Solar powered Bluetooth Thermometer with Supercap

This projects provides hardware & software modifications to run the Xiaomi Thermometer LYWSD03MMC with a supercapacitor and mini solarpanel.

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I stumbled on the XIAOMI Mijia Bluetooth Thermometer LYWSD03MMC as i wanted to monitor room temperature and humidity via bluetooth low energy.

I found two really awesome projects which enable custom software on the LYWSD03MMC.
As i really dislike the usage of batteries in low power devices (Even if they last months or years) i wanted to power this tiny devices with a mini solar panel and a supercapacitor.

Software sources can be found on GitHub: ATC_MiThermometer

  • 1 × Xiaomi Mijia Bluetooth Thermometer LYWSD03MMC the main part
  • 1 × Coin Cell Supercapacitor 1.5F 5.5V (e.g. Korchip DCL5R5155VF EDLC)
  • 1 × Germanium Diode should be prefered over Silicon Diode due to less voltage drop
  • 1 × Solar Panel for ambient light conditions a amorphous solar panel works best (up to 5 Voc )

  • Small solar panel charging curve on 2 sunny days

    MartMet02/25/2021 at 20:52 0 comments

    The devices is placed on the balcony and charged with a small solar panel BR-243318C.

    In direct sunlight the small version of the solar panel gives max. ~3.45V (- 0.3V of the Ge-Diode)

    In ambient light the voltage is about 2.5V so the supercapacitor is not fully used .... this solution is suboptimal for indoor use.

  • 24 MHz vs 48 MHz

    MartMet02/25/2021 at 20:30 0 comments

    To run a more powerful solarpanel the power waste cycle should consume as much power as possible. So i changed the MCU clock speed to 48MHz.

    I was curious what the impact would be on the overall power consumption i plotted a comparsion between the voltage drop via 24 MHz & 48 MHz.

  • Runtime test with 1.5F Supercapacitor

    MartMet02/25/2021 at 20:12 0 comments

    Voltage drop over time  (mV / hours of day)

    Charged 1.5F to 3.6V and let the thermometer run with 48MHz. Advertising interval 10s, measurement is done via plotting internal ADC values.

    Below 2.0 V, the LCD is barely readable, but the Bluetooth transmission still works. This small voltage gap at around 2380mV seems to occure everytime, so something must drain power at this voltage.

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  • 1
    Case Modifications

    The case needs minor modifications to be able to connect the solar panel with wires.

    Note: When viewing the LCD the wires should be exit the case on the left side. The BLE antenna is on the right and i expierenced a worse RSSI while the first attempt to exit the wires on the right side.

  • 2

    Just solder GND and Voltage wires a shown. Dont forget to place in a diode, this is necessary that the solar panel does not discharge the Supercap overnight.

  • 3

    Remove the "-" pin from the supercapacitor. And carefully bend the "+" pin down. The capacitor fits into the slot of the CR2032.

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