A 55 mm round, four-layer sensor node that runs from a solar cell and a 14500 LiFePO4 cell: 868 MHz LoRa, 2.4 GHz BLE, and an environmental sensor suite on one board.
| Schematic | Board |
|---|---|
| Schematic, six pages (PDF) | Open the project in KiCanvas |
KiCanvas loads the whole project in the browser: the board opens first, and the five sheets are one click away in its Project panel. No KiCad installation required. The PDF is the quickest way to read the schematic end to end.
The node is meant to sit in a field and report microclimate data for months at a time without a battery change. A solar harvester tops up a single LiFePO4 cell, a boost holds 3.3 V, and the MCU wakes to sample temperature, humidity and soil moisture before transmitting over LoRa.
Three choices shaped the board more than anything else:
- The radio is split. A bare nRF52840 runs BLE from a PCB antenna, while an SX1262 carries the long-range link through a U.FL connector, so the 868 MHz front end can be measured and re-matched without touching the MCU.
- The MCU is a bare aQFN73 at 0.4 mm pitch, fanned out directly on the top layer. No module, no fanout board.
- The RF feed is impedance-controlled. The 868 MHz path, the 2.4 GHz antenna feed and the U.FL launch all run in a 50 Ω net class over an In1.Cu plane, on a stackup chosen for that geometry.
The outline is the last constraint: Ø55 mm, three mounting holes, and an antenna keepout that no layer may cross.
- 868 MHz LoRa, discrete SX1262 with a switchless match/filter/balun and a U.FL measurement point
- 2.4 GHz BLE from a bare nRF52840 (aQFN73) with an on-board PCB antenna
- BQ25570 solar harvester with PMOS reverse-polarity protection and TVS
- TPS61099 3.3 V boost from a single-cell 14500 LiFePO4
- Battery PTC, reverse-polarity protection, current-measure jumper and NTC divider
- BME280 environment sensor and a switched analog soil probe
- SWD debug header, eight test points, three fiducials
- 96 parts across five hierarchical sheets
| Item | Value |
|---|---|
| Board size | Ø 55.0 mm, 4 layers |
| Stackup | JLC04161H-7628, FR4, Er 4.6, 1.6 mm nominal |
| Copper | 1 oz outer and inner, ENIG finish, green mask, white silk |
| Minimum feature | 0.20 mm track, 0.15 mm clearance, 0.20 mm drill |
| Vias | 0.30/0.20 mm and 0.20/0.20 mm, tented, no blind or buried |
| Impedance | 50 Ω ±10 % single-ended, In1.Cu reference — design value, see below |
| Assembly | JLCPCB Standard PCBA, 2 prototypes |
The 50 Ω is the requested impedance control, not a measurement: JLC's impedance
report and a test coupon are still outstanding, and nothing on this board has
been measured — docs/impedance.md tracks that.
Line colour and weight separate the five signal classes: power, digital, analog, RF and control. The 3V3 bus feeds the radio and the sensing block from the same boost converter, and the soil probe is switched so it draws nothing while the node sleeps.
The radio sheet carries a switchless match, filter and balun between the SX1262 and the U.FL, laid out as a 50 Ω coplanar waveguide over the In1.Cu ground plane. The geometry was chosen from a quasi-static 2D sweep before layout:
| W (mm) | S (mm) | h (mm) | Z₀ (Ω) | εeff |
|---|---|---|---|---|
| 0.38 | 0.20 | 0.210 | 50.4 | 3.51 |
The sweep and its caveats are in
simulation/results/cpw_sweep.md. The
full-wave model and the VNA coupon are still to be run.
The PCB antenna sits in a keepout with no copper on any layer, fed from the
nRF52840 through the same 50 Ω class. The keepout is the fragile part, and it is
documented with the stackup in docs/stackup.md.
The BQ25570 runs as a boost charger only — its buck converter is deliberately unused — feeding a single LiFePO4 cell through a PTC and a reverse-polarity PMOS. The TPS61099 then holds 3.3 V for the MCU, radio and sensors, and the soil probe is switched so it cannot leak current while the node sleeps. The full pin list and the reviewed net table are in the schematic.
Four layers carry the aQFN73 fanout on the top, a solid ground below it, and the supplies on the inner layers. Assembly is Standard PCBA with X-ray, because the aQFN73 and the BME280 both have pads underneath.
| Front copper | Back copper |
|---|---|
 |
 |
| Inner 1 — ground plane | Inner 2 — power |
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| Path | Contents |
|---|---|
hardware/rev-a/ |
KiCad 10 project: five schematic sheets, PCB, symbol and footprint libraries, 3D models |
simulation/ |
Coplanar-waveguide pre-check for the 868 MHz feed, and the plan for the 3D model |
docs/ |
BOM, assembly, fabrication and PCBA notes, stackup, impedance, schematic PDF |
firmware/ |
Reserved for the node firmware |
Open the project with KiCad 10:
kicad hardware/rev-a/lora-node.kicad_proThe board is drawn for JLCPCB 4-layer fabrication and Standard PCBA, with X-ray
inspection for the aQFN73 and BME280. Paste, stencil and panel choices are in
docs/fab-notes.md,
docs/assembly-notes.md and
docs/pcba-notes.md.
The fabrication package for a release — gerbers, drill file, job file, BOM and CPL — is attached to the release rather than committed here.
Rev A is frozen and has not been fabricated yet. The design is complete through DRC, ERC and schematic/board parity.
Released under the CERN Open Hardware Licence Version 2 – Permissive.