BQ2550r power harvester
A ultra-low power harvester.
Uses the Texas Instruments part BQ25504; see the datasheet.
It is designed: - for small size since many applications are portable. - to harvest indoor lighting (not full sun.) - pulsed loads
The design "programs" the BQ25504 for: - solar cell input - super capacitor storage (rated 5V) - all-or-nothing load: output is 0V, or [2.5-5V], pulsed.
Connections: - energy source across Vin and Gnd (typically one or more solar cells or panels in parallel) - load across Load+ and Load- (the load is switched on the low side) - storage device across Bat+ and Gnd (typically a 5V supercap)
You might change the design but if you use a battery or different energy source, be sure to read the datasheet: this design doesn't use the battery management function of the BQ25504 in the recommended way.
The design is for applications that use as much energy as available (harvested) in pulses as it becomes available. When the storage reaches 5V, the load is switched ON. The load stays ON as the storage voltage drops to 2.5V under load. The load is switched OFF when storage reaches 2.5V (partly because harvesting works better if the storage is not completely discharged, and partly because most loads don't work well at low voltages.) The load remains OFF until storage voltage reaches 5V again, as energy is harvested. Load ON time depends on the load and the capacity of the storage. Load OFF time depends on the rate of energy harvested and the capacity of storage.
You could also omit the load switching FET and tap into the VBAT_OK signal, which is high for the same conditions. Then your load could always be powered from Bat+, and use the power as you see fit.