Implementations of the present disclosure are directed to a module for attachment to an injection device that includes a flexible carrier foil configured to be attached to a surface of the injection device independent of a geometry of the surface, at least one light emitting diode attached to the flexible carrier foil in a first position and configured to emit a light signal in a direction based on the first position through a transparent wall of the injection device, at least one photodiode attached to the flexible carrier foil in a second position relative to the first position, the at least one photodiode being configured to detect at least a portion of the light signal from a direction based on the second position and being configured to emit an electrical signal based on the light signal to a microprocessor, and wherein the microprocessor is configured to receive the electrical signal emitted by the photodiode and to determine the position of a stopper and the amount of the fluid within the injection device based at least in part on the electrical signal.
