Thin‑Pod Telemetry Logger

Low‑power VBATT sensing and Wi‑Fi logging for the Thin‑Pod energy‑harvesting bench rig.

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Overview

This mini‑project streams the supercapacitor VBATT from an AEM00941 EVK v1.2 + 5 F cap to a host over Wi‑Fi using a Raspberry Pi Pico 2 W. A high‑impedance divider and small RC feed the RP2350 ADC; a MicroPython script serves data over TCP; a tiny Python client logs to CSV on your laptop/desktop (or Pi Zero 2 W).

Highlights

• ~2 µA divider overhead (1.0 MΩ // 660 kΩ) — harvester‑friendly.
• 5 s default cadence; LED heartbeat on send.
• TCP stream → CSV (timestamp,elapsed_s,vbatt_v) with auto‑reconnect.
• Fault‑tolerant bring‑up (clear wiring signatures and firewall‑proof flow).

This repo is a self‑contained “telemetry side‑quest” to support Thin‑Pod harvester tuning. It doesn’t require the UWB radio or ADXL1005 to operate.

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System Architecture

AEM00941 EVK ── 5F/6V supercap (BATT)
        │
        ├─ BATT+ ─ 1.0 MΩ ─┐
        │                  ├─ (ADC node) ── GP26/ADC0 (Pico 2 W)
        └─ BATT− ─ 660 kΩ ─┘
                          ║
                       100 nF → BATT−

Pico 2 W (MicroPython) ──TCP:5007──▶ Host (Python client) → vbatt_log.csv

See also: docs/fig/fig1_architecture.png (placeholder).

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Hardware

• Energy harvester: e‑peas AEM00941 EVK v1.2 (BATT terminals) (Alt. for later experiments: Smart‑Material CL‑54; wiring differs.)

• Supercapacitor: 5.0 F / 6 V (e.g., Eaton PTV‑6R0V505‑R)

• Telemetry MCU: Raspberry Pi Pico 2 W (RP2350 + Wi‑Fi)

• Divider & RC:

  • R_TOP = 1.0 MΩ (±1%)
  • R_BOTTOM = 660 kΩ (±1%) (680 kΩ also fine; update code)
  • C_NODE = 100 nF (C0G/NP0 preferred)

• Breadboard/jumpers, common ground (AEM BATT− ↔ Pico GND)

Design numbers

• Scale factor: VBATT_SCALE = (R_TOP + R_BOTTOM) / R_BOTTOM = 2.515 (for 1M / 660k)
• ADC node at VBATT=3.34 V: ~1.33 V
• RC settle τ ≈ 39.8 ms → 6τ ≈ 0.24 s (well below 5 s cadence)
• Divider current at 3.3 V: ~2.0 µA

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Wiring

Choose one single breadboard row as the ADC node and place all of these into that row:

• GP26 (Pico ADC0) jumper,
• One leg of 1.0 MΩ (other leg to BATT+),
• One leg of 660 kΩ (other leg to BATT−),
• One leg of 100 nF (other leg to BATT−).

Common mistakes: landing parts across the center trench (node becomes two rows), missing the common ground, or swapping the divider legs — see Troubleshooting.

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Software

Requirements

• Pico 2 W with MicroPython firmware (latest stable).
• Python 3.10+ on the host (Windows/macOS/Linux/Raspberry Pi).
• Optional: Thonny for Pico file transfer/console.

Files

thin-pod-telemetry/
├─ README.md 
├─ LICENSE # MIT license
├─ requirements.txt
├─ docs/
├─ host/
├─ images/
├─ pico/

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Quick Start

1. Wire it exactly as in the diagram (single node row; common ground).

2. Pico: open pico/main.py and edit:

   SSID = "<your_wifi_ssid>"
   PWD  = "<your_wifi_password>"
   R_TOP = 1_000_000
   R_BOTTOM = 660_000   # 680000 if using 680 kΩ
   INTERVAL = 5         # seconds between samples

   Save to the Pico as main.py. Reboot or power from a wall adapter. Thonny will print Pico IP and listening on 5007.

3. Host: open host/tcp_client_csv.py and set the Pico IP printed by Thonny:

   PICO_IP = "192.168.x.y"  # from Pico console
   PORT = 5007

   Run:

   cd host
   python tcp_client_csv.py

   You should see Connected to ... and rows streaming; a vbatt_log.csv appears in host/.

On Windows, no firewall changes are normally required because the host initiates the outbound TCP connection.

Notes

Keep AEM BATT− ↔ Pico GND common.

With 1M/660k and 100 nF at the node, start-up blips are normal; the Pico script discards the first few samples and waits SETTLE_S.

Calibrate by updating R_TOP/R_BOTTOM, ADC_REF, or CAL_GAIN.

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Calibration (optional but recommended)

Pick one of these simple options:

A. Measured resistors Measure R_TOP and R_BOTTOM with a DMM and set those exact values in main.py.

B. One‑line gain trim If DMM says 3.340 V while CSV shows 3.303 V:

CAL_GAIN = 3.340/3.303  # ≈ 1.0113
v_batt *= CAL_GAIN

C. ADC ref tweak Treat 3V3 as ADC_REF and set:

ADC_REF = 3.337
ADC_SCALE = ADC_REF / 4095.0

D. Two‑point fit Capture readings near 0 V and ~3.5 V; compute v_batt = a*raw + b and apply.

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Troubleshooting

No data / connect errors

• Ensure Pico prints an IP and listening on 5007.
• Host must use that Pico IP.
• Check: netstat -an | find ":5007" (should see an ESTABLISHED connection while streaming).

CSV ~0.00 V

• ADC node is effectively at GND: open 1.0 MΩ leg or node landed on a GND row. Check BATT+↔node ≈ 1.0 MΩ.

CSV ~8.09 V (flat)

• ADC node clamped near 3.3 V: open 660 kΩ leg / node not pulling down. Check node↔BATT− ≈ 660 kΩ.

Start‑up blips/zeros

• Normal while RC settles; the first 2–3 samples can be discarded or add a 0.2 s settle before averaging.

Windows inbound rule confusion

• Not needed with this flow (host is the client). If roles are ever inverted, use PowerShell:

  New-NetFirewallRule -DisplayName "Pico_TCP_5007" -Direction Inbound -Action Allow -Protocol TCP -LocalPort 5007 -Profile Private

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Acceptance (ready‑to‑fill)

Req	Description	Target	Evidence	Result	Pass/Fail
R1	VBATT accuracy vs DMM	≤ ±2%	Results §	(fill)	(fill)
R2	Sampling cadence	≥ 1/5 s	Config §	(fill)	(fill)
R3	Divider overhead	≤ 25 µA	Design §	(fill)	(fill)
R4	Robust logging	UTC CSV; reconnect	Demo §	(fill)	(fill)

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Roadmap / Future Work

• Auto‑plots and thresholds (Matplotlib) and daily log roll‑over.
• Optional lower‑Z divider (100 kΩ/68 kΩ + 100 nF) or op‑amp buffer for precision.
• Integrate telemetry with harvester duty‑cycle experiments (weights/position sweeps).
• Containerised host tools (uv/pipx, minimal venv) and Makefile targets.

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Safety & Notes

• Supercaps can deliver high peak currents — avoid shorts; respect polarity.
• Share ground between AEM EVK and Pico.
• This is a bench‑test tool; field firmware may gate or duty‑cycle the divider to save energy.

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License

MIT for software, CC‑BY for docs

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Acknowledgements

• e‑peas AEM00941 EVK v1.2, Eaton PTV supercapacitors.
• MicroPython & Raspberry Pi Pico team.

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Credits

Built by Neil Thomson as part of the Thin‑Pod project. Feedback and PRs welcome.