An electronic device (104) comprising: a housing (203) for attachment to an injection device (102); a first sensor assembly (304) comprising a first coil (406a) and two magnets (402a, 404a) of opposite polarity, wherein the first coil (406a) is configured to provide a first voltage pulse as a first Wiegand wire (224) of the injection device (102) moves from a first position proximate to one of the magnets (402a) of the first sensor assembly (304) to a second position proximate to the other magnet (404a) of the first sensor assembly (304); and one or more processors (308) configured to enter an enabled state from a sleep state after receiving the first voltage pulse from the first coil (406a).