A compact, battery-powered tinyML audio and environmental data-collection board based on the RP2040. The device records digital microphone data to microSD, can attach a DHT11/DHT22 sensor, and reports status or summarized data over LoRaWAN through a LoRa-E5 module. It supports USB or solar-assisted charging of a single-cell Li-ion/Li-polymer battery and is packaged in a two-piece FDM enclosure.
Intended Use
Community development and field data collection for tinyML audio applications.
Local audio capture and storage where Wi-Fi is unavailable.
Periodic low-bandwidth LoRaWAN telemetry rather than streaming raw audio over LoRa.
Prototype and validation platform; the current revision is not yet production-qualified.
Status: Preliminary engineering budget for review. It is grounded in the live schematic, but several load currents are planning allowances because the exact SD card, DHT sensor, firmware duty cycle, battery, and solar panel are not selected.
The power path is:
Confirmed Electrical Facts
Main regulated system rail: +3V3.
Internal RP2040 core rail: +1V1, generated by the RP2040 internal regulator.
Battery rail: VBAT, a single Li-ion/Li-polymer cell charged to 4.2 V by U5.
Charge-current intent: approximately 900 mA according to the schematic annotation and R7 = 2 kOhm.
U4 RT8059 is rated in project component data as a 1 A buck converter with 2.8 V to 5.5 V input.
Debug: USB serial for logs, SWD through J8 for recovery and source-level debugging
Flash: W25Q128JVSIQ external QSPI, 16 MiB
Pin MappingRecommended Firmware State MachinePlatformIO Configuration
The exact Arduino-Pico release and SDIO/I2S libraries must be pinned after a compile-tested implementation. The microSD wiring is 4-bit SD, not conventional SPI; do not silently substitute an SPI example without rewiring hardware.
Initial Bring-Up Source
This source is intentionally limited to peripherals that can be initialized with stable Arduino-Pico APIs from the confirmed schematic: USB logging, user LED, DHT, LoRa UART/reset,...
A compact, battery-powered tinyML audio and environmental data-collection board based on the RP2040. The device records digital microphone data to microSD, can attach a DHT11/DHT22 sensor, and reports status or summarized data over LoRaWAN through a LoRa-E5 module. It supports USB or solar-assisted charging of a single-cell Li-ion/Li-polymer battery and is packaged in a two-piece FDM enclosure.
Intended Use
Community development and field data collection for tinyML audio applications.
Local audio capture and storage where Wi-Fi is unavailable.
Periodic low-bandwidth LoRaWAN telemetry rather than streaming raw audio over LoRa.
Prototype and validation platform; the current revision is not yet production-qualified.
Indoor or sheltered outdoor deployment only until ingress, condensation, UV, thermal, and battery safety testing is completed.
What the Device Should Do
Boot from external QSPI flash and expose USB device/serial functionality.
Acquire mono digital audio from the ICS-43434 I2S microphone.
Store recordings, features, logs, and configuration on microSD.
Communicate with the LoRa-E5 module using UART and hardware reset.
Optionally read a DHT11 or DHT22 through the external sensor connector.
Indicate user, source, and charging states through LEDs.
Operate from a single-cell battery and charge from USB or a solar source.
Support SWD debugging, reset, and bootloader entry.
Micro-USB input, solar terminal, CN3063 single-cell charger, battery connector, RT8059 3.3 V buck regulator
Debug/UI
SWD header, BOOT and RESET switches, user and power/charge LEDs
Mechanics
42 x 57 mm, 4-layer PCB; 50 x 65 x 13.52 mm PETG enclosure with four M2-class board mounts
Mechanical System Architecture
Diagram
Mechanical baseline:
Enclosure envelope: 50 x 65 x 13.52 mm, rounded rectangular shell.
PCB envelope: 42 x 57 mm, 1.52 mm nominal thickness.
Four mounting locations at approximately (+/-17.66, +/-25.18) mm from the PCB datum.
Current enclosure is designed for PETG FDM with a 0.40 mm nozzle.
Physical fit, cable insertion, connector latch access, antenna strain relief, water ingress, and acoustic response remain validation gates.
Electrical System Architecture
Diagram
Hardware Subsystems
Power input and charging
J3 provides Micro-USB power and USB 2.0 data.
J5 accepts an external solar source.
Q2 and D1 form the input-selection path into VIN_CHARG.
U5 CN3063 charges one Li-ion/Li-polymer cell on VBAT.
R7 is 2 kOhm and the schematic note states a nominal 900 mA charge-current intent.
D4 and D5 indicate charge and done states.
Battery chemistry, capacity, protection-board presence, NTC strategy, and solar panel rating are not specified and must be selected before release.
3.3 V supply
U4 RT8059 with L2 generates +3V3 from VBAT.
U3 SY6280 is placed between the buck output and the system rail as a current-limited load switch.
The rail powers U1, U2, U6, MK3, J1, and J6.
Critical risk: this is a buck-only architecture. As a single-cell battery falls near or below the required 3.3 V rail plus converter headroom, the rail can drop out of regulation. Firmware brownout behavior and usable battery cutoff must be characterized.
Compute and memory
U1 RP2040 at up to 133 MHz.
Y1 12 MHz crystal with 15 pF load capacitors.
U2 W25Q128JVSIQ, 128 Mbit external QSPI NOR flash.
Native USB D+ and D- are routed through 27.4 ohm series resistors.
Standby target: below 3 mA at the battery, pending measurement of the whole board.
U6 can draw 111 mA at maximum 22 dBm transmit power for the 868/470 MHz versions.
U6 sleep current is specified as 2.1 uA with WDT enabled.
RP2040 typical average DVDD current in dormant mode is 0.18 mA; this does not include all board-level 3.3 V loads.
Manufacturing and Assembly Expectations
Four-layer, 42 x 57 mm PCB with all electrical components on the top side.
Prototype assembly is suitable for professional SMT assembly; connectors and mounting hardware require mechanical inspection.
Preserve controlled 90 ohm USB differential routing and 50 ohm RF routing.
Add or retain accessible measurement points for VUSB, VSOLAR, VIN_CHARG, VBAT, +3V3, +1V1, GND, RUN, SWD, SWCLK, LoRa UART, and SD clock.
Production release requires BOM lifecycle/stock audit, impedance and RF review, final ERC/DRC, programming test, and manufacturing panel review.
Firmware-Relevant Hardware Requirements
PlatformIO with Arduino-Pico is the current development direction.
USB serial diagnostics must start before peripheral initialization.
Initialize safe GPIO states before enabling peripherals.
Control LoRa reset and verify AT response before joining a network.
Mount and benchmark the SD card before starting audio capture.
Use DMA/Pio-based I2S capture and buffered SD writes to prevent data loss.
Store configuration and fault logs; recover cleanly after card removal or brownout.
Implement watchdog, brownout-safe file closing, and explicit low-battery policy.
Support SWD recovery even if USB firmware is broken.
Physical Design Expectations
Maintain the current 42 x 57 mm R3 board outline and four mounting points unless the enclosure is revised concurrently.
Preserve connector and button access defined by the enclosure CAD.
Keep the microphone acoustic path clear and validate response with the device placed on its intended surface.
Provide external strain relief for the IPEX pigtail.
The enclosure is not sealed and must not be marketed with an IP rating.
Important Design Decisions
RP2040 selected for low-cost audio/tinyML experimentation and flexible PIO/DMA.
LoRa-E5 selected for long-range low-data-rate connectivity.
Local microSD storage carries bulk audio; LoRaWAN carries summaries and events.
Single-cell solar/USB charging uses CN3063.
A buck regulator is used for 3.3 V, accepting limited battery utilization near end-of-discharge unless the power architecture changes.
Enclosure uses two FDM-printed PETG solids and direct user access rather than moving button plungers.
Assumptions
Region is provisionally 868 MHz unless the deployment region specifies 915/470/434 MHz.
DHT sensor and microSD card are optional external loads and may vary widely.
Battery includes appropriate cell protection or an external protection PCB; this is not visible in the schematic.
No raw audio is transmitted over LoRaWAN.
Power values not directly specified by datasheets are marked as planning allowances and must be replaced by measurements.
Acceptance Criteria
All rails start reliably from USB, solar-assisted input, and battery across the intended voltage range.
No regulator or protection component exceeds thermal limits during charge, SD write, and LoRa TX overlap.
USB enumerates and SWD detects the MCU.
Audio capture has no dropped buffers during sustained SD writes.
LoRa joins and transmits through the selected regional band and antenna.
The enclosure allows all intended cables/cards and does not obstruct the microphone.
Measured standby, recording, and transmit currents are documented and meet the selected runtime target.
Change Notes
Initial comprehensive specification created from the live Rev 1 schematic, BOM, PCB layout, prior firmware discussion, and current R3 enclosure record.
Unknown battery, panel, schedule, reporting interval, and runtime values remain explicit release inputs rather than invented specifications.