Abstract: A device for producing a detection signal when interfering, preferably metallic conductive pieces occur in an at least largely non-conductive conveyor stream. An alternating electromagnetic field is built up by an alternating current generator through a transmission coil in a segment of the conveyor stream to be monitored. The changes in amplitude and phase of the alternating electromagnetic field are detected by a coil system that feeds an evaluating circuit for deriving the detection signal, which, when an interfering piece appears, causes a stoppage of the conveyor stream for the purpose of removing the interfering piece. A resetting device (reset button) is also provided which is able to cancel the conveyor stoppage. A device is also provided for blocking the resetting device, controlled by the evaluating circuit, which disables the resetting device as long as the evaluating circuit still emits a detection signal.