user1006: I have HC-SR04 Ultrasonic Sensor with me. I couldn't understand the "Trig" pin's working structure and the working technic. Do we really need it while we use? 
(With Arduino examples, I generally use Echo pin more than trig pin.)


user7991: 
There are two versions of this device. In the threepin version you trigger the measurement and receive the echo pulse on the same pin. In the four pin version trigger and echo are on separate pins. 
Possibly your Arduino example is written for the three pin version. 

user4143: Yes, you do need it.
The purpose of the TRIG pin is to cause the module to send out a "Ping".  It's what makes the whole thing work.  As soon as the TRIG pin goes HIGH then LOW (for a period of not less than 10µs) the internal clocks start ticking.  8 cycles of 40KHz audio are sent out of the transmitter and it starts counting how long it takes for the echo to arrive.
If you don't use the TRIG pin the module won't be able to do anything at all.

1006: <p>I have HC-SR04 Ultrasonic Sensor with me. I couldn't understand the "Trig" pin's working structure and the working technic. Do we really need it while we use? </p>

<p>(With Arduino examples, I generally use Echo pin more than trig pin.)</p>

<p><img src="https://i.stack.imgur.com/X2dK8.jpg" alt="image"></p>
 7991: <p><img src="https://i.stack.imgur.com/p3fjW.jpg" alt="Two versions of ultrasonic sensors"></p>

<p>There are two versions of this device. In the threepin version you trigger the measurement and receive the echo pulse on the same pin. In the four pin version trigger and echo are on separate pins. </p>

<p>Possibly your Arduino example is written for the three pin version. </p>
 4143: <p>Yes, you do need it.</p>

<p>The purpose of the TRIG pin is to cause the module to send out a "Ping".  It's what makes the whole thing work.  As soon as the TRIG pin goes HIGH then LOW (for a period of not less than 10µs) the internal clocks start ticking.  8 cycles of 40KHz audio are sent out of the transmitter and it starts counting how long it takes for the echo to arrive.</p>

<p>If you don't use the TRIG pin the module won't be able to do anything at all.</p>
