In this clip the Chernobyl Dice set to continuously generate numbers in the range [1,26]. You
can check out a longer YouTube clip showing various modes of here:



The Chernobyl Dice is a quantum random number generator [0] that uses nuclear decays from a weakly radioactive sample
as a source of entropy. It consists of four primary components:

  • An Arduino Nano microcontroller
  • A Geiger counter
  • Six uranium glass marbles
  • Nixie tube display

Geiger counter events (“clicks”) are converted into random bits using the following algorithm:

  1. In a ring buffer, for each millisecond record either a 0 or a 1, depending on whether or not a Geiger event occurred
  2. Perform an initial de-bias of this 0-dominated stream using von Neumann’s method [0]
  3. Further de-bias by XOR-ing bits generated in the previous step with the mod2 of elapsed 4 microsecond intervals since
    since the device was powered on

The uranium glass sample is illuminated by an array of ultraviolet LEDs at each Geiger event, which makes them
fluoresce bright green. This has nothing to do with the radioactivity of the sample, but it does, however, look
really cool


The device has three modes of operations, which can be selected via the rotary switch:

Clock Mode

Displays the current time, with the Geiger board unpowered. The time can be set by flipping the toggle switches on and off
(the ’16’ toggle increments the hour, the ‘8’ toggle increments 10 minutes, the ‘4’ toggle increments 1 minute, and the
‘1’ toggle resets the seconds).

Streaming Mode

Repeatedly generates random numbers of a size specified by the toggles (or random bytes from 0-255 if no toggles are set). Numbers
generated in this mode are transmitted over serial via USB. This mode also has a “turbo” setting to facilitate statistical testing,
which can be enabled by holding down the pushbutton. When “turbo” is enabled bit generation will be indicated by LEDs in the
display, and the Geiger “clicks” will be silent.

Dice Mode

In dice mode random number generation is initiated via the pushbutton, and the size of the random number to be generated is set
by the sum of the toggled switches (no switches are set the device will generate random byte in the range 0-255). Press
once to generate the number, and once again to clear the display. The size of the number to be generated is displayed in
blinking digits.

Statistical Performance

Further testing is required to confirm the consistency of results, but currently the Chernobyl Dice is capable of generating a
1.5+ megabit file that passes a Python implementation of the NIST statistical
test suite [1]. This means the Chernolbyl Dice is likely a very fair dice.

monobit_test                             0.279698915238     PASS
frequency_within_block_test              0.404035783453     PASS
runs_test                                0.0688862287393    PASS
longest_run_ones_in_a_block_test         0.959135200804     PASS
binary_matrix_rank_test                  0.532456847429     PASS
dft_test                                 0.000155432528185  FAIL
non_overlapping_template_matching_test   0.999998184707     PASS
overlapping_template_matching_test       0.55898054206      PASS
maurers_universal_test                   0.224223569722     PASS
linear_complexity_test                   0.672504584189     PASS
serial_test                              0.424389760139     PASS
approximate_entropy_test                 0.425255814114     PASS
cumulative_sums_test                     0.348500456103     PASS
random_excursion_test                    0.0753308212732    PASS
random_excursion_variant_test            0.207160448911     PASS
monobit_test                             0.105735760191     PASS
frequency_within_block_test              0.436487225319     PASS
runs_test                                0.648059641506     PASS
longest_run_ones_in_a_block_test         0.184787158208     PASS
binary_matrix_rank_test                  0.310400523277     PASS
dft_test                                 0.156504142574     PASS
non_overlapping_template_matching_test   1.00000015958      PASS
overlapping_template_matching_test       0.629208901365     PASS
maurers_universal_test                   0.938296605093     PASS
linear_complexity_test                   0.0880093291441    PASS
serial_test                              0.131826155057     PASS
approximate_entropy_test                 0.137234909215     PASS
cumulative_sums_test                     0.112328349057     PASS
random_excursion_test                    0.17207234069      PASS
random_excursion_variant_test            0.299605480729     PASS

Statistical test suite results for two 212667 byte random binary sequences (‘rand.binary.1’ and ‘rand.binary.2’)
generated by device. EXPERTS NOTE: The last two tests of the first sequence generate the message: ‘J too small
(J=181 < 500) for result to be reliable' so these tests may be ignored for that dataset.

The directory statistical_testing contains some utilities for assessing and gathering data from the device, including sample
random binary data files, rand.binary.1 and rand.binary.2.

CAD Drawing

A Fusion 360 CAD drawing of the device can be viewed and downloaded at this URL:

Thanks to the following GrabCAD users for their models:

Overview and Parts List

WARNING: These instructions and resources are not polished and are somewhat untested. This is a project for advanced makers, and
you should fully expect to have a bit of an adventure while building your own Chernobyl Dice! That said: Shoot me a message if
you run into trouble, and I’ll try to help you out and improve the instructions as well.

Here’s a rough outline of the steps required for assembly:

  1. Print or fabricate the following custom parts from files in the GitHub repository

    • Enclosure (using 3D printer or 3D printing service)
    • Logic, Nixie Display, and Control Panel Custom PCBs (using a board fabrication service such as OSHPark)
    • Stainless steel front panel (using a service such as OSHCut)
    • Acrylic back panel (using a service such as Sculpeo)
  2. Order other components (see URLs in the parts list)

  3. Embed brass standoffs into enclosure (this is how front and back panels and custom PCBs will be mounted)

    • TIPS
      • A single drop of cyanocrylate (superglue) should be placed in the standoff holes in the enclosure before they are pressed
        in (the tip of a Phillips head screw driver works well for this task)
      • If the tops some holes came out of the printer a bit distorted then they can be widened easily by twisting a largish
        Phillips head screwdriver in them until the top of the hole is wide enough.
  4. Assemble custom PCBs and ‘exixe’ nixie tube driver boards using build photos as a guide

    • TIPS
      • The female headers on the Nixie Display Board for the nixie tube driver PCBs can be assembled using only four long female
        header strips (e.g. there’s no need to separately attach sixteen 7-pin female headers—see photos)
      • The “CS” 8-pin male header is a currently a tight fit, so you will probably need to use needle noise pliers to push the pins
        through the board
      • For the control panel PCB you can temporarily mount the toggle switches in the front panel and then press the PCB on top of
        the back of the switches to ensure good alignment—you can then pull the whole board-and-toggle assembly off of the front
        panel to solder it, or even solder it in place and then pull it off
      • The file ‘nixie_holder.stl’ is a provided as a useful place to rest the nixie tube while soldering them to the ‘exixe’ driver
  5. Attach the acrylic back panel and panel mount USB cable to the rear of the enclosure

  6. Mount custom PCBs inside enclosure and perform wiring (see wiring schematic and build photos)

  7. Partially dis-assemble rotary switch to attach it to the “lock plate” (lock_plate.stl)

  8. Perform any necessary finishing to the stainless steel front panel

    • TIPS
      • Some mounting holes for toggles and switches may need to be widened (use a round file for this)
      • Surface finish of a laser cut part can be improved by sanding with the grain using ~300 grit sandpaper (test on the back
        side first)
  9. Mount toggle-and-PCB assembly, LED-with-holder assemblies, rotary-switch-and-lock-plate assembly, and pushbutton on front panel

    • TIP
      • While attaching the LED to the holder, apply a drop of cyanocrylate (superglue) to prevent the LED from falling out of the
        front of the holder
  10. Perform wiring of the control panel (see wiring schematic and build photos)

  11. Fit the eight nixie display driver boards into the female headers of the Nixie Display Board

  12. Wire the Control Panel to the Logic Board

  13. Mount the front panel to the enclosure, taking care to insure that the hole in the rotary switch lock plate lines up with the
    upper-left standoff on the front of the radioactive sample holder

Standoff Size Guide

Location Standoff Length Type
Rear of enclosure 10 mm Female-Female
Front of enclosure 10 mm Female-Female
Nixie Display Board mount 40 mm Female-Female
Logic Board mount 20 mm Female-Female
Logic Board-to-Geiger Board 30 mm Male-Female
Geiger Board-to-Bottom of Uranium Sample Box 15 mm Male-Female
Bottom of Uranium Sample Box-to-Top of Uranium Sample Box 6 mm Male-Female
Top of Uranium Sample Box-to-Front Panel 6 mm Male-Female

NOTE: For large internal standoff distances (e.g. the Nixie Display mount) two standoffs can be stacked to achieve
the desired distance.

Wiring Diagram

How connect the internal wiring of the Chernobyl Dice.

Build Photos

Enclosure with Standoffs Inserted

You can ignore the white breadboards on the left of the enclosure. These assisted in an earlier iteration of the
project and are not needed for assembly.
Click here
for larger photo.

Logic Board Mounted

Note the wires for UV LED array are run beneath the logic board.
Click here
for larger photo.

Nixie Display Board Wiring

Click here
for larger photo.

Nixie Display Board Mounted and Logic Board Wiring

Click here
for larger photo.

Geiger Board Mounted

NOTE: You must pull the JMP2 jumper near the center of the Geiger board (this turns off the built-in speaker of the
board—we want these clicks to be optional and controlled by firmware instead).
Click here
for larger photo.

Uranium Sample Holder Lower Half Detail

UV LEDs and piezo speaker are glued mounted into place using cyanocrylate (superglue) adhesive. Uranium glass
marbles will be held in place mechanically when the top half is mounted.
Click here
for larger photo.

Uranium Sample Holder Lower Half Mounted

The middle standoff is attached on the underside with a nut.
Click here
for larger photo.

Uranium Sample Holder Top Half Mounted

Click here
for larger photo.

Control Panel Board Wired

Click here
for larger photo.

Control Panel Board Wiring Detail

Click here
for larger photo.


Many thanks to Emily Velasco (@MLE_Online) for advice on stainless steel surface sanding,
and for generally being enthusiastic as heck about this project.


[0] “Quantum Random Number Generators.” M. Herrero-Collantes and J. C. Garcia-Escartin.

[1] “A Statistical Test Suite for Random and Pseudorandom Number Generators for Cryptographic Applications.”

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