Ultrasonic sensor: Trigger-Echo Cycle Overlap

We (@Ashish_Pardeshi, @jtd , @Farhan , @_siddiquisubhana, @sandysat @Ambika, @msakeenah @Ambika_Indore) started with exploring Ultrasonic Sensor (HC-SR04). @_siddiquisubhana is interfacing this with Arduino.

Screenshot from 2020-09-04 17-16-15

We started looking at its Data Sheet and understanding its functional range.

We managed to understand the working frequency (40 Hz ~25 ms) and its relation to the maximum distance (425 cm: 400 cm in datasheet). (See the calculations on the right in the figure below.)

Now the question was - what happens if the echo of trigger pulse t1, is received after the next trigger t2? Does it confuse the sensor, or how does the sensor resolve it? See the figure below. (The blue lines traces the path of the light)

Screenshot from 2020-09-04 17-02-55

This involves clarifying reflection of sound, properties of material…

Possible explorations:

  • @sandysat says We can use a signature (send pulses of diff amplitude, frequency, and number of pulses) for trigger signals. But we are unsure how the material responds to each of these?

More clarification is needed. Let’s talk!



My wild guess is that, if the object is beyond the specified max-range(as mentioned the datasheet), then the echo won’t come back. If the object is within the max range, then the echo of t1 would certainly be received before sending t2.


@Ambika posts https://courses.washington.edu/bioen508/Lecture6-US.pdf

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I doubt that this would be an all or none effect. But it would be possbile to make the reciever analog opamp - in this case LM324 - to operate only above a certain amplitude threshold.

However such an approach raises new problems. If the obstacle is ultrasound absorbent the range should drop substantially.

Here is a circuit I found on the net - the data sheet provides no schematic.
R21-R16 sets the operating point of the 4 opamps at 1/2 Vcc -2.5v. U2D provides a gain of 6. U2C is a bandpass filter. Someone can try and simulate it using Lush projects. U2B is a high pass filter with gain. U2A is the comparator that is also gated to block recieve when the transmitter is transmitting - can anyone figure ot why?
U2A output is inverted with Q1 and fed to the micro’s interrupt. The max232 which is a TTL to RS232 drive is used as a capacitive voltage multiplier. RS232 - common in older comps and once also used for networking before TCPIP ethernet became common - uses +12V and -12V on the Tx and Rx pins. These voltages are generated intenally by the Max232 from the 5v supply using a capacitive charge pump. The total voltage of 24V is fed to the ultrasonic transducer. Can anyone figure out what signal is generated by the micro to drive the max232?

A far more sophisticated reciever is in most animals ears. The bat too has this but additionally has a transducer.
Does anyone know if other mammals use such sonar echo location?

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Yes, there is some cool stuff that some Dolphins and Whales do, underwater though!

https://www.youtube.com/embed/28Ap4ove3Fg This video shows how Dolphins generate the sounds.

https://en.wikipedia.org/wiki/Animal_echolocation This link gives some interesting Cochlear apparatus in the ears of the bats for further processing!

Actually, these animals appear to use this for more interesting things! More than just measuring or perceiving distance! [btw #offtopic @G_N can perception be thought of as qualitative measuring by the SMN??, where measuring is differentiating the difference that is already there and hence an active process - i.e., an action]

@JTD And regarding the Max232 and other stuff that you mentioned, some of it requires more elaboration to actually engage with it! Can you guide us to the page where you saw this picture of maybe the datasheet or any relevant video?

And regarding the ultrasonic absorbing properties, I am eager to learn from @Ambika_indore!



An interesting note by @Farhan today was that - even two entirely different ultrasonic sensors (USS1 and USS2) kept at close proximity at particular angles can confuse each other.

For example, receiver of USS2 can receive the reflected signal transmitted by USS1, and this could lead to unreliable measurements!

This again indicates a need for signatures to the transmission signals as hinted by @sandysat the other day!