Wireless Acoustic Communication Btwn 2 Arduinos

[davidace7]

Howdy everyone. I am stuck on a project and would really appreciate some help.
The project consists of 2 arduinos communicating with eachother using sound at 40kHz.

The setup: I have 2 Arduino Nanos. One is a reciever, one is transmitter. Each has an identical 40kHz piezo transducer attached. The transmitting circuit utilizes a transistor to power the piezo, and the recieving one has an analog amplifying and digitizing circuit. (Please see attached diagrams.) These are slightly modified versions that I got from Eduard Zola http://www.zolalab.com.br/eletronica_projetos/ultrasonic_talk.php

I am having trouble getting a clear back and forth, until i decided to carry out some tests, having the arduinos share a button, ground, and a digital pin (See circuit 2).
In this case, the signal transmission worke perfectly, where the Rx was able to determine length of signal independent on the range (This is unerwater btw).
The signal wasa definetely from sound, not shorting, as when I took the 2 transducers out of water, signal transmission ceased, as it should.

Can someone please explain WHY the system works when the arduinos share buttons but not when they are separate?? Is it a grounding issue? I am completely lost.
Thanks

Circuit2:

[ephitran]

Without sharing GRND D2 of RX & D9 of TX are not on the same phase to be read correctly.

[davidace7]

Do you mean D5 Rx and D3 Tx (the pins connected to Piezo)? Because D2Rx and D9Tx are removed when using the 2 devices separately, and the Rx begins reading whenever it receives a HIGH at D5, from the Piezo.

[ephitran]

D2 & D9 and digital communication. Example >3V=HIGG, otherwise =LOW, t must know 'that' 3V over then what? Ground? if you don't want to share GRND then both GRND must be grounded to shared earth-

[davidace7]

Thing is, the GND is shared in the configuration above. However, when I separate them, I removed the digitala connections D2-D9 as well as D12-D3. In that case each arduino has its own GND, and the only communication between them is via sound.

[MarkT]

Without posting your code its hard to know, but the guesses above about synchronization seem plausible. Clock recovery is often the hard part of digital comms.