medium brightness state is desired one of the lines is high and one of the lines is low when the full brightness state is desired both lines are low the control lines 37 and 38 are coupled to a 74act244 31 integrated circuit a 74ac244 will also work this circuit is normally used as a bus driver however when its control lines are high its outputs 34 see a very high impedance to ground tri state if its inputs are low grounded 35 and its control lines are low enabled its outputs 34 see a very low impedance to ground thus this device performs very well in the role of a grounding switch the output line 36 is a current control lead of a constant current source similar to the lp2951 manufactured by micrel pin 7 this constant current device is used to drive the optical source in a manner where the amount of current on line 36 determines the brightness of the source two resistors 32 and 33 are placed in parallel with each other but in series with line 36 these resistors are connected to the outputs of the 74act244 34 thus when both enable lines 37 and 38 are high the 74act244 is open and no current flows on line 36 this.