Disclosed herein is an auto injector for administering a medicament, the auto injector comprising: a housing; a cartridge receiver configured to receive a cartridge comprising a first stopper and a cartridge compartment containing the medicament, the cartridge compartment having a first cartridge subcompartment containing a first medicament component of the medicament and a second cartridge subcompartment containing a second medicament component of the medicament; a drive module coupled to move a plunger rod between a retracted plunger rod position and an extended plunger rod position, the plunger rod being configured to move the first stopper; a processing unit coupled to the drive module, wherein the processing unit is configured to control the drive module to move the plunger rod from a first plunger rod position to a mix plunger rod position with a mix plunger rod speed, wherein the mix plunger rod position is selected to position the first stopper in a position wherein the first medicament component is mixed with the second medicament component, and to provide an onset signal after a number of completed inversions of the auto injector has been performed.