the preferred embodiments of the blade retracting mechanism that are shown and described herein are operable with a detection system that is designed and configured to be able to detect the presence of a person either by use of a mechanism that detects the proximity of a person &# 39 ; s hand or possibly by virtue of detection of contact of a person &# 39 ; s body with a portion of the saw or other tool that is designed to be a sensor . the most likely component to be used as a sensor is the saw blade , which would be used to detect the person &# 39 ; s proximity or contact with a saw blade . if contact with a saw blade is necessary for detection , there would necessarily be at least some injury to a user depending upon the speed of approach and the speed in which the entire system operates . the preferred embodiment of the present invention is shown in fig1 - 6 , with fig1 showing a side view of a blade retracting mechanism indicated generally at 10 , with the blade 12 being in a normal operating position , where the blade is near a riving knife 14 . in fig3 , the blade 12 is shown to be partially retracted as would occur during operation of the mechanism 10 and in fig5 , the blade 12 is fully retracted below the surface of the table saw top which is approximately at the elevation shown by the dotted line t of fig1 , 3 and 5 . the blade 12 rotates on a shaft 16 that is journaled in a generally triangularly shaped arm 18 that has a curved lower surface 20 . the arm 18 rotates around another shaft 22 that is attached to a large plate 24 that is mounted to the saw by mounting brackets 26 and 28 located on opposite ends of the plate 24 . the plate 24 carries a motor base 30 that is mounted above the elevation of the plate 24 and carries a motor 32 for driving the saw blade 12 . the output shaft of the motor 32 is not shown , but it carries a pulley which drives a belt 34 and a pulley 36 , with the pulley 36 being connected to another pulley or having an extension for driving a belt 38 and in turn drives a pulley 40 that is operatively connected by the shaft 16 to an arbor ( not shown ), but which drives the blade 12 . since the arm 18 is pivotable about the shaft 22 , it should be understood that the motor 32 is configured to be able to drive the belts 34 and 38 via the motor pulley and pulley 36 regardless of the vertical position of the blade 12 . stated in other words , the distance between pulleys 36 and 40 remains constant as does the distance between pulley 36 and the motor drive shaft regardless of the vertical position of the blade 12 . when the blade is in the normal operating position as shown in fig1 , the top right surface of the arm 18 abuts against a stop member 41 that is mounted to the plate 24 by a mounting bracket 42 with bolts 44 . the arm 18 is held in the upper position as shown in fig1 by a detent assembly , indicated generally at 46 , that comprises a main bracket 48 that is bolted to the plate 24 by bolts 50 and wherein a detent rod 52 engages a v - shaped recess 54 . the rod 52 is biased into engagement with the recess 54 by a spring 56 that is seated on a bolt 58 and which is adjustable to vary the biasing force that is applied to the rod 52 . the detent assembly 46 is therefore designed and configured to maintain the arm 18 in its normal operating position unless it is rotated downwardly with sufficient force to depress the detent rod 52 away from the v - shaped recess 54 and release the arm 18 for rotation . the force that is necessary to overcome the detent assembly 46 is provided by a firing mechanism that is indicated generally at 60 . a cross - section of the mechanism is shown in fig2 , 4 and 6 , and comprises a cylindrical barrel structure 62 having a cylindrical bore 64 in which a piston 66 is positioned . the mechanism 60 has a replaceable cartridge 68 that is placed in a smaller bore 70 with the cartridge 68 being retained by an end cap structure 72 through which a metal pin 74 is located with the pin extending to the cartridge 68 so that it can be fired . the pin 74 is connected to a wire 76 which in turn is connected to a capacitor 78 . during operation , the capacitor 78 is switched to release its electrical charge which travels through wire 76 to heat the pin 74 to a temperature that fires the cartridge 68 and this causes high pressure gases to be produced and cause the piston 66 to be moved to the left out of the bore 64 . the piston 66 is guided by a support 80 having a cylindrical opening therein sized to receive the cylindrical piston 66 . as it is driven in the leftward direction , i . e ., away from the barrel 62 , a hammer 82 is driven toward a hammer damper 84 that is mounted to a skid 86 that also has a mounting structure , indicated generally at 88 , which holds the damper 84 and cushions the hammer 82 . during its travel the hammer 82 drives the arm 18 in the clockwise direction around the shaft 22 . the piston 66 has an enlarged end portion 90 that defines a shoulder 92 that engages the support member 80 at the end of its travel . it should also be understood that the skid 86 and support member 80 are effectively connected together . an end bracket 94 has a narrow center portion 96 that extends rearwardly and it is retained by a pair of support members 98 ( fig1 ) located on opposite sides of the center portion 96 . the support members 98 are bolted to the plate 24 by bolts 100 . an anvil 102 is connected to the arm 18 by a pair of pins 104 and a pair of bolts 106 as well as by a strap 108 that is bolted on opposite ends to the anvil 102 and the arm 18 . it should be apparent that the front surface of the hammer 82 is in contact with the adjoining surface of the anvil 102 so that when the firing mechanism 60 is activated , the piston 66 will cause the hammer 82 to move the anvil 102 and arm 18 in a clockwise direction so as to retract the blade 12 below the surface t before the user is seriously injured . when the microprocessor triggers the pyrotechnic device , a high amperage current is sent through the wire 76 to the electrode or pin 74 , which directs the current to the back of the cartridge 68 . the cartridge 68 fires causing a rapid pressure rise in the barrel 62 behind the piston 66 that produces approximately 10 , 000 pounds of force that propels it toward the anvil 102 . the piston 66 is forced to move along the barrel axis pushing the anvil 102 as it does so . this pushing force overrides the detent assembly 46 and the arm 18 rotates in a clockwise direction . when the arm 18 moves to its position of maximum travel as shown in fig5 , the hammer 82 fully compresses the hammer damper 84 . in this position , the barrel 62 vents the combustion gases because the piston 66 moves past the end of the barrel 62 thus relieving the barrel pressure in preparation to stopping the piston 66 . since the arm 18 and hammer 82 are not physically connecting , i . e ., they only touch , the arm 18 is free to continue rotating even though the piston 66 stops . the arm 18 continues to rotate until it contacts a mechanical stop that is not shown , at which time its movement ends . the hammer damper 84 then relaxes and pushes the hammer 82 and piston 66 back into the barrel 62 . the cartridge 68 is a commonly available 22 caliber nail gun cartridge that is used in construction and costs about $ 0 . 05 . an advantage of the preferred embodiments of the present invention is that once the cartridge 68 is fired , the end cap 72 can be removed enabling the spent cartridge shell to be removed . the firing mechanism 60 can be rearmed by placing another cartridge 68 in the bore 70 , inserting another pin 74 in the end cap 72 , connecting the wire 76 to the pin and recharging the capacitor 78 . in a second preferred embodiment shown in fig7 - 10 , reference numbers that have been given to the first preferred embodiment shown in fig1 - 6 are used where the components are similar . in the embodiment shown in fig7 , the arm 18 has structure that interacts with a detent assembly 46 in a slightly different manner , i . e ., the v - shape recess 54 is not directly contacted by the detent rod 52 , but contains an extension piece 112 that extends beyond the curved surface 20 of the arm 18 and contacts the end of the detent rod 52 as shown . this extension is necessary because of a slight modification to the placement of the detent assembly 46 to accommodate the addition of an anti - bounce mechanism , indicated generally at 114 that is located below the anvil 102 . the outer surface 20 of the arm 18 has a number of serrated teeth 116 along a portion of the surface which are positioned to engage a ratchet assembly 118 that pivots about a shaft 120 that is operatively connected to the plate 24 . a compression spring 122 has one end engaging an extension 124 of the ratchet assembly and the opposite end bearing against a bracket 126 so that a pawl 128 of the ratchet assembly 118 is biased toward the arm 18 . as shown in fig8 , the saw blade 12 is mounted to the arm 18 through an aperture in the plate 24 by the shaft 16 . the aperture in the plate comprises an arcuately - shaped track 21 such that when the arm 18 rotates clockwise about the shaft 22 , the shaft 16 moves downwardly along the arcuately - shaped track 21 . after the firing mechanism 60 rotates the arm 18 in a clockwise direction , its left surface 130 will contact a stop or damper 132 . because of the tremendous force having been applied to the arm 18 , the arm 18 is moving rapidly and bounces off the stop 132 . as show in fig8 , as the arm 18 bounces off of the stop 132 , the pawl 128 will engage one of the teeth 116 and stop the counterclockwise movement of the arm 18 and prevent the blade 12 from moving upwardly above the tabletop to potentially strike the user . while not shown , a pull lever is preferably provided to release the pawl 128 from the teeth 116 as the arm 18 is returned to its normal operation position . while adjustable , it is preferred that the amount of force needed to release the pawl 128 be in the range of about 10 to about 20 pounds . the amount of force needed to lift the arm 18 to its normal operation position is within the range of about 10 to about 20 pounds and the amount of force to overcome the spring 56 in the detent assembly 46 to latch the arm 18 in the normal operating position is preferably within the range of about 15 to about 25 pounds . while various embodiments of the present invention have been shown and described , it should be understood that other modifications , substitutions and alternatives are apparent to one of ordinary skill in the art . such modifications , substitutions and alternatives can be made without departing from the spirit and scope of the invention , which should be determined from the appended claims .