They are added in function draw().

The function draw runs again and again automatically, about 60 times per second.

They are added each time. In draw() you could have a for-loop and add 5 or 10 each time. Could be faster, dunno.

But they also die. So they don't increase in number.

Maybe you have to stop them dying.

When you want to keep them, change the dead() function in
http://processing.org/learning/topics/multipleparticlesystems.html

or remove these lines:

1.     if (psys.dead()) {
         psystems.remove(i);
       }

Also they get thinner;
so instead of

1.     stroke(255,timer);

better have

1.     stroke(255);

And instead of

1. void timer()
   
2. {
   
3. timer -= 0.5;
   
4. }

have 

1. void timer()
   
2. {
   
3.    // timer -= 0.5;
   
4. }

They disappear also because they leave the screen. To avoid this, let them bounce from the screen borders so they stay on the screen.

Greetings, Chrisir

Thanks for your answer ! I didn't think about the for loop in the draw() function to add more and more particles, thanks for the advice.

I made some additions in my humble posting. Maybe

instead of

1. void timer() {
   
2. timer -= 0.5;
   
3. }

have 

1. void timer()
   
2. {
   
3. // timer -= 0.5;
   
4. }

is best

I just tested with the loop method in the draw() to add particles. The problem is that it's very heavy.

oh, I am not a native speaker either...

you could draw the stuff in draw every time, but get the update from arduino only every 500 millis() or so....