Solar-Powered Field Sensor

Learning Goals 5 min

Cluster G's capstone: a sensor node that runs forever. Solar panel charges a LiPo through a TP4056; the LiPo runs the L04-39 wake-sleep node; gateway picks up the readings via LoRa. The unit can be sealed, dropped in a field, and forgotten. By the end of this lesson you will:

1. Wire a solar panel + LiPo + TP4056 charger to power a low-power Arduino node.
2. Compute the solar energy budget — does the panel produce more energy than the node consumes?
3. Build the complete weatherproof package — sealed enclosure, sensors poking through, antenna out the top.

Warm-Up 10 min

Hardware:

• 5 V, 1–5 W small solar panel.
• TP4056 USB charging module (most have a USB-C port + JST connector for the LiPo).
• 500–2000 mAh LiPo cell.
• Low-power node from L04-39 (Pro Mini + sensor + LoRa).
• Waterproof enclosure (IP65 ABS box).
• Cable glands for wires going in/out.

Energy budget — does it work?

Solar panel: 1 W × 5 hours of sun per day (UK / Malaysia average) = 5 Wh/day.

Conversion losses + LiPo charge efficiency: ~70%. Usable: ~3.5 Wh/day.

Node consumption: ~50 µA average × 3.3 V = 165 µW = 4 mWh/day.

Energy in / out: 3500 / 4 ≈ 900× surplus. The node can run forever and the LiPo will stay topped up even through cloudy weeks.

The math is dramatically in your favour with a tiny panel + a properly sleeping node.

New Concept · Solar charging + power management 25 min

The wiring

The TP4056 takes 5 V from the panel, charges the LiPo (when light), and continuously supplies the load (whatever the LiPo can give). The module with the "DW01" chip has under-voltage protection — won't kill the LiPo if it gets fully drained.

MPPT — beyond TP4056

The TP4056 is dumb: it tries to draw ~1 A from the panel always, even in cloudy weather when the panel can only give 200 mA. The panel's voltage drops, efficiency suffers. For tiny solar projects this is fine.

For larger panels (10 W+), use a real MPPT (Maximum Power Point Tracker) — adjusts the load impedance to extract maximum power. Modules: CN3791, BQ24650, LT3652.

Voltage chain

Solar (~6 V open / 5 V loaded) → TP4056 → LiPo (3.0–4.2 V) → 3.3 V LDO regulator → Pro Mini.

Drop the LDO if your Pro Mini is the 3.3 V version that accepts 3.0–4.2 V directly on VCC. Each conversion costs efficiency.

Sealing for outdoor use

• IP65 ABS box from any electronics supplier.
• Cable glands (PG7 size) for wires.
• Silicone sealant on seams.
• Vent membrane to allow air pressure changes without water ingress.
• External antenna on the LoRa for range.

Sensors poking out

For a soil moisture probe: short cable from inside the box to the probe in soil. Capacitive probes are essential — resistive ones electroplate in days.

For a temperature sensor: drill a small hole, push DS18B20 probe through, seal with epoxy. Keep the chip inside; the probe is outside.

Worked Example · Build a perpetual weather sensor 30 min

Step 1 — energy estimate

Pro Mini + BME280 + RFM95 LoRa, sampling every 15 min.

• Active cycle: 30 mA × 200 ms = 6 µAh.
• Sleep: ~10 µA × 14.8 min = ~2.5 µAh.
• Per cycle: ~8.5 µAh. 96 cycles/day = ~0.8 mAh/day. ~300 mAh/year.
• 2000 mAh LiPo handles 6+ years of consumption alone. Solar tops it up daily.

Step 2 — pick the panel

500 mA / day surplus over consumption. Even a 100 mA average from a small 1 W panel exceeds requirements. A 1 W panel suffices.

Step 3 — assemble inside the enclosure

1. Mount TP4056 + LiPo + Pro Mini on a small piece of perfboard.
2. Route solar wires + sensor wires + antenna out through cable glands.
3. Add a small power switch (optional but useful for service).
4. Apply silicone where lid meets body.

Step 4 — gateway

An ESP32 inside your house listens on LoRa, forwards to MQTT (L04-14), Home Assistant displays the readings.

Step 5 — field test

Mount outside (garden, balcony, roof). Verify packets keep arriving for ≥ 1 week through varying weather.

Step 6 — monitor the LiPo

The node publishes its V_BAT with each reading. Watch it on the dashboard. After a cloudy week, does it drop? Solar surplus working as expected?

Try It Yourself 15 min

Mini-Challenge · Mount and forget 10 min

1. Mount the unit outside.
2. Leave for a week.
3. Check the data continuity — any packet drops?
4. Check the V_BAT trend — stable? Climbing? Falling?
5. If V_BAT is falling, undersize the panel or oversize the consumption.

This is the "deploy and forget" bar. Get it right once, run it for years.

Recap 5 min

Solar + TP4056 + LiPo + L04-39 sleep node = a sensor that never needs charging. The math is dramatically in your favour with proper sleep. Weatherproof enclosure + cable glands + external antenna = field-ready. Cluster G done. Cluster H starts tomorrow: writing your own libraries.

Solar panel

PV cells producing DC current under light. 5 V, 1 W small panels are sufficient for low-power IoT.

TP4056

Cheap USB-C LiPo charger module with optional protection circuit. Standard for hobby solar projects.

MPPT

Maximum Power Point Tracker. Adjusts load impedance to extract max power from the panel. Worth it for > 5 W panels.

Voltage chain

Solar → charger → battery → regulator → chip. Each conversion has efficiency loss.

IP65 / IP67

Ingress Protection ratings. IP65 = dust-tight + water-jet-resistant. IP67 = immersible to 1 m for 30 min.

Cable gland

Compression seal allowing wires to enter a sealed enclosure without compromising the seal.

Vent membrane

Permeable to air, not to water. Equalises pressure when temperature changes; prevents seal failures.

Adaptive sleep

Dynamically adjusting the sleep interval based on V_BAT or other conditions. Extends lifetime during low-charge periods.

Homework 5 min

1. Mount your solar node outside. Note V_BAT baseline.
2. Check after 24 h. Did it gain or lose charge?
3. Read ahead to ARD-L04-41 (Anatomy of a Library). Cluster H starts.