  The measurement of cosmic-ray individual spectra provides unique information
regarding the origin and propagation of astro-particles. Due to the limited
acceptance of current space experiments, protons and nuclei around the "knee"
region ($\sim1\ PeV$) can only be observed by ground based experiments. Thanks
to an innovative design, the High Energy cosmic-Radiation Detection (HERD)
facility will allow direct observation up to this energy region: the instrument
is mainly based on a 3D segmented, isotropic and homogeneous calorimeter which
properly measures the energy of particles coming from each direction and it
will be made of about 7500 LYSO cubic crystals. The read-out of the
scintillation light is done with two independent systems: the first one based
on wave-length shifting fibers coupled to Intensified scientific CMOS cameras,
the second one is made of two photo-diodes with different active areas
connected to a custom front-end electronics. This photo-diode system is
designed to achieve a huge dynamic range, larger than $10^7$, while having a
small power consumption, few mW per channel. Thanks to a good signal-to-noise
ratio, the capability of a proper calibration, by using signals of both
non-interacting and showering particles, is also guaranteed. In this paper, the
current design and the performance obtained by several tests of the photo-diode
read-out system are discussed.
