Nobody pays much attention to automatically
opening and closing sliding doors nowadays. In view of the complex 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.
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. . . 18V

It will be sufficient to loop the nylon wire a couple
of times around the motor spindle. Correct tension is obtained by incorporating a tensile spring in the wire.
A small push button switch is fitted in the left- and right-hand
door frames so that when the door The wire open or closed, a switch contact is closed.
You also need a light barrier ora similar device that transmits a positive pulse of suitable length on the
approach of a person. Such devices have been pub-
lished in Elektor Electronics before, and there is also
one elsewhere in this book.
The diagram in figure 1 contains a bridge circuit, consisting of transistors T1 . . .TO which, depending
on the logic level at the bases of T1-Ta or T2-T4,
determines whether the motor is at standstill, rotates clockwise, or turns anti-clockwise. When the
circuit is being tested, the motor may be replaced by
.D1 and D2 (with limiting resistors R8 and R9 respectively).
The choice of transistors depends on the current
drawn by the motor, which should not exceed
500 mA. T1-Ta and T2-T4 form complementary pairs,
for instance, BD239-BD240.
A short pulse at pin 6 of bistable FF1 sets the door in motion:
the first time, it may be necessary to reverse the connections to the motor! When the doo is fully open, it touches switch So. It does not mat- ter whether it is just a touch or whether the door
keeps the switch depressed: the motor stands still for a
short time, which is adjustable with PI, and
then rotates in the opposite direction so that the
door closes. If, while the door is closing, the light
barrier is actuated, the motor changes direction
again, and the operation repeats itself. When the
door is closed, switch $1 provides a pulse which
causes the motor to be switched off until the next
time the light barrier is actuated.