The first iteration of the BoatDetect system architecture is complete. The device is mounted underwater and tethered to the bottom. A piezoelectric diaphragm senses the noise generated by boats passing by overhead. An active low pass filter removes all frequencies above 1000Hz and amplifies the signal. An envelope detector outputs a positive peak amplitude signal. The Tiny85 microcontroller periodically reads the analog signal. If the voltage of the signal is above the detection threshold, the Tiny85 wakes up the Arduino Nano. The Nano writes a timestamp to a file on the SD card and goes back into power down mode. The Tiny85 continues to wake up the Nano every time the voltage signal is above the detection threshold. The Tiny85 stops waking up the Nano after about four weeks when the battery state of charge drops below 10%. At this point, the device has to be retrieved for service. The data file is downloaded from the SD card and the batteries are removed and replaced with a fully charged set. A quick analysis of the data will show hours of the day and days of the week when boat traffic is heaviest in the vicinity of the device.
Total estimated cost of the system is approximately $37. The cost breakdown by component is detailed in pie chart below. Almost 50% of the cost is in the batteries and water bottle.