A ratchet screwdriver includes a handle receiving a rod coupled with a coupling sleeve. The controller may also perform, during each of a repeating sequence of time periods, analog to digital conversions on the analog signals, and further provide command signals to the electric motor and the VVC based on the ATD conversions and readings. The outer conveyor unit includes a pivotable conveyor section that is mounted for pivoting movement about a substantially horizontal pivot axis. The counter doping causes the total net charge in the first region to be approximately equal to the net charge in the second region. The organic light-emitting device includes a cathode; an anode; and a light-emitting layer interposed between the cathode and the anode, wherein a buffer layer is disposed on at least one surface of the light-emitting layer.