  We report electronic transmission properties of a tight binding Aharonov-Bohm
ring threaded by a magnetic flux, to one arm of which a finite cluster of atoms
has been attached from one side. we demonstrate that, by suitably choosing the
number of scatterers in each arm of the quantum ring and, by decoupling the
ring from the atomic cluster, the transmission across the ring can be
completely blocked when the flux threading the ring becomes equal to half the
fundamental flux quantum. A transmission resonance then occurs immediately as
the coupling between the ring and the impurity cluster is switched 'on'. It is
shown that the delta-like transmission resonances occur precisely at the
eigenvalues of the side coupled chain of atoms.Thw 'switching' effect can be
observed either for all the eigenvalues of the isolated atomic cluster, or for
a selected set of them, depending on the number of scatterers in the arms of
the ring. The ring-dot coupling can be gradually increased to suppress the
oscillations in the magneto-transmission completely. However, the suppression
can lead either to a complete transparency or no transmission at all,
occasionally accompanied by a reversal of phase at special values of the
magnetic flux.
