Building an anchor watch

It's not only Marie who has many good recipes, I do as well! Here is how to bake an "anchor watch", which is a small device telling you, when you are anchoring, that your anchor has lost grip and that your boat starts drifting.

In other words, this device wakes you at night with a horrible sound if the boat drifts out of the circle whose centre is your anchor, and whose radius is roughly the length of chain you have been pulling out. Having such a device makes us sleep much better (especially Marie, with a factor 2 she says) since we know that we'll be warned if something goes wrong.

Ingredients:
- our hoover's batteries (or any 12V source of power, your boat batteries for instance),
- a cheap car GPS antenna with USB plug
- a couple of small electronic components: one NPN transistor, one 12V buzzer, one 1kOhm resistor
- the magic ingredient: a Raspberry Pi, a small computer of the size of a cigarette package, a that costs only $35

Recipe:
- The Raspberry Pi receives the position of the boat through the GPS antenna, and calculates the distance to the anchor (specified) using a simplified version of the Haversine formula for small distances. It deduces if the boat has drifted or not.
- If she did, the voltage of one of the GPIO pins (simple electronic interface of the Raspberry Pi), goes from 0V to 3.3V. And that's where the NPN transistor lets the current pass through the buzzer and...
- BEEEEEEEEEEEEEEEEEEEEEEEEP. Wake up and put your boots on sailor, it's time for some action!

Picture of the final product (well, that's a prototype, OK?)

"Chuck Norris approved" during the night of June 9th 2013, in a 35knots gale!