7 and secured with lock nuts 63 and 64 shown in fig1 e tightening adjustment bolts 4 and 5 place steel cables 37 and 38 in tension machine screws 41 and 44 protrude into reduced diameter shank of bolts 4 and 5 these act as stops to prevent disengagement of bolts 4 and 5 from their respective couplings 39 and 42 four phototransistors 49 50 51 and 52 shown in fig1 are mounted through holes in block 48 block 48 is fastened to circuit board 45 with machine screws 74 and 75 fig1 circuit board 45 is fastened to end plate 2 with machine screws 46 and 47 the path of the light beam 79 is shown as a phantom line fig1 b details spacers 15 and 16 concentric with holes through tube 1 which provide a mounting point for an external support bracket cables 37 and 38 and light beam 79 pass through this portion of the tube 1 fig1 c details cables 37 and 38 passing through slots in cable guide blocks 56 and 60 guide blocks 56 and 60 provide stability to the cable tube system when the cables are in tension guide block spacer 59 separates guide blocks 56 and 60 forming an assembly with bolts 58 and 62 and flat washers 57 and 61 guide blocks 56 and 60 are a close sliding fit inside tube 1 to permit assembly spacers 13 and 14 are concentric with holes through tube 1 which provide an external handle attachment point spacers 13 and 14 locate the guide block assembly in the tube aperture plate 53 is fastened to guide block 56 with screws 54 and.