Abstract: A blood collection bag is suspended from a leaf spring, which leaf spring, on being repositioned a specified distance by the weight of a properly filled blood collection bag, energizes an electrical circuit. The electrical circuit generates a pulse to momentarily energize a coil in proximity to a permanent magnet and momentarily disrupt the magnetic field of the magnet. A clamp for terminating the flow of blood to the blood collection bag is actuated on release of a weight retained by the magnetic field of the magnet, which release is achieved by the disruption of the magnetic field on energization of the coil.