A first capacitor is discharged with every pulse of a train of pulses of a frequency proportional to the actual speed of a rotary machine under control and is charged up during intervals between these pulses. The voltage of the first capacitor provides a signal to a threshold circuit which operates to provide a second signal whenever the voltage of the first capacitor exceeds a predetermined voltage which may be set in order to set the speed at which the machine is to be controlled. During the presence of the second signal a second capacitor is charged, beginning at a predetermined voltage, preferably zero. The regulating magnitude for the machine to be controlled is derived from the voltage of the second capacitor either directly or indirectly through a controller circuit and a power stage. The circuit is particularly useful for control of the speed of machines that run slowly or can conveniently provide only relatively few speed-indicating pulses per revolution.