Abstract: The top, flashing lights and steady-burning side lights placed on an elevated obstruction to warn aircraft are monitored by a failure-detecting circuit. The circuit includes circuit components which detect and indicate the failure of either or both top lights, the latter case possibly indicating the alternative of continuous operation thereof, and for detecting and indicating one or more failures of the side lights. Provision is made for remotely actuating an internal latching relay which so changes internal connections that all detecting and indicating components should response.