JPH0440954B

A commutator-type d-c motor has its line current connected through a sensing resistor (18) or transformer (18'), the pulses of which, upon energization, are counted in a counter (C) and also passed to a speed sensing circuit (S) in a control circuit (30), energization of the motor being disconnected if the frequency of undulations or waviness of the motor current drops, thus indicating that the positioned element, for example an antenna, window, or sliding roof of an automobile has reached a limiting position. The counter can be used to establish intermediate positions, upon suitable control by a selector switch (27) or a reference count number stage (C-Ref) controlled, for example, by a potentiometer (P) or from the selector switch (27). A timing circuit (T) prevents short-circuit through the motor upon manual sudden reversal of command of direction of rotation. The main switch can be coupled to a car radio to extend the car radio antenna upon energization thereof, retract the antenna upon de-energization, the selector switch also permitting manual retraction or projection, and disabling of the system if, for example, the radio is used for stored, e.g. tape reproduction.