  We propose an ac current source that can be tuned from a pure charge to a
pure spin current source. The device consists of two mesoscopic capacitors
attached to a two-dimensional strip of a topological insulator. The change from
charge to spin current is controlled by an offset in the top gate potentials
that drive the capacitors. In addition to this setup, which anticipates the
experimental realisation of quantum point contacts in topological insulators,
we propose an analogous source in the quantum Hall regime which only relies on
presently available building blocks. To this end, we calculate the band
structure of a topological insulator in a magnetic field. The intrinsic
spin-orbit coupling, together with a split gate, allows for an analogous
source, where charge and spin current can be manipulated. The realisation of
the device as well as the detection of the ac spin current are within reach of
present experimental technology.
