First of all, thanks to Christoph Tack for the extensive reviewing and the valuable feedback on the schematic. One major change came from his advice: the new architecture drops the Pi Pico and runs instead on an ESP32-C3 MINI, which is much more suited for ultra-low-power profiles.

The power path starts from a 20–30 V pack of alkaline batteries. The input is protected against reverse polarity and surges by an LM74610 ideal diode and a TVS diode (SMAJ36AH). A MAX16956 buck converter then provides a regulated 5 V rail, always on.

On this 5 V line sits a TPS2553 current limiter, set to about 250 mA (R_ILIM ≈ 118 kΩ). This limiter is mandatory because the supercapacitors are placed after it, directly on the 5 V bus.
During LoRa TX bursts, the E22-400T33D can draw up to 1.3 A. Thanks to their low ESR, the supercaps deliver this current. But when they recharge, the limiter ensures the source current stays below 250 mA (~70 mA on the battery side).
This low recharge current is critical: it is compatible with end-of-life alkaline packs or very cold conditions, and it even improves autonomy—lower current means more usable energy can be extracted from the cells.

The supercap charging is supervised by a TLV6700 comparator (previously a MAX8212 in V1). It cuts charging at 4.90 V and re-enables it below 4.75 V, stabilizing the bus around 4.8 V. This is high enough for reliable E22 operation but low enough to minimize supercap leakage, which increases with voltage and temperature.

From this 5 V rail, a TPS62840 ultra-low Iq buck generates the permanent 3.3 V rail. This powers the ESP32-C3, the TPL5010 watchdog, the PCF8523 RTC, and the FRAM memory.

• The RTC is always powered, mainly to use its alarm output, wired together with the TPL5010 WAKE through an open-drain buffer.

• The FRAM (≈ 27 µA continuous consumption) is physically disconnected when not logging, using MOSFET switches to avoid phantom powering through the I²C bus.

Two high-impedance resistor dividers allow monitoring of both the input (30 V) and the 5 V bus. The high-voltage divider is switched by a P-MOS/NPN combo for ESP32 safety.

The LoRa E22 module runs directly from the supercap 5 V bus. The ESP32 supervises the radio: if the module becomes unresponsive, a hard reset circuit can force recovery (still pending in this version).

The watchdog (TPL5010) resets the ESP32 if it fails to toggle DONE within 10 s. The firmware kicks it every 8 s, and the DONE line also drives a small green LED, flashing 100 ms every cycle—visible but still energy-friendly.

Protections are layered:

• SMAJ36AH TVS for surge,

• Polyfuse 2016L050MR set at 500 mA for the input,

• LM74610QDGKRQ1 ideal diode for reverse polarity and blocking back-feed to the cells,

• 2 A fuse on the supercap path.

Finally, a USB-C connector allows ESP32 programming and log extraction. Two tactile switches let you force BOOT mode or reset, just like on a standard devkit.

PCB

The board is designed to slide into a 50 mm PVC tube. Target outline is 45 mm wide × 125 mm long to leave insertion clearance.

• Assembly: all components are placed on the front side; the supercapacitors are mounted on the back side.

• Minimum trace width: 0.256 mm (~10.1 mil).

• Keep-out & fit: maintain a small edge clearance for the tube wall; avoid tall parts near the edges to prevent friction during insertion.

Next steps

For the next steps, I’ll wait until September 20 to launch fabrication — just enough time to add the missing power switch on the E22 line and double-check everything.
The build will go through PCBWay — thanks for supporting this project! They’ve been awesome.

I also scattered debug pads all over the board to make troubleshooting easier.

And hopefully, I’ll be able to run new field tests out during this winter!

PS : I put new gerber files on the projet