We all have a doorbell at our homes. When a
visitor comes to our house, he searches for
the doorbell switch and then rings it to let us
know his presence. If the who came to our
house cannot find the doorbell or else if the
person is so short that he cannot reach the
doorbell, what can be done? How will it be if
we use an automatic doorbell which rings as
soon as a person arrives at our place? There
are no more hassles. The person who comes
to our house need not search for the doorbell
and press it any more. If we install this
automatic doorbell using object detection
circuit, the circuit will automatically sense the
presence of the person and it rings the

Circuit Diagram of Automatic
Door Bell Using Object


This circuit operates using a pair of
ultrasonic transmitter and receiver modules
which are used to detect the person and then
if the person is detected, the door bell is
automatically turned ON when the person is
in-front of the door.

The ultrasonic transmitter operates at a
frequency of about 40 Kilo-Hertz. That means
it continuously transmits the ultrasonic
waves of about 40KHz. The power supply
should be moderate such that the range of
the transmitter is only about one or two
meters. If the transmitting power is less than
one meter, then there is a chance that the
person who is one meter away is not
detected. Also, if the range is set to be very
large, then it may lead to false triggering,
meaning that, the objects far away from our
door are considered as the visitors and the
alarm rings. This can be a nuisance for us if
the alarm rings for every object or person far
away. So, to avoid both the problems, the
transmitting power is kept to an optimum

The ultrasonic receiver module receives the
power at the frequency same as that of the
transmitter’s so that noise will be eliminated
and we get less false triggering. The
sensitivity of the receiver can be tuned by
using the 500K-ohm variable resistor
arranged as a pot in the circuit. By tuning
this properly, we can achieve the desired
results. The output of our circuit is given to a
buzzer circuit which acts as a doorbell in our
case. The receiver in this circuit uses IC
LM324 which is internally has four op-amps.
Out of the four op-amps, we are using only
four of them and leaving the other one
unused as it is not much required in our
case. The three op-amps are used in
cascaded arrangement to provide high gain
as well as noise free output.

An opto coupler is used at the 9output to
avoid any interaction between our circuit and
the door bell.

Assemble the circuit on a PCB as compactly
as possible and then attach it to your main
door. Thats it! You may provide a power
supply using a 9V DC adapter with filtered
and regulated output. If the 9V adapter with
regulated output is not available, then we
recommend you to use a 12V unregulated DC
adapter with 7809 voltage regulator.

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