  We study the scattering in a quantum star graph with a F\"ul\"op--Tsutsui
coupling in its vertex and with external potentials on the lines. We find
certain special couplings for which the probability of the transmission between
two given lines of the graph is strongly influenced by the potential applied on
another line. On the basis of this phenomenon we design a tunable quantum
band-pass spectral filter. The transmission from the input to the output line
is governed by a potential added on the controlling line. The strength of the
potential directly determines the passband position, which allows to control
the filter in a macroscopic manner. Generalization of this concept to quantum
devices with multiple controlling lines proves possible. It enables the
construction of spectral filters with more controllable parameters or with more
operation modes. In particular, we design a band-pass filter with independently
adjustable multiple passbands. We also address the problem of the physical
realization of F\"ul\"op--Tsutsui couplings and demonstrate that the couplings
needed for the construction of the proposed quantum devices can be approximated
by simple graphs carrying only $\delta$ potentials.
