void shift(byte gear) {

  if (gear == 1) {
    digitalWrite(RELAY_1, RELAY_ON);
    digitalWrite(RELAY_2, RELAY_OFF);
    digitalWrite(RELAY_3, RELAY_OFF);
  }
  else if (gear == 2) {
    digitalWrite(RELAY_1, RELAY_ON);
    digitalWrite(RELAY_2, RELAY_ON);
    digitalWrite(RELAY_3, RELAY_OFF);
  }
  else if (gear == 3) {
    digitalWrite(RELAY_1, RELAY_OFF);
    digitalWrite(RELAY_2, RELAY_ON);
    digitalWrite(RELAY_3, RELAY_OFF);
  }
  else if (gear == 4) {
    digitalWrite(RELAY_1, RELAY_OFF);
    digitalWrite(RELAY_2, RELAY_OFF);
    digitalWrite(RELAY_3, RELAY_OFF);
  }
  else if (gear == 5) {
    digitalWrite(RELAY_1, RELAY_OFF);
    digitalWrite(RELAY_2, RELAY_OFF);
    digitalWrite(RELAY_3, RELAY_ON);
  }

}