Provided is a boosting circuit having a small circuit scale. The prism assembly has a first light incident surface, a first light emitting surface and a second light emitting surface. Additional curved and straight sections can be linked to provide various ring configurations. The display is divided into a first area that is not overlapped with the first main body when the second main body is slidably open, and a second area that is overlapped with the first main body when the second main body is slidably open. The calibration is done gradually in order to differentiate between sensor drift and a transition in current due to a differential fault.