I liked the idea to monitor room temperature and humidity via bluetooth low energy. Therefore i used the XIAOMI Mijia Bluetooth Thermometer LYWSD03MMC. A really nice tiny device, unfortunatly powered by a single CR2032 coin cell.
Although batteries last for months or even years, I'm not a big fan of powering small devices with such polluting equipment. I had the idea to power the thermometer with a mini solar panel (and a supercap as backup for night / cloudy times).
I wanted to keep the hardware modifications as simple as possible, so there are basically only 3 components needed:
a solarcell, a supercap and diode to prevent discharge at night.
Also i wanted to provide a wider voltage range and a accurate power display of the actual charge of the supercap. Therefore i modified the open source custom software (links below) and added a power display which takes the non-linearity of the supercap into account.
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.
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.
Remove the "-" pin from the supercapacitor. And carefully bend the "+" pin down. The capacitor fits into the slot of the CR2032.