an a c induction motor 1 which is preferably built as a geared motor and laid out for a voltage of 220 v at a power input of 25 w on the drive shaft 2 of the geared motor 1 sits on the one hand a crank disc 3 whose crank pin 4 engages with an elongated slot 5 of a slide 6 which in turn is in communication with push rods 7 for the movement of the not depicted closing member also keyed to the shaft 2 is a cam plate 8 with a peripheral cam 9 which cooperates with two microswitches 10 and 11 which are mounted for example at the housing of the geared motor 1 the arrangement of the micro switches 10 and 11 relative to each other and to the cam plate 8 is so made that the microswitch 10 above the peripheral cam 9 is only opened at the moment when the closing member of the ventilating device has reached its opening position it then remains open until the closing member has again been put back into its locking position on the other hand the microswitch 11 is opened and is only closed after the closing member has reached its opening position it then remains closed until the closing member has again reached its locking position and is then opened through the peripheral cam 9 of the cam plate 8 on the circuit of fig1 the two microswitches 10 and 11 are selectively energized through a switch contact 12 when the switch contact 12 connects the closed microswitch 10 then the closing member of the ventilating device is moved through the geared motor.