HOW IT WORKS:
This circuit operates using a pair of ultrasonic transmitter and receiver modules which are used to detect a person or object and then if the person or object is detected, the door bell will automatically turn ON when the person is in-front of the door. The RED LED light is just an indicator light which shows that the circuit is active.The basic principle of operation is like that of Radar. The Module automatically sends 40 kHz and detect whether there is a pulse signal back.
From figure C, can clearly see that Ultrasonic Tx/Rx is a complete RADAR system. NB: Radar means Radio Detection And Ranging. There are two types of Radar; the Primary Radar and the Secondary Radar. So our design works like Primary Radar system. Primary Radars are designed to transmit pulses of short or long square waves and receive the transmitted in return when they hit or reflect from an object. The distance of the object will determine the strength of the Echo. The higher the power transmitted the better of the Echo/Reflected signal. More details of how the automatic doorbell operates using object detection is explained using Ultrasonic Transceiver modules as shown below;
The ultrasonic transmitter:
It operates at a frequency of about 40 Kilo-Hertz. This transmits continuous ultrasonic waves (ping) of 40 KHz. Typically they are short square waves generated by 555 Timer IC. The two Transistors connected to the transmitter enable the signals of both half cycles to be transmitted. At positive half cycle, PNP transistor conducts and at negative half cycle, NPN transistor conducts. The power supply to the circuit determines distance in meters, so we use 8V moderate power supply 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 high, 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 problems, the transmitting power is kept to an optimum level. We also have to note that the speed of the Ultrasonic wave is the same as that of the sound which is; velocity of sound = 340 meters per seconds. Taking that to consideration, the behavior of the Transmitted pulses, Echo or reflected wave will be the same as sound waves. Frequency is calculated as follows; using the formula above with respect to 40 KHz the values of R and C in the circuit can be calculated. The resistor R of 20Kohms Variable is used to adjust or trim to 40 KHz with 0.001uF fixed ceramic capacitor, C. By our calculation, resistor R is trimmed to 17.85kohms
The ultrasonic receiver:
It detects the Echo or reflected signal from object at the frequency same as that of the transmitter. The received signal is Echo/Reflected waveform look like rectangular square waves (ie. echo effect). The typical example of Ultrasonic Tx/Rx waveform is shown in fig.C. This is so because when waves hit an object or surface at velocity (340m/s) of sound, there?s slight change in their properties. The sensitivity of the receiver can be adjusted by using the 500K-ohm variable resistor (VR2) arranged as a pot in the circuit. By tuning this properly, we can achieve the desired results. The output of our circuit is coupled to a buzzer circuit through opto coupler to a doorbell in our case. The receiver in this circuit uses IC LM741 which is three (3) different op-amps. The three op-amps are used in cascaded arrangement to provide high gain as well as noise free output. The Alarm circuit is made up of phototransistor, buzzer or sound generator, 680ohms resistor and 6V DC supply from 3xAAA batteries. However, the total impedances to phototransistor is 2kohms, which is the sum of 680ohms and internal resistance present in the buzzer. These arrangements as shown in fig.D below; provide compensation to phototransistor and prevent it from damage.