*The purpose of the TRIGGER pin is to cause the module to send out a Ping i.e request. It makes the sensor works. As soon as the TRIGGER pin goes HIGH then LOW 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.
*In 4 pin ultrasonic sensor seperate pins are used for TRIGGER to transmit and ECHO to recieve
*In 3 pin ultrasonic sensor uses pin SIG which is used for both TRIGGER and ECHO.
*For complete details on Generation of 40khz you can look up for design of ultrasonic senor.