This device monitors the correct functioning of a ﬂashing light by opto-electronic means. The flash from the light is fed via an optical ﬁbre to a phototransistor, which converts the optical impulse to an electrical impulse. The electronic circuit evaluates the pulse and its regular repetition. As soon as the operating voltage is applied, the evaluation relay closes the changeover contact. If the operating voltage fails, the relay opens immediately.
This method of operation represents the fail-safe normally-closed circuit function and guarantees an alarm even if the operating voltage fails. On the other hand, the changeover contact serves to continue an alarm, e.g. in a failure message line, or the direct blocking of further machine processes.