For this moment I don't have much to say. It will be use NRF24L01+ RF module to transmit data and receive commands. Node itself will be based on STM32F or STM32L with M0 core. Solar cell or solar panel is a source of energy for operating. MPPT system is also can be implemented using ST SPV1050 high efficient energy harvester. I prefer to use supercaps as energy storage.
This project would be a long story...
Ultralow power energy harvester and battery charger
STM32L011D3P6 TSSOP14 or STM32F072F4P6 TSSOP20
EECS0HD224H Supercap 5.5V 0.22F
Capacitors / Electric Double-Layer (EDL)
Now I have energy harvester schematic configured in boost mode. The main purpose - energy harvester testing. Stil I don't sure is SPV1050 right choise. So need to create test board and figure it out. Only problem - very fine pitch (0.4mm) so I need to use 0.15\0.15mm traces PBC (can't be done DIY). And shipping from china would take a lot of time.
Fast shipping costs 3 times more than boards itself ;(
In schematic there are possible to use one of four different inductors, four 10uF ceramic 0805 or one 47uF polarised cap, one of two different litium rechargable batteries or up to six 5.5V 0.22F supercaps. Some kind of flexibility.
Next step: measure max power point to evaluate resistor values for harvester. Not complex but require time to measure VACs.
Next steps: 1) Create schematic for SPV1050 in boost configuration. 2) Measure VAC of different solar cells (currently I have two, but planned to have 2-3 more to find the best one). Software and hardware for measuring also can be created (or I can use keithley source meter and Labview program). 3) Write a program to calculate passive components for SPV1050. Can be useful, I think.
Pics: Solar panel made of two 0.5V 140mW solar cells ($2 10pcs).
VAC of three solar cells (140mW 0.5V again) connected in series. Source of light - 60W light bulb.