Exploders of the magneto type that are operated by the twist action given by a half-turn of the firing key. A magneto exploder consists essentially of a small armature that can be rotated between the poles of a set of permanent magnets. The armature is rotated by means of toothed gear wheels actuated by the movement of the firing key. The electric circuit between the exploder and the detonator is completed by means of an automatic internal switch operating at the end of the stroke, or contact may be made by means of a pushbutton.