Best as I can see the device looks like it should work when plugged into separate arduino. To eliminate software I have attached simple transmitter & receiver sketches that should just pulse the on board LED of the receiver when the transmitter is running. When it's not running the LED could be on, off or fluttering due to random noise but should not pulse in a fixed manner. This will hopefully eliminate hardware problems and point the finger at software.
Transmitter:
void setup() {
pinMode(10,OUTPUT); // TX Pin
}
void loop() {
digitalWrite(10,HIGH);
delay(100);
digitalWrite(10,LOW);
delay(100);
}
Receiver:
void setup() {
pinMode(2,INPUT); // ISR 0 Pin
pinMode(13,OUTPUT); // Onboard LED
attachInterrupt(0,rxISR,CHANGE);
}
void rxISR(){
digitalWrite(13,digitalRead(3)); // Echo pin state to LED
}
void loop() {
}