automatic sliding door

Nobody pays much attention to auto- matically opening and closing sliding doors nowadays. In view of the com- plex mechanics involved, not too many people have so far attempted to fit an automatic sliding door in their living room. If you are happy with a relatively slow movement, such a door can, however, easily be realized with the aid of a DC motor and a small electronic control unit. The basic mechanical construction of the automatic door is shown in fig- ure 1. A suitable length of stranded nylon wire is attached to the left- and right-hand sides of the door and strung across four nylon roller guides as shown. The wire is attached to the spindle of a DC motor, the rotational direction of which depends on its polarity. Such motors are available in variety in many model building shops or from electrical suppliers, and should be suitable for operation from 6...18 V. It will be sufficient to loop the nylon wire a couple of times round the motor spindle. Correct tension is obtaine...