Embodiments described herein include a controller (22). The controller includes at least one interface (62) that couples the controller to stimulation circuitry (24), which includes a pair of electrodes (26a, 26b), and to measuring circuitry, and further includes controller circuitry (64). While the electrodes are coupled to skin (60) of a subject, the controller circuitry, via the interface, drives the stimulation circuitry to (i) pass a plurality of stimulating pulses (32) between the electrodes, and (ii) pass one or more reference pulses (34) between the electrodes, the reference pulses being different from each of the stimulating pulses. The controller receives, from the measuring circuitry, an impedance between the electrodes that is measured using the reference pulses. In response to the impedance measured using the reference pulses, but not in response to any impedance measured using the stimulating pulses, the controller circuitry controls the stimulation procedure. Other embodiments are also described.
