Rotary dosage sensing module for and in a disposable pen drug delivery device

A rotary dosage sensing module for a pen drug delivery device comprising a drug-filled cartridge with a displaceable piston and an outlet and a piston rod to be axially advanced in a direction towards said piston to displace the piston and thereby expel a dosage of drug from the cartridge through the outlet, the piston rod rotating relative to the piston around an axis of rotation (A) during axial movement,

• said module comprising:
• a power source unit with a (−) and a (+) terminal,
• a processor unit connected to the (−) and (+) terminals of the power source unit,
• a sensor unit comprising;
• a first sensor part adapted to be directly or indirectly fixed to a part of the pen drug delivery device not rotating during dose expelling, the first sensor part comprising a flexible printed circuit board sheet having a first surface on which is disposed a plurality of individual electrical conductive sensor areas arranged in a pattern, some of which being electrically connected to the (−) terminal of the power source unit and some of which being connected to the processor unit, wherein the flexible printed circuit board sheet is folded around and adhered to the power source unit and where said first surface is supported by a surface of the power source unit,
• a second sensor part arranged opposite to the first sensor part and adapted to be directly or indirectly fixed to the piston rod to follow the rotation of the piston rod during dose expelling, the second sensor part comprising a plurality of electrically connected contact structures, and
• wherein the plurality of electrically connected contact structures are connected to the (−) terminal of the power source unit via those electrically conductive sensor areas that are connected to the (−) terminal of the power source unit, the plurality of electrically connected contact structures being adapted to, upon relative rotational movement between the first and second sensor part, electrically connect different individual electrically conductive sensor areas to the processor unit to thereby close an electrical circuit between the (−) terminal and the processor unit for the different conductive sensor area, each electrical connection generating an electrical signal to the processor unit being indicative of the rotational position between the first and second sensor part, and wherein the processor unit is adapted to process said electrical signals to determine the amount of relative rotations between the first and second sensor part and thereby calculate the expelled dosage size based on the determined amount of relative rotations.