Abstract: A DC current level detector is provided for monitoring the heater current of a pitot tube. The detector includes a toroidal core defining a bore through which can be passed a line carrying the current to be detected whereby an amount of flux is generated in the core according to the current to be detected. A reference circuit is provided to generate in the core a reference amount of flux against which the first flux is compared in an algebraic combination. An oscillator circuit is inductively coupled to the core to saturate the core in opposite directions. The algebraic difference between the fluxes is employed to generate a signal to operate a relay. The relay is included in a circuit in which are connected a power source and an indicator such as an alarm lamp. The core is of toroidal shape and a casing is provided which houses the core along with the associated circuits and which is provided with an opening corresponding with the bore of the core.