There is lot of ic's that can produce ultrasonic Freq. plus. Is it possible to catch the bounce back and trigger relay?
Is it possible to triggering a relay by freq. shift or tone? Some of those ant-barking curcuits that use 555's is it possible that they could they be modified for proximity detector use? Could transmitter catch the signal or tone on the return the echo bounce and trigger the relay. Also can some of those Max Bot's run directly to a relay and work?