The data sheets that are available for this device are quite minimal , so here is some more data that I have worked out my self.
Although the HC-SR04 runs on 5V , the trigger pulse can be 3.3V. Thus it can be run from a 3.3V pic chip. Be aware though, the out put pulse is 4V, which is higher than the 3.6V maximum input voltage of a 3.3V pic chip. A diode is series will bring this down to an acceptable 3.35V.
On receipt of a 10USec positive going pulse on the trigger pin , the HC-SR04 will send 8 25USec pulses from the transmitter ( the one on the left in the picture ) these pulses are nearly 15V in size .
A while later an echo may be received in the receiver ( the one on the right )
This signal is much smaller . In this case it was a strong signal of around 400mV at its maximum.
A closer look shows the 40Khz echo.
The electronic on the board examines this signal and will produce a pulse that is proportional in length to the time taken for the echo to return. Note there is a fixed delay before the pulse is generated. In this picture the yellow trace is the trigger pulse and the red the start of the “Echo” pulse. This pulse is NOT the echo its self. The time from the trigger pulse to the echo pulse is ALWAYS 468USec. No matter how close or far the echo.
Here is the pulse width for 50mm distance.
For 100mm distance.
For 150mm distance.
For 200mm distance.
The pulse for
50mm = 264Usec
That should be around 5.8usec per mm . My distance measurement weren’t too accurate. Note the pulse width is proportional to the distance . Double the distance is double the pulse width.