 
 void loop()
 {
+  delayMicroseconds(50);
+  
   // check for measurement request on UART
   if (Serial.available() > 0) {
     // read the incoming byte:

  pinMode(pingPin3, INPUT);
  duration = pulseIn(pingPin3, HIGH);
  cm3 = microsecondsToCentimeters(duration);
  
  // probe sensor 4
  pinMode(pingPin4, OUTPUT);
  digitalWrite(pingPin4, LOW);

  pinMode(pingPin4, INPUT);
  duration = pulseIn(pingPin4, HIGH);
  cm4 = microsecondsToCentimeters(duration);
 */
 
  // output result on serial 
  Serial.print(cm1);
