Abstract: A neon sign circuit is disclosed having a neon tube forming the lighted portion of the sign, a source of electrical power, and a high voltage transformer between the electrical power source and the neon tube. The circuit also includes a neon sign circuit malfunction detector circuit. This circuit includes a ground fault detector connected in the circuit between the source of power and the transformer, to ground, and to the neutral load side of the primary windings of the transformer. A pair of adjustable spark gaps are each connected to one of the secondary windings of the transformer between the transformer and the neon tube. A light source is connected between each spark gap and ground and light sensitive resistors are positioned adjacent to each light source.