This automatic door
opener can be made using readily available components. The
electromagnetic relay at the output of this gadget can be used to
control the DC/AC door-opener motor/solenoid of an electromechanical
door opener assembly, with slight intervention in its electrical wiring.
A laser diode (LED1) is used here as the light transmitter.
Alternatively, you can use any available laser pointer. The combination
of resistor R1 and diode D1 protects the laser diode from over-current
flow. By varying multi-turn trimpot VR1, you can adjust the sensitivity.
(Note that ambient light reflections may slightly degrade the
performance of this unit.) Initially, when the laser beam is falling on
photo-transistor T1, it conducts to reverse-bias transistor T3 and the
input to the first gate (N1) of IC1 (CD4001) is low.
The high output at pin 3 of gate N1 forward biases the LED-driver transistor (T4) and the green standby LED
(LED2) lights up continuously. The rest of the circuit remains in
standby state. When someone interrupts the laser beam, photo-transistor
T1 stops conducting and transistor T3 becomes forward-biased. This makes
the output of gate N1 go low. Thus LED-driver
transistor T4 becomes reverse-biased and LED2 stops glowing. At the
same time, the low output of gate N1 makes the output of N2 high.
Instantly, this high level at pin 4 of gate N2 triggers the monostable
multivibrator built around the remaining two gates of IC1 (N3 and N4).
Values of resistor R8 and capacitor C1 determine the time period of the
The second monostable built around IC2 (CD4538) is enabled by the
high-going pulse at its input pin 12 through the output of gate N4 of
the first monostable when the laser beam is interrupted. As a result,
relay RL1 energizes and the door-opener motor starts operating. LED3
glows to indicate that the door-opener motor is getting the supply. At
the same time, piezo-buzzer PZ1 sounds an alert. Transistor T5, whose
base is connected to Q output (pin 10) of IC2, is used for driving the
relay. Transistor T6, whose base is connected to Q output of IC2, is
used for driving the intermittent piezo-buzzer. ‘On’ time of relay RL1
can be adjusted by varying trimpot VR2. Resistor R9, variable resistor
VR2 and capacitor C3 decide the time period of the second monostable and
through it on time of RL1.
The circuit works off 12V DC power supply. Assemble it on a general-purpose PCB.
After construction, mount the laser diode and the photo-transistor on
opposite sides of the door-frame and align them such that the light beam
from the laser diode falls on the photo-transistor directly. The motor
connected to the pole of relay contacts is the one used in
electromechanical door-opener assembly. If you want to use a DC motor,
replace mains AC connection with a DC power supply.