Drive unit for a medicament delivery device

A drive unit for a medicament delivery device, the drive unit comprising:

• an elongated plunger rod extending in a longitudinal direction, where the plunger rod comprises a plurality of sound or vibration elements on an outer surface and a co-acting element at a distal end of the elongated plunger rod and where the elongated plunger rod is hollow and has an open distal end and a closed proximal end;
• a plunger rod holder comprising a beam positioned transverse to the longitudinal direction, where the beam sequentially contacts individual sound or vibration elements of the plurality of sound or vibration elements when the elongated plunger rod moves axially and proximally relative to the plunger rod holder;
• a drive force element capable of applying a drive force on the elongated plunger rod, where a proximal end of the drive force element is positioned within the elongated plunger rod;
• an actuator operably connected to the co-acting element on the elongated plunger rod for releasably holding the elongated plunger rod in an energized state when the drive force element is exerting the drive force on the elongated plunger rod;
• an activator operably connected to the actuator for releasably holding the actuator in a holding position, where axial movement of the activator causes the actuator to move from the holding position when the co-acting element on the elongated plunger rod forces a distal portion of the actuator to move in a direction generally transversal to the longitudinal direction resulting in the axial and proximal displacement of the elongated plunger rod.