  Feedback flashing ratchets are thermal rectifiers that use information on the
state of the system to operate the switching on and off a periodic potential.
We discuss different strategies for this operation with the aim of maximizing
the net flux of particles in the collective version of a flashing ratchet
consisting on N overdamped Brownian particles. We show the optimal protocols
for the one-particle ratchet and for the collective ratchet with an infinite
number of particles. Finally we comment on the unsolved problem of the optimal
strategy for any other number of particles.
