This is just a fun project I have been working on in the last little while. If you like this instructable, consider voting for it in the Arduino Challenge!

Anyone who has experience with programming will have probably used random numbers in their code. These random numbers aren't actually random though. They are actually generated using an algorithm that produces numbers that appear to be random, but the numbers actually follow a sequence. While these "pseudo-random" numbers are fine for some purposes, they just won't do for others. Cryptography is a case where the predictability of pseudo-random would allow someone to break codes with ease.

True random numbers on the other hand rely on quantum phenomena, or chaotic systems. Quantum systems include things like radioactive decay, and shot noise in electronic circuits, and are fundamentally random processes. Chaotic systems are things like atmospheric noise, which is so chaotic, that it can effectively be used as a source of randomness.

I wanted to build a true random number generator just for fun, and to see how well it would work considering that it is fairly cheap and easy to set up.

Step 1: Materials

Arduino - ~$30

MightyOhm geiger counter - $100 without case, $115 with acrylic case (or any geiger counter with a pulse out)
http://mightyohm.com/blog/products/geiger-counter/

22 gauge solid core copper wire - $4 (for a whole roll. You can probably even use some multi-strand wire)

3 pin female header - found for free

salt substitute (optional) - $4

Step 2: Assemble the Geiger Counter

A geiger counter is a device that is capable of detecting radioactive particles. A particle passes through the geiger tube and ionizes the gas inside. The tube is connected to a high voltage supply, and the ionized gas allows a small current to flow. This pulse is detected, indicating that a radioactive decay has happened. Because it is impossible to predict when a radioactive atom will decay, we can use the time between pulses to generate random bits.

I bought my geiger counter from mighty ohm (www.mightyohm.com). It comes as a kit, so you need to solder it up yourself. It didn't take me long to put it together and it worked perfectly on the first try. Full assembly instructions can be found on their website. Although the geiger counter doesn't come with a display, it does have an LED indicator and a peizo speaker that beeps whenever it detects a particle. The board also has a pulse out pin that we'll use to hook it up to the arduino.

Step 3: Connecting to the Arduino

As I said in the previous step, the geiger counter board has a pulse out pin. The pulse pin is a part of three pins, and the other two provide access to ground and to power. I used a 3 pin female connector to connect to the pins, and soldered a piece of wire to each pin. The geiger counter typically runs off two AAA batteries and draws about 10mA. I chose to save the batteries and run the board off the 3.3v line on the arduino board, which can handle about 50mA of current. I have let it run overnight and it works just fine.

These instructions are specific to the mighty ohm geiger counter, but should be similar to connecting any other geiger counter with a pulse out.

If you want to run the geiger counter without batteries

1. Locate the 3 pin connector labeled PULSE
2. Connect the first pin (marked with an arrow on the board) to the 3.3v output on the arduino.
3. Connect the middle pin (pulse out) to digital pin 8 on the arduino.
4. Connect the last remaining pin to a ground pin on the arduino.

If you want to run the geiger counter with batteries, only do steps 3 and 4, leaving the first pin unconnected

Step 4: Connect the Arduino to a Computer

To interface the arduino with the computer, connect it using a USB port. I chose to program the software in a language called processing, which has built in libraries for communication with an arduino. Its also cross platform, so you should be able to use all the software for this project without a problem.

First download the arduino sketch (make sure you have the newest version of the arduino software) and burn it to your arduino.
Then, download one of the processing sketches.

A couple of notes about the software:

The arduino sketch constantly reads pin 8, checking for a pulse from the geiger counter. When it detects a pulse, it checks the time in milliseconds that the arduino has been running for. It does this four times and then finds the length of time between the first and second pulses and the third and fourth pulses. If the first length of time is greater than the second length of time, then the bit will be a 0. If the second length of time is greater than the first length of time, then the bit will be a 1. Once the bit is generated, it is sent to the computer. This is based of the algorithm used by http://www.fourmilab.ch/hotbits/

There are two different versions of the processing sketch, one with biased bits, and the other with unbiased bits. The difference is that the biased one may be biased slightly due to the hardware, but produces a bit every four counts of radiation. The unbiased sketch preforms an XOR operation on two incoming bits to remove hardware bias, but produces a bit every 8 counts, doubling the time it takes for bits to be produced.

Regardless of the version you chose, the processing sketch will run until it makes 1000000 bits (or you turn it off). Every 500 bits, it saves the bits to a text file that you specify when the sketch starts up.

Step 5: Detect Radiation

Once the arduino has been programmed, you can run the processing sketch. The geiger counter should detect about 20 counts of radiation a minute, as natural background radiation. This equates to about 5 bits a minute, which is pretty slow. To increase the number of counts per minute, I bought salt substitute, which contains potassium chloride. A small percentage of potassium atoms are naturally radioactive (its still perfectly safe) , and we can actually measure the increased radioactivity with the geiger counter. I was able to get up to 60 counts per minute, or a bit over twice background radiation (again perfectly safe). The increase in radioactivity allows the counter to produce bits at a faster rate.

Step 6: Using the Bits

Now you should have your random bits, but what can you do with them? Well you can group them up and convert them into binary numbers. A group of 8 bits will give random numbers between 0 and 255. You can then use scaling algorithms to generate any range of random numbers. You can convert the numbers into characters and make your own one time pad to send unbreakable encrypted messages (just use the processing sketch below!). You can use the bits like a coin, but based on quantum randomness.

At this point you should be able to generate random numbers for whatever you want. If you do something cool with it, post an instructable about it!

Note: I have not subjected the bits to any statistical test to verify how random they are, beyond testing 1000 bits to see if the number of 0s occured as often as 1s (very close to 50/50) Also, I cannot assure that the bits will be random enough for real world serious applications where random numbers are needed, but they should be good enough for whatever a hobbyist would have in mind.