Design Goals
The Petal Radio is our proof-of-concept and testing prototype for FloraNet. We designed it with reliability in mind so that we could confidently develop our software and firmware, and gather valuable testing data. ​
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The board includes two power supplies, low-voltage disconnect (LVD), micro-processor unit with SD card, LoRa module, buttons, an RGB LED, and USB and UART connections. The redundancies we included have come in handy several times this semester!
PSU
High Efficiency Power Supply
Efficiency is key for solar- and battery-powered devices like the Petal Radio. Using less energy means longer battery life, smaller and cheaper solar panels, and fewer network outages.​ The current version of Petal Radio is a prototype, so we wanted to test two power supply options. ​One power supply unit (PSU) is a pre-made module, and the other is a custom unit we designed.
Custom PSU
The custom PSU is cheaper, offers higher performance, but requires a more complex design.
Module PSU
The module PSU is easy to install on the board and works reliably, but it is expensive (~20% of the board's total cost for one part).
We tested and compared the efficiencies of both power supplies at a high load (650mA) and a low load (10mA) across the three LVD battery voltages and 5V. As shown in our data below, the custom power supply outperformed the module, especially at low loads. The low load performance is particularly important because the Petal will be in a low-power mode for most of its operation.
Supply efficiencies at high load.
Supply efficiencies at low load.
LVD
Low-Voltage Disconnect
Batteries can be ruined if they are discharged too much, so the Petal Radio has a low-voltage disconnect (LVD) function. The power supply monitors the battery voltage, and when it drops too low, the power supply shuts down the Petal. The battery starts charging again before an excessive discharge happens. The power supply won't turn back on until the battery has safely recharged.
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The Petal Radio LVD is designed to work with 6V and 12V LiFePO4 and 12V lead-acid/AGM battery types. The LVD is configured using the LVD battery selection pin headers (P1 and P2) next to the power supply, shown in the image adjacent.
LVD battery selection pins
MPU
Cutting-Edge Microprocessor
The Petal Radio uses an Espressif ESP32-S3-MINI microprocessor unit (MPU) module. The S3 is a dual-core 240MHz microprocessor with Espressif's industry-leading embedded Wi-Fi modem built-in. The Petal uses a module because it includes an antenna for the Wi-Fi modem.
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The SD card slot next to the MPU hosts a card for our web app files. Having these files in external, removable storage means we can modify these files without re-programming the MPU firmware.
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Lastly, we broke out the spare GPIO pins to headers so we could use them in case of emergency.
SD card slot and ESP32-S3-MINI
Buttons
Power-Saving User Button
The Petal Radio includes a user button that, when pressed, activates the Wi-Fi network hosted by the MPU. When the Wi-Fi network isn't needed, the firmware puts the board into a low-power listening mode, cutting power consumption by over 90%. This extends the battery life of the unit. The unit will continue to act as a LoRa repeater and storing messages as it receives them in the SD card, ready and waiting for the next time the unit is woken up by a user.
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If the Petal is in a water-tight enclosure, the user button pins on the MPU can be wired to an external button outside the enclosure through the adjacent terminal block.
User button and breakout terminal block
