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