Reverse engineering Jacob Jensen P-watch: Part 1

by Mik on March 15th, 2011

Jacob Jensen's P-watchI should actually have started this serie one month ago, as I started looking into it there. But better late than never, so here it goes:

This christmas I got this Jacob Jensen’s P-watch as a gift and thought about whether or not to install it in my car. I decided to try it out – but after three weeks of hell I took it out and bought an old manual P-watch. The automatic P-watch works by measuring vibrations of the car and deciding the parking state from the internal algorithm. However it does NOT work as originally thought! Every single day when I stop at the end of the highway, the parking watch beeps and think that I have parked – no, I’m not just saying every day for fun. It was every, single, day when I stopped at the end of the highway .. I could just sit in my car and say “now it’s gonna beep” – and it would. Beside the wrong parkings the people behind the electronic P-watch have decided to “lock” the P-watch after a while, so you cannot manually set it. So often when I was out driving and needed to park I had to reset the P-watch, manually set the time correct and then manually set the parking time afterwards! This is NOT good for a parking watch costing you almost 200$.

I wrote to Jacob Jensen about my 90% error rate and irritation. I got redirected to the manufactoring company (from the design-company) and was promised an answer in a very short time – now almost two months later I have still not received the response.

So what to do with a shit-watch that is to more annoyance than help? Well.. if I could juuuust reverse engineer it :-)

  1. Hello fellow hacker.. you are on hack a day. Tillykke!

  2. Hi,

    Nice job you did, did you use any RTC to keep time? Or is it just the GPS data for that?
    I’ve thought of this, but never made it so far ;-)

    Anyways, good job!


  3. Hi!

    I used a 32.768Hz external crystal to keep time on the parking disc and the internal 8MHz to run the code (actually the disc doesn’t depend on the GM862-module). However as a feature if you press the up+set button simultaneously, the clock will “request” the real-time clock over rx/tx from the GM862 and the GM862 will respond to this request with the GSM-clock. This way the parking watch will be working even if the GM862-module is turned off :-) .. If you want you can also set the time manually on the parking disc by holding the set button (just as you set every other digital watch!) ;-)


  4. Well, I just don’t trust those xtals.

    I need the time to come from a product i can trust,
    cause when u use a crystal of 32.768 Hz, the coupling capacitors can change this value (not much, I know, but they’re able to).

    That’s why I was asking.

    Thanks for the response.


  5. That’s right and that’s why I have the GSM-time-feature built in. However if you look at my schematics (part 6), there is no coupling capacitors for my crystal – if that’s what you meant! But yes .. if you have the clock running non-stop for a long time, you need to consider this :-)

  6. Sindijs permalink


    I would like to suggest a new automatic parking timer. It work without any error and its stable. I use it almost 6 month and without any problems. This device measure electromagnetic field of car’s engine. So in that way device know when to to turn on time on screen. For more information search in internet for site parkingtimer [dot] eu.

    Hope this help!

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