int led = 5;
int button = 7;

int val = 0;
int old_val = 0;
int state = 0;

void setup ()
{
  pinMode(led, OUTPUT);
  pinMode(button, INPUT_PULLUP);

}

void loop() { 
  val = digitalRead(button);

  if ( (val == HIGH) && (old_val == LOW)) {
    state = 1 - state;
  }
  old_val = val;
  if (state == 1) {
    digitalWrite(led, HIGH);
  }
  else {
    digitalWrite (led, LOW);
  }

}