illuminated by a pair of bias lamps 36 and 38 fixedly mounted within the sensing device 12 the bias lamps are preferably 7370ps available from sylania miniature lighting products inc hillsboro n h these lamps are connected in series to a power supply voltage of fifteen volts d c so as to provide seven and one half 7 5 volts per lamp the bias lamps 36 and 38 fully illuminate the encodings 20 and 22 upon registration of the workpiece holder 10 with respect to the sensing device 12 referring now to fig3 the circuitry used to control the light emitting diodes 32 and 34 and to respond to the photosensitive transistors 28 and 30 is illustrated the light emitting diodes are subject to a drive current having a frequency defined by a nine kilohertz oscillator 40 operating through a base activated transistor 42 the drive current is defined by a voltage source 44 as well as a resistor 46 upstream of each light emitting diode each light emitting diode is selectively subjected to the aforementioned drive current by a selectively activated downstream transistor 48 or 50 the base of each downstream transistor is connected to an output of a d type flip flop circuit 52 the negation output of the flip flop is fed back to the d input so as to cause the flip flop 52 to toggle in response to a five hertz clock signal from a clock circuit 54 in this manner a logically high signal is always present at either the base of the transistor 48 or the base of the transistor 50.