The present invention relates to an electronic circuit for harvesting energy from ultra-low power sources, said circuit comprising a capacitor module (1), a microcontroller (2), two analog switches (3 and 7), an auxiliary battery module (4), a terminal (5), and an operating mode module (6). The circuit is characterised in that: at least two sources (FE) from a plurality of sources (FE) are connected to the capacitor module (1); and the microcontroller (2) transmits the RC digital signals [1.4] at the capacitor module (1) to the first set of analog switches and changes from an idle state to an active state. In addition, the microcontroller (2) is configured to: perform an energy storage step, by connecting the energy sources in parallel with the capacitors; perform a first amplification step, by reconnecting the set of capacitors in series with one another via the first set of analog switches connected in parallel, by means of a switch, with the array of auxiliary batteries that are connected in parallel with one another; perform a first step of transferring energy from the capacitors to the auxiliary batteries; perform a second energy amplification step, by reconnecting the array of batteries connected in series with one another via the second set of analog switches; perform a second step of transferring the energy amplified by the auxiliary batteries by means of a switch, by means of which the auxiliary batteries connected in series with one another are connected in parallel with a main external battery, transferring the energy to same.