// square wave generator (U_eff = 5 V;  f = 12.5 Hz)

void setup()                      // preparing pins 0 and 2 for output use
{ 
  pinMode(0,OUTPUT);
  pinMode(2,OUTPUT);
} 


void loop ()                      // generating square wave voltage
{
  digitalWrite(0,LOW);            // setting pin 0 to ground (0 V)
  digitalWrite(2,HIGH);           // setting pin 2 to +5 V
  delay(40);                      // waiting for 40ms, i.e. half time of oscillation
  digitalWrite(0,HIGH);           // setting pin 0 to +5 V
  digitalWrite(2,LOW);            // setting pin 2 to ground (0 V)
  delay(40);                      // waiting for 40ms, i.e. half time of oscillation
}