From $4 Solar Light to a Smart Wireless Sensor Node: MSP430 + nRF24L01+ for Garden Monitoring

The goal of this project is to repurpose a low‑cost solar driveway light (available for $3–$4 at hardware stores like Lowe’s and Home Depot) into a wireless remote sensor node. The node uses an MSP430G2553 microcontroller and an nRF24L01+ transceiver (with the spirilis library) to transmit light intensity from the light’s built‑in photoresistor, soil moisture from a custom sensor inspired by gardenbot.org, and soil temperature from a 10 kΩ silicon‑coated thermistor. The data is sent wirelessly to a second MSP430G2553/nRF24L01+ pair mounted on a Raspberry Pi.

The Raspberry Pi receives the data via UART, and a Python script logs it into CSV and JSON files (drawing inspiration from the Uberfridge project). An Apache server serves the JSON, and the data is visualised in real‑time using the JavaScript Flot chart library. Additionally, the Pi’s node controls a transistor that drives a solenoid valve on the garden hose, turning it on or off based on the soil moisture reading.

I taught myself Eagle (Autodesk) and designed a custom PCB to replace the stock board inside the solar light. The board incorporates a Texas Instruments TPS61097 boost converter (I obtained a free sample) to step the 1.2 V from the battery up to 3.3 V needed to power the MSP430.

The first prototype (shown in Photo 6) includes only the power circuitry. I temporarily connected the EN and VIN pins on the TPS61097 because the bypass switch (see page 16 of the datasheet: https://www.ti.com/lit/pdf/tps61097-33) was not functioning correctly. I used two 100 kΩ, 1 % resistors in series to create a 200 kΩ network, but the bypass remained stuck on, so I jumped EN to VIN to obtain the full 3.3 V output. I’d like to enable the bypass feature correctly—any guidance on the resistor values for a 1.2 V battery would be appreciated.

For more detail, see the full project write‑up: Solar Driveway Light to MSP430 Wireless Sensor Node.