A data acquisition device (100) that can be detachably connected to an actuator member (205) of a drug delivery device (200) includes a detector unit having a sensor element configured to detect an actuation action to be performed on an actuator member (205) to dispense at least a portion of a drug contained within the drug delivery device. A push-button (120) is configured to exert the actuation action, wherein the sensor element (132) detects pressing the push-button (120) as part of the actuation action. An electronic subassembly (130) includes a power source (134) and is connected to the sensor element (132) and to the power source (134). The electronic subassembly (130) is configured to store at least a piece of information related to the detected actuation action and to transmit said piece(s) of information in compliance with an actual charge level of the power source (134).
