AJ’s Invisible Infrared LED Geocache

April 10, 2012 admin Cache TypesMulti-Cache

This multi-cache uses Infrared LEDs which can’t be seen by the naked eye. You can however see them by using a camera or other device such as a cell phone camera which is sensitive to Infrared light.

Enegizer Crank FlashlightFirst a power source is needed. I picked up a crank flashlight from Walmart for  about $8.00 and gutted it for its dynamo. A dynamo is a type of power generator that outputs voltage when turned.  

Charging circuit

There is also a charging circuit in the flash light which regulates the voltage to the LEDs to 1.2 volts. The circuitry includes 6 diodes for the dynamo to ensure no voltage coming in from the dynamo is being wasted. A storage cell to retain the power being generated (Green) and a 1.2v zener diode to limit the voltage from the 3.5v battery. You may notice three wires coming off the circuit board above.  The white wire is ground, the red wire supplies ~1.2v to the parallel infrared LED array, and the red/white striped wire is battery voltage.
Blinking LED eyesWhen the striped wire voltage becomes greater than 3v it powers two self blinking LEDs that are wired in series. This lets the user know that the system is near full charge. The remaining spaces consist of wired and un-wired (filler) LEDs.

IR leds

These LED’s will light with the three digit number to unlock the combination lock later in the cache. They show up purple in the video and photos when powered on but to the naked eye alone they cannot be seen. The rest of the LED’s are soldered open to the back of the breadboard so the user cant guess the number from shape alone. The red wire connects to a momentary push button before it goes to the IR LED array. When the button is pushed the the voltage is dumped into the IR LED’s.

At this point the project is a ball of wires. It needs to be put into some kind of enclosure that would keep it safe and make it look cool.

milling the geocache box AJ came across this nice aged box. As nice as the box was with it’s metal hinges and latch, some modifications had to be made. One of which was to mill the edge thinner to accommodate the short crank handle. Two holes were also drilled, one for the crank and the other for the momentary push button. Circuit board ledsPlaned 2×4 wood was glued to the sides to hold the plexiglass cover and LED array. The dynamo crank was also glued to the wood for support. The charging circuit, LED array and wires were also hot glued into place. The left bottom side of wood needed filled in due to cutting the side wall shallow for the crank. It was sanded down to match the original side height.

LED matrix boxNext a tinted piece of plexiglass was caulked into place keeping the circuitry safe from the elements and mostly peoples curiosity. Caulk was used instead of hot glue in case the unit would ever need worked on. A razor blade and  screw driver could open it right up. Lastly and eye was painted on the plexiglass as to match the caches description. Acrylic paints were used and Top Coat sealer was used to keep it fresh looking.Painted Eye on Cache The LEDs cannot be seen through the tinted plexiglass until the dynamo is turned with the hand crank. The color scheme matches the age and style of the old wooden box. This is what the box looks like closed with the hand crank inserted.


Crazy Geocache BoxEvil Gocache Box

Dyno With CrankCache Box Size comparison

antiqueDynamoelectricevil geocachehand crankinfraredledoldsteampunkwooden

5 Responses to “AJ’s Invisible Infrared LED Geocache”

  • [...] [AJ] and [Brian] are making sure the geocache challenges they set up take some ingenuity to solve. They’ve just rolled out a two-part cache which uses a code hidden in infrared light. [...]

  • [...] [AJ] and [Brian] are making sure the geocache challenges they set up take some ingenuity to solve. They’ve just rolled out a two-part cache which uses a code hidden in infrared light. [...]

  • Robotguy says:

    This is great! I’ve been interested in these types of physical puzzles for a while now, but had a hard time getting any traction because geocachers in my area tend to avoid puzzle caches. I thought about applying the ideas to an ARG, but that limits things to local participation, so the ARG guys tend to avoid that.

    I have considered setting up a members-only forum or an IRC channel for discussing physical puzzles of this type (so players can’t just search Google for the answer). It would be nice to be able to share ideas and maybe even hardware so that players in different locations can experience these types of puzzles (i.e. everyone builds N copies of their puzzle, then exchange).Would anyone be interested in such a discussion?

    • WaltMcK says:

      I’ve been building microcontroller based caches for a couple of years now and placing them around Rancho Cordova, CA. There is a fan base that really likes them, but they are fairly difficult so there aren’t a lot of caual geocachers that attempt the.

      The traction problem is also in finding others that are doing this kind of thing. There is a book mark list on geocaching.com where the owner is trying to bookmark the ones he knows about. There are a fair number on this. I started a thread in the geocaching forums, but there isn’t much activity on it.

      I really like the dynao idea. Powering these things is a big problem.

  • Vballhjs says:

    looking for more of these… tried looking in the forums on geocaching.com w/out much luck… any links you guys can provide?

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