Here's some folks who did it using a lava lamp!
   [1]http://www.lavarnd.org/
   Obviously I wouldn't take this too seriously, but could be a fun
   weekend hack.

   On Mon, Sep 9, 2013 at 12:58 PM, Bill Stewart
   <[2]bill.stewart@pobox.com> wrote:

   At 12:32 PM 9/9/2013, Juan Garofalo wrote:

     very naive question here :
     Wouldn't it be possible to build a RNG using something like a zener
     diode and a $2 microcontroller?

     Sure, and if you like playing with Arduinos and similar electronics,
     it'd be a good experiment.
     The main issues you run into (besides getting a decent fast USB
     interface without having to use surface-mount electronics, which are
     annoying to solder),
     are validating the quality of the randomness and getting enough
     speed to be useful (which depends not only on your noise quality but
     also on whether you're using a USB-enabled chip or just bit-banging.
     DieHard [3]http://en.wikipedia.org/wiki/Diehard_tests is probably
     still the standard quality test - your noise is almost certain to be
     biased, rather than purely uniform, so you'll need to whiten the
     data and adjust your entropy estimates appropriately.

   --
   Rich Jones
   OpenWatch is a global investigative network using mobile technology to
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References

   1. http://www.lavarnd.org/
   2. mailto:bill.stewart@pobox.com
   3. http://en.wikipedia.org/wiki/Diehard_tests
   4. https://itunes.apple.com/us/app/openwatch-social-muckraking/id642680756?ls=1&mt=8
   5. https://play.google.com/store/apps/details?id=org.ale.openwatch&hl=en