An alarm unit for signaling impending overflow of a urine or ostomy (waste) bag includes an external magnet held at the end of an arm. An internal ferrous armature attracts the magnet which provides flux holding a switch element open when the magnet provides at least a predetermined flux to the switch. A clamp allows the user to fasten the unit to an edge of the waste bag with the bag walls between the magnet and the armature. When the bag contents reach the magnet, fluid force of the bag contents separates the magnet and the armature, causing the switch element to close and conduct power from a power supply to a signal element. The arm may be flexible. The switch element preferably comprises a reed switch controlling an electronic power switch.