Cadmium sulfide flame detector for use with protect relay oil primary controls. Can be used with controls such as R4166, R4184, R7184, R8182, R8184, R8185 and R8404. Glass-to-metal hermetic seal in plug-in cell prevents deterioration by humidity, soot or oil fumes. On flame failure, the light sensitive cadmium sulfide cell, in conjunction with flame sensing circuitry, causes the Protectorelay control to shutdown the main oil burner.