A timepiece power source is provided, including a barrel containing a mainspring mounted between a drum and an arbor rotating integrally with a ratchet wheel, including a differential gear coaxial to the barrel, with a first input driven by the ratchet wheel, a second input which is the drum, and a pivoting output wheel, coaxial to the arbor, arranged to form or to drive a power reserve indicator, the differential gear including balls driving the output wheel, cooperating with an upper surface of the drum, and with a lower surface, which is opposite thereto, of a differential plate forming the first input, pivoting coaxially on the arbor and connected to the ratchet wheel by a friction means cooperating with a plate, and the ratchet wheel forms the cover of the barrel.