Finding An Active TX Pin On Cheap GPS

Twenty Euros will score you a small, self-contained GPS keychain. Crack that case open and you can have a lot more. [j3tstream] explored the guts of the thing and found that the NMEA data can be streamed out of the TX pin on the GPS chip.

First off, check out that miniscule GPS antenna module, crazy! But we digress. For testing purposes the asynchronous UART of the GPS was probed, proving that the data can be acquired. From there [j3tstream] moved to an Arduino Pro Mini with an SD card for data logging. The uC is powered from the GPS board but this will quickly exhaust the stock battery so [j3tstream] swapped it out for one from an old cellphone.

That little dot-matix LCD that comes with the unit also caught our eye. If you can hack a headless interface for the GPS that could be repurposed for your next project. May we suggest a wearable gaming project for it?

33 thoughts on “Finding An Active TX Pin On Cheap GPS

  1. holy crap real-time GPS trackers have gotten extremely cheap!! I started searching around ebay trying to find this device, but i found others that seem more interesting to me.
    check this out, less than $20:
    (by no means affiliated with this seller or product)

    Add a SIM that supports SMS and you can register a phone number and password. send commands via sms to retrieve current coordinates and change various configuration settings.
    If the SIM is provisioned with voice calling capability, you can even call this device and listen in with a built in mic, or have it call you when it detects a sound above 60 decibel….


    1. That one you linked to doesn’t have real GPS in it. It’s often hard to tell, but the keyword is LBS positioning. It uses Land Based Services (cell tower stuff) to do its thing. Also there’s no mention of what GPS chipset it uses.

    2. 15 years ago I paid a fortune (more than £200 IIRC) for a three sat PCMCIA card that took 45 minutes to get a fix. Since then it has lived and is about to die.

      The world is moving too fast, I want to get off please.

      1. You should sell it on ebay. I found a nice person of the “religion of peace” that paid more than I did new (in 1998) for a simple hiking gps with a serial IO. Ended up dropping a dime to dhs just to make sure as looking at the dude’s purchases, he wasn’t in it for hiking *touches finger to nose* -my csb out of the whole thing. We will end with me saying that his account was frozen in 36 hrs and he never returned to ebay. I doubt they let them use it in gitmo.

        BUT one of you ee types can maybe help me with this: What would/could someone do with the addition of a breathalyzer to a gps unit? That was the other odd purchase history-100s of them in single purchase form. Are they making some kinda flying pee detector like we sorta used in ‘nam or what? This could be lots of fun so any idea is a good one. Feel free to let me know :)

        1. No. I highly doubt that did happen, quite sure not the ‘religion of peace thing. Why would a ‘terrorist’ (yes, haha, you were only implying that) do stupid things like that? That you don’t like people with that religion does not instantly make them stupid.
          On the other hand, you posing a patriotic saviour, not unlikely. Perhaps you saw something on ebay that you did not understand (a company buying up out of production GPS chips that their product needs so they don’t have to redesign) and you played little snitch to the DHS. Do not make it more than that and certainly don’t be proud of yourself helping your country another step towards fascism/police state.
          If suspicious ebay users are enough to send someone of to gitmo (as you seem to imagine) then you certainly do not live in the ‘land of the free’ anymore.

          1. Wow. You are the one we need to worry about, apparently.
            A) It’s true
            B) Sorry you don’t read internets, sometimes we use things called memes. You probably would still be jizzing if they were kiddy diddling cat’lics…hypocrite
            C) Do you have any useful suppositions on the breathalyzers?
            D) Are you 17 or 18 years old? Such angst rarely lasts past wisdom.

  2. I want someone to write software that will directly read the NMEA data that streams in plain text via USB from the old DeLorme Earthmate LT-120.

    DeLorme’s old software only works on 32bit Win 9x or XP and requires a virtual RS232 port driver. Is this lazy programming or unnecessary complication? I used a USB probing program and saw the raw text NMEA data along with what appears to be other info added by DeLorme coming from the dongle.

    Is there a Windows GPS program that can DIRECTLY read NMEA data from a USB port instead of having to have a pointless fake RS232 emulation shoehorned into the works? If it uses a plugin model then it would only need a plugin to filter out any non-NMEA information from the DeLorme LT-20 and their other old GPS devices not supported past XP.

    Windows, 32 or 64 bit, will download and install a driver for the LT-20 but Windows just ignores it thereafter and apparently the data goes nowhere. In Device Manager it’s only visible by viewing devices by connection and it shows as a USB Input Device > HID-Compliant Device.

    1. I don’t know if it runs on Windows, but the gpsd daemon that underpins most of the GPS stuff on *nix is designed to process plain-text NMEA from UARTs (this being how 95% of off-the-shelf implementations seem to operate).

      Whether they’re USB UARTs or RS232 doesn’t matter much either way… it’s just a convenient way to get text streams into a PC in a semi-standard way, without having to reinvent the wheel with a new protocol each time. You can use raw HID of course, but when almost every other GPS device is pretending to be a serial port and software already exists for it, most integrators take the easy route :)

      1. What should work is a universal USB/HID to virtual RS232 driver with setup options to select which USB port to capture and set the COM port number. If it had some way to monitor the incoming USB data that would make it easy to figure out which port the GPS dongle is plugged into.

        As long as all map software needs is the raw incoming data from a GPS dongle, any map software that must connect to a real or virtual COM port would work with any USB GPS dongle – as long as the software doesn’t require any proprietary information inserted into the NMEA data stream.

    2. virtual serial is not pointless, its standard way of making serial communication accessible over usb
      there should be a win7/8 driver for that virtual serial somewhere

      what you describe is some hacked serial over hid kludge

      anyway, if you really “want” one I can write you a python winusb/libusb module for a small small price of few hundred dollars.

    1. Raw GPS module with antenna can be mail ordered for about $10 with “free shipping” from reseller instead of $40 from Spark&Fun. Someone in China can certainly get better pricing for a production run.

      Seeing that they can make decent looking ipod clones i.e. case, LCD, battery, some cheap uC in the $10 with “free shipping”, there is plenty of room in the $40 price point to make a GPS gadget.

  3. interesting, looks like Venus638FLPx has fully programmable (flash memory inside) core, few trackers people listed (on DX and ebay) all have different UIs, meaning they all have custom firmwares.
    I wonder if there is chinese SDK leaked somewhere.

    1. Yes, probably there is, but it is not leaked. Copyright is lax in China, but people there know the advantages of sharing as well. So instead of formal open source licences, you can get the code if you know the right people. Or at least, that is how I understand it, read Bunnie Huang’s blog if you want to hear it from the horses'(bunnies’?) head.

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