tricky situation: USB charging 5V, LiPo batterias 3.7V, e-ink controller needs 5V, solar panel gives 7V
diverse circuits for voltage increase (booster, step-up), decrease (buck, step-down) are needed, or current stabilization (photovoltaics) are needed
WittyPi allows us to get current/voltage information, have inner clock, have device with its own boot on, boot off cycles
cute little Voltmeter connected to the third state of the main button