A vital safety monitoring circuit including an electromagnetic transformer for feeding periodic check signals into one leg of a balanced bridge network having a pair of input and output terminals. A solid-state switching circuit is connected across the pair of input terminals. An operational amplifier circuit is connected across the pair of output terminals. An optical coupler circuit is connected to the operational amplifier for producing output signals corresponding to the periodic check signals when the solidstate switching circuit is nonconductive and in the absence of a critical component or circuit malfunction.
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