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Using Random Numbers with Arduino

This video tutorial talks about using the random() and randomSeed() functions with Arduino.  It is pretty straight forward, but there are some intricacies worth noting.

Creating truly random numbers in Arduino is harder than you might think.  The closest we can get in Arduino, and just about anywhere else, is using pseudo random numbers.  That is, numbers that mimic randomness, but in fact do have a pattern if analyzed for a long enough period.


Why are Random Numbers with Arduino All the Same?

The most important thing to understand when using the random() function with Arduino is that it will generate the exact same list of pseudo random numbers every time.

So if you build a slot machine, and the first crank of the handle is a winner, then you can be sure that if you reset the Arduino board and pull the handle again – it will still be a winner the first time.

Using random numbers with Arduino breadboard image

The easy way to overcome this is using the randomSeed() function.

This function takes a value (an integer for example), and uses the number to alter the random list generated by the random() function.  The number you pass to the randomSeed() function is called a ‘seed’.

You might put randomSeed() in the setup, and then use the random() function in the loop.  Something like this:

But there is still an issue – even though the sequence of random numbers is different when using the randomSeed() function – it will still be the same every time the sketch is run.  It is just a different list of pseudo random numbers!


One Solution to the Random Problem

So, what to do?  Lucky for us the Arduino reference has a great solution.  Use the analogRead() function to read a value from an unused analog pin.  Since an unused pin that has no reference voltage attached to it is basically ‘floating’, it will return a “noise” value. This noise value can seed the randomSeed() function to produce differing  sequences of random numbers every time the sketch is run.

Random numbers with Arduino supplies list


Below is the sketch from the video using analogRead() and randomSeed() in unison:

Well, hopefully that was random enough for you! If you enjoy this tutorial, I highly recommend checking out our free Arduino Crash Course (You can sign up below).