I'm using Hc-Sr 04 ultrasonic sensor for my project and I want to know that if I could or not utilize an slope. Have you an idea?

  • Do you mean "chirping"? No, the device outputs a fixed 40KHz frequency (8 cycles worth per ping).
    – Majenko
    May 20, 2015 at 13:05
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    Do you mean detecting the distance to a non-perpendicular surface? May 20, 2015 at 13:27
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    Still don't understand quite what you are asking. They say a picture is worth a thousand words. Can you draw one for us depicting what you mean?
    – Majenko
    May 20, 2015 at 13:59
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    A perfect surface at an angle to the perpendicular would give no reflection back to the module (it goes off at another angle, just like a mirror or a ball kicked against a wall), but most surfaces aren't perfect. In practice you will likely get some reflection but at greatly reduced range. If your sloped surface is smaller than the beam width you may also get a reflection off of its perimeter. May 20, 2015 at 17:18
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    I'd suggest running a google search using "hc-sr04 reflection angle" there are a number of existing thoughts and posts on this idea. One of the most interesting items are the diagrams that show reflected signal drop-off as a function of angle of incidence to object. The bottom line is that this is not an accurate measurement tool for non-perpendicular objects. Adding multiple receivers ... for example at -45o, 0o and +45o may give you better data.
    – Kolban
    Jun 19, 2015 at 18:48

2 Answers 2


I have a couple of these running right now, and they will often work at a slope when the surface is complex (not flat). However, the detection angle, as it comes from the sensors, appears to be about 30deg from centre in each direction. Hence, the sensors do need to be relatively pointed at the object being detected.

When placing my hand directly above it, it doesn't really matter what angle I tilt my hand, presumably since I have fingers that are not flat. On the other hand, if I put a paper towel in front of it, it will stop seeing it if I angle the paper towel more than 20 or so degrees.

Overall, if you keep the object within 30 degrees from the sensor's fixation point, it will work, granted the object, if flat, is "pointed" at (facing) the sensor flat-wise within about 20 to 25 degrees. For irregularly surfaced objects, it does not seem to matter too much unless the angle is pretty steep, such as 60 degrees.

Here is what I mean:

enter image description here


Slope surfaces may cause reflections so you need to try. I think 30 degree going to make reflection and gives wrong value

  • And even more important, reflections that go away from the senor, not back at it. Fortunately surfaces are not perfect and some tiny imperfections will reflect some in the correct directions, but the threshold will be very low. Correct answer though, as you say, is to try.
    – user6569
    May 20, 2015 at 18:36

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