Abstract: A signal for a conventionally constructed mail box is provided having a distinctive visual effect wherein coaction between a stationary and movable member shows a totally black field indicating no mail received, but shows a series of distinct white bars when a mechanism is automatically operated upon opening the door of the mail box to insert mail. The construction is substantially enclosed and very compact to be attached by a consumer on the back wall of a mail box with a signal control mechanism within the mail box extending from the back wall to the hinged door of the mail box. The invention is readily attachable to existing mail boxes with simple tools and sheet metal screws.