The reminders about Amazon Prime day got me thinking about Prime Numbers. Whilst I was coding a power efficient “blink” for a new 4Tronix board I joked that it was not calculating primes. But then I thought, “why not?” I remembered back to my ZxSpectrum days and thought, it that can calculate primes then there’s no reason it could not be done on an Arduino. The piece of code I used on the Spectrum was called the Sieve of Eratosthenes. I’m not sure that I understood it properly at the time but it works by taking all of the numbers in an array and removing all the multiples of prime numbers from that array.  I found an example in C and tweaked it to work on the Arduino.


void setup() {
//Based on

#define LIMIT 32700 /*size of integers array*/

byte primes[LIMIT];
unsigned long i,j;
int z = 1;

for (i=2;i<LIMIT;i++)

for (i=2;i<LIMIT;i++)
    if (primes[i])
        for (j=i;i*j<LIMIT;j++)


for (i=2;i<LIMIT;i++)
    if (primes[i]) {
        Serial.print(“th prime = “);

void loop() {
// this loop is left intentionally blank

Let me know if you come up with a faster version or one that can calculate bigger primes.


Sieve of Erathoshenes In C

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