post16/ultrasonic/circuit
I say, NE555 is not stable at all, don't use it, cause 40Khz frecuency will always moving from central value, then you will have many headaches.
Use LM567 for generate the 40kHZ frecuency, and other LM567 for receiver, it is a good tone decoder. You can use it as a oscilator or a tone decoder, and it have 18mV of sensibility. Only thing you must care is the type of capacitor, it's recomended use a NP0 capacitor, because it don't change the value due to temperature variations.