Captain Crunch Geocache

Construction Guide

[   by LinuxOnTheBrain   ]


    This is an electronic geocache with an audio triggered lock inspired by John Draper, a.k.a. 'Captain Crunch'. In the early 1970's John and his phone phreaker friends discovered that a toy whistle being included in boxes of Cap'n Crunch® cereal played a tone which could be used to control the (now retired) in-band signaling long distance telephone network. The phone companies used this tone to control long distance line connections, when played it would disconnect one end of the call leaving the remaining caller in Operator Mode. John and his friends quickly realized that Operator Mode = Free Phone Calls and the rest is telecom history.
    To open this geocache the user has to figure out the frequency of the toy whistle (variable frequency setting), then record a sample of that tone and bring it to the cache. At the cache the user must find the 'key' located on the cache, insert the key into the keyhole to power it up then play the pre-recorded tone to open the lock.
    Most of the materials used to build this unit were recycled from something else or I fashioned them myself from scraps. (To be earth friendly of course). I hope this inspires others to build electronic geocaches since disposable electronics are everywhere and can be easily reused. Feedback is appreciated. Thanks and enjoy!

Table Of Contents

    Legal Notice
    Materials List
        All Other Materials
    The Electronics Pack
        Hacking The Voice Module
        Circuit Assembly
    The Casing
    The Center
    The Tall Side Back
    The Short Side Back
    The Short Side Front
    The Gearbox
    The Door
    The Short Side Lid
    The Tall Side Lid
    Assembly And Testing
    Final Assembly
    Recommended Changes


    Please read this document completely before beginning the project. The builder needs to be aware of a number of things to avoid any problems and to take advantage of the recommended changes. It should be noted first that because most of the materials used were recycled the builder is encouraged to use what's creative. Also important is that many of the measurements used in the design documents were taken indirectly (inferred) because they were taken after assembly so please double-check measurements and fittings during construction.
    Distributed with the construction guide pack are the original Xfig design documents. Xfig is a wonderful open-source (free) program similar to CAD and is available primarily for Linux. There is also a ported version for Windows. Xfig is an excellent tool for anybody interested in drafting project designs. If errors are found please let me know so the construction pack can be corrected.

Legal Notice

(I don't know if I really need this but just in case here's one that works...)
This construction guide and its contents are distributed under the GPL license:
Captain Crunch Electronic Geocache

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program. If not, see <>.

Materials List

    Below are the materials that were used to construct this geocache. Replace with materials with you have where possible and/or appropriate.

Electronics Materials
Quantity Item Cache Location Notes URL
1 Electret microphone Electronics pack A standard pc mic works nicely. Link
1 470 ohm 1/4W resistor Electronics pack Link
3 2.2k ohm 1/4W resistor Electronics pack Link
4 10k ohm 1/4W resistor Electronics pack Link
3 100k ohm 1/4W resistor Electronics pack Link
1 5k ohm linear taper potentiometer Electronics pack Link
3 3mm LED Electronics pack Link
3 BC-170C npn general purpose transistor Electronics pack A 2N2222 might also work. Link
1 330 micro-farad capacitor Electronics pack Link
4 10 micro-farad capacitor Electronics pack Link
1 4.7 micro-farad capacitor Electronics pack Link
1 2.2 micro-farad capacitor Electronics pack Link
2 0.1 micro-farad capacitor Electronics pack Link
1 0.022 micro-farad capacitor Electronics pack Link
1 5VDC reed relay Electronics pack Link
1 Low-noise 6VDC motor with gear Gearbox Radio Shack hobby motors are very noisy.
1 Momentary contact SPST push button switch Horizontal front switch Link
1 SPDT switch with rolling lever Vertical bottom switch Link
1 Momentary contact DPDT push button switch Door switch
1 Radio Shack voice record/playback module Electronics pack Link
1 LM567CM tone detector chip Electronics pack Free samples available. Link
1 4-AA battery carriage Electronics pack Link
4 AA alkaline batteries Electronics pack
1 Project Enclosure (3x2x1") Electronics pack Link
1 Grid-style PC board with solder ringed holes Electronics pack