I enjoy both Geocaching and electronics and have made a couple of electronic puzzle caches using an Arduino.
My latest creation (with the help of a fellow cacher for the code) is using an Arduino and RFID reader to make a puzzle geocache we call RFIDuino.

The GeoCacher takes RFIDuino and has to find 3 RFID tags in order to get the location of the final geocache.
They are given the coordinates of the first tag, when they find and scan it they are given the coordinates of the second tag - and so on until they find the final.  After they find the final cache and sign the logbook RFIDuino is returned to the original hiding place for the next person.
Even if you don't geocache you could make your own RFID reader similar to the one in this Instructable and use it for a treasure hunt game.  It could be adapted to have a locked lid operated by a servo similar to the "reverse geocache box".

Step 1: Parts List

You will need the following:

- Arduino of some kind.  I have been building my own on stripboard, but recently bought some "Evil Mad  Scientist" ATmegaxx8 Target Boards to use in future projects.  If you build your own you should add a "protection diode" on the + side of the input voltage.  An IN4001 or similar will work.
- Parallax RFID Reader Module
- HD44780 compatible LCD display.  I find a black on yellow works best in daylight vs a blue on white. I get mine from DipMicro
- Small 10K trim-pot for the display contrast.
- 5v Boost Regulator For this project I wanted 2x AA to power the device as most geocachers carry them.  However with the code I use they drain in about 1 1/2 hours, so a 4x AA may be a better option
- hook up wire.  I use wire from cat5 stranded patch cable.
- case of some sort.  I used a clear blue case found at most electronics stores.
<p>I am new at building arduino projects, could you send me the schematic for this project if possible? It sounds cool and I would really like to have on in the area.</p>
<p>I'll see what I can come up with this weekend</p>
<p>I'd like to use a funduino RFID-RC522 board, instead of the parallax. I'm not clear if it will run on only the signal out pin (it has miso, mosi, sck, ss, and rst).</p><p>Your instructable is quite clear, and understandable. Perhaps I should just buy a parallax setup. </p>
Pin 14 to Arduino digital pin 12 (DS7) <br>Pin 14 to 5V (backlight +) <br>???
Fixed!, thanks
Hmmm, OK my mistake: <br> <br>pin 15 to 5v (backlight +) <br>pin 16 to ground via ~100 ohm resistor (backlight -)
I like what you did here. My family and I also enjoy Geocaching, and I may build one like this of my own. I would incorporate a solar charger and charge controller, but either way this is a great idea and a fun twist to the sport. Thanks for sharing!

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