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Digital Electronics : Practical

A cache by mjouk Send Message to Owner Message this owner
Hidden : 11/8/2012
3 out of 5
5 out of 5

Size: Size: regular (regular)

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Geocache Description:

A puzzle cache which you can only solve by building a small electronic circuit on a breadboard.

Caveat cacher

The coordinates above point to the middle of a field. There is nothing to see there.


Congratulations to TelemachusGB for being First To Find.

Digital Electronics : Practice

Although it's fun to play with electronics on paper, it's even more enjoyable to actually build some hardware.

To solve this puzzle, you'll need to build the simple co-ordinate decoder. Happily you'll find a kit containing the parts you'll need in Digital Electronics : Theory. Originally there were three kits in the cache, but some might be in use. In the best tradition of these things batteries are not included so you'll need a couple of AA cells.

Please do not open the kit of parts in the field in case they fall out and get lost. Instead, it's probably best to take the kit home and build it there. The kits have a few spare parts in them, so don't worry if you have some left over.

If you've solved the Theory puzzle but are unable to get a kit, please contact the cache owner, particularly if you're coming from afar.

A finished decoder

Once built, the decoder turns 8-digit hexadecimal numbers into pairs of coordinates.

When you've solved the puzzle, please disassemble the decoder and put the kit back where you found it so that others can use it.


To build the decoder just follow these instructions:

You should pay particular care to put the chip and the battery the right way round.

At stage 19, you should install a capacitor and not an orange wire.

If you've not done this sort of thing before, you might find the following notes helpful:

  • You will not need a soldering iron, nor any knives or cutters, though a small screwdriver might help you disassemble the kit afterwards.
  • The components need only to be gently pushed into place on the breadboard.
  • You will need to take care that you put the components in the right place, and that you don't bend any pins.

It will probably take you about 15 to 30 minutes to build the decoder.


To test the decoder enter F29A3969 : you should get N 52° 10.000 E 000° 13.000.

You'll have to work out how to enter the number into the decoder, but hopefully that won't be too taxing!

If it doesn't work, check your connections. If that doesn't fix it, then please contact the cache owner.


Finding the final cache is a two-stage affair:

  1. Decode FC5A5F87 with the decoder and visit the coordinates. You should find a new 8-digit code there.
  2. Decode the new code: these are the coordinates of the final cache.

In the hope that you'll want to do more of this sort of thing, there's an Arduino Uno for the FTF.

You can check both sets of coordinates on geochecker:

Please remember to return the kit to the Theory cache.

For the curious

The decoder schematic is shown below:

As you'd probably expect all the work is done by a micro-controller, specifically a Microchip PIC 16F886. The PIC simply encrypts the 32-bit input (with a 32-bit variant of skipjack) and treats the output as two 16-bit numbers. These can conveniently be interpreted as milli-minutes from some offset.

If you'd like more information, please feel free to contact the cache owner.

Additional Hints (Decrypt)

Uvagf sbe svaqvat gur pnpur ner vapyhqrq va gur trbpurpxre.

Decryption Key


(letter above equals below, and vice versa)



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