Abstract: This invention relates to an ignition circuit assembly for an explosive device of the kind in which an electric heating element is energizable from a transformer secondary winding to ignite the explosive. A transformer primary winding, which in use is magnetically coupled to the secondary winding, is contained in a casing, together with a generator circuit which is operable to energize the primary winding and an inhibiting circuit which inhibits operation of the generator circuit until a trigger signal is applied to the inhibiting circuit to cancel the inhibition.