Sensor device removably attachable to a drug delivery device

A sensor device removably attachable to a drug delivery device, the sensor device comprising:

• an array of optical sensors arranged within the sensor device such that, when the sensor device is attached to the drug delivery device, (i) the drug delivery device has a first movable element which is configured to move along a linear path parallel to a longitudinal axis of the drug delivery device, and (ii) each optical sensor is operable to detect light received at different locations along the linear path and to output a signal indicative of an amount of detected light;
• circuitry configured to receive the signals output from the optical sensors and, based on the received signals, determine information associated with a location along the linear path of the first movable element;
• and
• a sensing arrangement arranged within the sensor device such that, when the sensor device is attached to the drug delivery device, the sensing arrangement is operable to read encoded information that is visible from an exterior of the drug delivery device,
• wherein the encoded information comprises a mode indicator portion for indicating an operational mode of the drug delivery device, and
• wherein the mode indicator portion is provided on an internal element of the drug delivery device, the internal element being configured to move from a first position to a second position in response to an actuation of a drug delivery mechanism, and move from the second position to the first position in response to an actuation of a drug dialing mechanism.