A display arrangement for e.g. a medical substance delivery device includes a movable control member, at least one transducer responsive to movement of the control member to convert mechanical energy into electrical energy to output an electrical signal, a drive circuit for receiving the electrical signal and for outputting an output drive signal, and a display device for receiving the output drive signal and displaying a variable image that varies consequent on the relative movement. The display may be a bistable display, so that the display function is self-powered without a requirement for a battery or the like. The parameter displayed for a medical substance delivery device may be a dose volume, a count of doses delivered, progress and/or completion of a dose. The drive circuit is an absolute position encoder and detects the actual position of the control member and does not deduce the position by counting pulses.