We discuss a mesoscopic capacitor in the quantum spin Hall regime. One plate of the capacitor is represented by a quantum dot, tunnel coupled to the helical edge states of a two-dimensional topological insulator, the other plate by a top gate above the dot. Periodically modulating the top gate, the quantum dot emits pairs of particles propagating in opposite directions along the edge. We discuss two applications : The first involves threading the quantum dot with a magnetic field. This separates the emission times and the capacitor acts as a single particle source. The source can then be interpreted in analogy to a Hong-Ou-Mandel experiment, with the magnetic field playing the role of the time delay between the two involved particles. The second application involves two mesoscopic capacitors attached on the opposite edges of a two-dimensional topological insulator. The current emitted by this setup can be tuned from a pure ac charge to a pure ac spin current by an offset in the top gate potentials. As the required quantum point contacts are not yet available in topological insulators, we additionally propose an analogous source of ac spin current in the quantum Hall regime.