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.
Hello, nice project. I did something similar regarding the supercapacitor. I think that you can easily almost double the autonomy of your system.
You could place a cheap boost converter to 5V between the panel and the cap. This way even with weak sun it will charge the cap always to 5V.
And a LDO converter after the cap to bring the voltage down to 3.3V. Then you supercapacitor will store charge between 5V down to 2V or the minimum of the clock to work. This is almost double than now.
This is amazing what can be done with a very small solar panel and supercapacitor. And parts are now very cheap.