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
level.
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.
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