This Self-Contained System Gathers Outdoor Sounds for Ecological Research

The idea

From the smallest insect to much larger birds and mammals, the animal kingdom is full of creatures that create noises for a wide variety of purposes, such as finding food or a mate. However, human activity and loss of habitat dramatically reduced their populations, which is why tracking these noises is vital for conservation efforts. Daniele Pozzi has created a project inspired by the 2022 Klangnetze system that can both record sounds and play them back in a completely off-grid solution.

Capturing and playing audio

Because the entire focus of this project was to capture and play audio, plenty of attention went into ensuring the signal quality would be acceptable in a broad range of conditions. A Raspberry Pi Zero 2 W was selected as the primary controller due to its good balance of relatively low power consumption versus its processing speed and connectivity. Attached to its 40-pin header is a Raspiaudio MIC+ V2 board, which contains a built-in I2S microphone module, a pair of 5W stereo speakers, and a 32-bit I2S digital-to-analog converter to drive them. In addition to these, Pozzi also included a more powerful Dynavox Minibass PS-138 loudspeaker that could be placed outside of the main enclosure.

The solar panel and battery

Most off-grid power solutions use a combination of a solar panel(s) and batteries, and this project is no different. A single 2.5W 5V solar panel was connected to an Adafruit Universal Solar Charger board that contains the BQ24074 IC. After receiving power from either the solar panel or an 18650 Li-ion battery cell, it outputs a steady 3.3V that is boosted to 5V thanks to an Adafruit PowerBoost 500 Basic board. This solution ensures the power source can be intelligently switched between either the battery or the panel itself without unnecessarily degrading the Li-ion cell over time.

Power management

Beyond the previously mentioned components, Pozzi wanted to go one step further by ensuring whole system will correctly power off whenever either the battery and solar panel have insufficient voltage to keep everything running. This was achieved by first using one NOR gate to set a pin if the Solar Charger board is outputting a signal on its PGOOD pin and if the battery voltage is above 3.9V. Due to the Pi dropping the battery's voltage below 3.9V when it first boots, a monostable NE555 and secondary NOR gate were added to create a 10-second buffer between state changes to prevent rapidly toggling the Pi on and off.

Putting it all together

With the circuitry completed, Pozzi grabbed a waterproof outdoor enclosure and began installing each component, starting with the Li-ion battery holder and solar panel plug. After creating a small cutout for the MEMS microphone element and running the enclosure through a water resistance test, he finally added the Raspberry Pi Zero 2 W, the audio shield, and the other power management modules. To see more about this project, you can read Pozzi's write-up here on Instructables.