A setting mechanism for a triggering device employs a knob fixed to a rotating part of the triggering device. The knob carries a selector lever that is pivotable to radially project from the knob in angularly spaced directions. Arcuate walls surround the knob to define gaps corresponding to alternative rotational periods for the knob. The selector is pivoted to travel in one or the other gap. The selected rotational period may correspond to a predetermined time.