which responds to a falling pulse and emits an output pulse for a period of e g 10 ms and this output pulse in turn passes to a third 3 one shot 165 which again responds to a falling pulse and is adjustable in its duration for example 35 ms the output next passes to a fourth 4 one shot 167 responsive to a falling pulse with a duration of 10 ms for example which passes its output signal to the s input of an s r flip flop 169 which upon receipt of a positive pulse latches the pulse in the positive mode until it is reset by a signal at its r input resetting is accomplished with the output of the 1 one shot 161 after inversion at the inverter 171 of its polarity so that the flip flop is reset at the end of the output signal of 1 one shot 161 the opposed windings 89a b of the solenois unit are for convenience designated right and left according to their relationship in fig3 and each winding is connected in parallel to each of a 30 volt and a 12 volt source through corresponding control relays relay cr 1 controls the 30 volt line 173 and the coil 163 of this relay is connected to the output of 2 one shot 163 while relay cr 2 is in the 12 volt line 175 for the right winding and its coil is connected to the output of 3 one shot 165 relay cr 3 connects the left solenoid winding to the 10 volt source via line 177 and its coil.