rotate on differential output shaft 88 and the top and bottom gears of 78 and 80 to rotate about differential pivot shaft 93 without causing the whole differential 82 to rotate pivot shaft 93 is fixedly pinned to the differential output shaft 88 in such manner so that the longitudinal axis of the differential output shaft 88 is normal to and lies in a plane which passes through the longitudinal axis of the pivot shaft 93 a gear train 94 96 98 meshes with an output shaft gear 100 which is pinioned to a spring biased output shaft 102 when the device is in a 34 safe 34 position the tension on spring 104 is sufficient to create a torque on output shaft 102 which holds rotor blades 106 and l08 in an open position when the output shaft 102 rotates 90 u00b0 the rotor switch arms 106 and 108 make electrical contact with switch output terminals 110 and 112 respectively thereby placing the switch in an 34 arm 34 position referring now to fig1 and 2 in operation when two pulsating dc 180 u00b0 out of phase electrical signals 34 a 34 and 34 b 34 are applied to first and second solenoids 10 and 12 respectively and a dc voltage is simultaneously applied to lock solenoid 42 the pulsed actuation of solenoid 10 and 12 drives the first ratchet wheel 74 incrementally via drive pawls 70 and 72 each solenoid 10 and 12 must be pulsed alternately otherwise the first ratchet wheel 74 will not advance because of the solenoid and ratchet tooth juxtaposition the first ratchet wheel 74 is machined with 72 teeth or 5 u00b0.