A drug delivery apparatus including a barrel to contain a substance, a piston slidable in the barrel, a supply circuit to provide input currents, an antenna affixed along the barrel to receive the input current, generate inductance with the piston, and provide output currents commensurate with a position of the piston along the piston course, and processing circuitry configured to receive the output currents, and determine a quantity of the substance inside the barrel based on the output currents.