignition cylinder is essentially a "switch", insert the correct key and the tumblers set correctly to enable the user to turn the key and start the car - closing a 12 volt circuit. If I find the right 2 wires, I could create the circuit myself without the need of a key. So if I build a "black box" with some logic and led displays, this black box could mimic the turning of the key by closing the circuit of the two ignition wires for every sequence from 001 through 999 - and it can do it a lot faster than me sitting in the car flipping the key. I'd have to figure out a way on having the black box stop entering codes once the right one is found, and to display the working code, which is the whole point of this exercise.

    I have the idea, I can find the right two wires, I have a programming background (it's been years), but I don't know where/how to start with assembling this "black box".
     
  6. hevans1944

    hevans1944 Hop - AC8NS

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    If your automobile does in fact use the Texas Instruments encoder/decoder, there are way more than 999 codes. The key fob transmits a 40-bit code, which is something north of one trillion different codes.

    Since you have access to the fob and the car, there should be a way to pair the two of them again. As I mentioned earlier, the datasheet does state there is a way to do this. It involves placing the fob in learn mode to store a new 40-bit code and then placing the decoder (wherever that is!) in learn mode to receive and store the 40-bit code from the fob.

    I know nothing about manipulating the ignition key to enter a code. Where did you find that information? The owner's manual for the car?
     
  7. PhilB

    PhilB

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    Oct 29, 2014
    Yes, the owners manual. The car wants one of two inputs to disable the immobilizer: the 3 digit code + the sequence numbers that are transmitted by the fob; or the three digit code entered from the ignition key.

    I have multiple working fobs, and multiple ignition keys. The whole operation works great. But, if I should ever loose or break my fobs (which are now about 20+ years old), without the 3 digit code my car will essentially become a good looking planter box for some ferns.

    All I want is the code. But I don't want to pay the manufacturer four figures for it.
     
  8. Gryd3

    Gryd3

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    Jun 25, 2014
    What possible car do you have that uses these keyfobs at 20+ years old?
    Go take a stroll by some car audio shops and see if they have an 'alarm' module that will interface with your ECU. This would replace the factory security with aftermarket security and let you use your wheels without the original key-fob.

    An additional note, is that many times these keys have RFID tags, or other passive components inside that interface with an additional circuit within/near the lock cylinder. You may need to worry about that too.
     
  9. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    I don't think it requires "four figures" to replace or re-program an inoperative key fob. See this website for example of an on-line service.
     
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