Here are some circuit diagrams (leaving the power source abstract):
Acoustic transmitter circuit diagram (click for full size).
Acoustic receiver circuit diagram (click for full size).
Acoustic modem microcontroller circuit diagram (click for full size)
Transmitter (top) and receiver (bottom) circuits.
I’ve previously described the power supply and transmitter for my acoustic modem.
The receiver circuit completes the modem’s hardware design. It is simply a two-stage amplifier that gives a total gain of about 2000, in series with a band-pass filter.
The acoustic transducer produces a beautiful sine wave with nearly no noise. I found that I could apply a huge gain to its output and the amplifier’s output would still be clean: the raw (unfiltered) output has a 5 V offset, and the signal fits into the 5 V above the offset. In other words, the output is at -5 V when idle and peaks at around 0 V when the receiver receives a signal.
The goal is for the receiver to output a signal that is around -10 V when idle and peaks at -5 V when it receives a signal. These voltages correspond respectively to logic low and high on the microcontroller.