3.9k will be alright. The goal is to obtain a voltage between 2.5V and 3.3V at the digital input of Bela.
A smaller value may not trigger the digital input, while a higher level may break the Beaglebone.
At the same point, you want to keep the current flowing to ground small enough to avoid drawing lots of current for no good reason.

Given this general schematic

V_in-/\/\/--|--/\/\/--gnd
      R1    |   R2
            |
          V_out

the first Kirchoff law tells you that

$V_{out} = V_{in} \frac{R_2}{R_1 + R_2}$

The current to ground $I$ is

$ I = \frac{V_{in}}{R_1 + R_2}$

so we can aim at keeping $50000\Omega > R_1 + R_2 > 1000 \Omega $. A value smaller than $1000\Omega$ would potentially draw too much current, a value much higher than $50000\Omega$ may affect the actual $V_{out}$ value, depending on the input impedance of the digital input.

The values I suggested give the current requirements (giving a total resistance of 6.4k, and a $V_{out} = \frac{4.2k}{2.2k+4.2k} 5V = 3.28V$.
Your values are also suitable: $R_1 = 2.2k$ and $R_2 = 3.9k$ give a total resistance of 6.1k and a $V_{out} = \frac{3.9k}{2.2k+3.9k} 5V = 3.20V$.

2 months later

I'm now in the final stage of my project.
But I've noticed that the Bela doesn't send any messages any more if I connect more then 3 sensors to it.
My plan is to connect 5 of those sensors.
Is it possible that the 4th sensor is pulling the last bit of juice so that the overall Watt is not enough from the 5V supply?
...If so, could I then just use a 5V DC power supply and plug it to the bela....would it get more juice and distribute it as needed? (I've already read that the power plug is needed if the internal amp's need to be powered.)

Are you using the 5V from the analog out header?

As per the schematics, that 5V output has a 100ohm resistor in series. This is to make sure that the board is not damaged if it that pin is accidentally shorted to ground, and this pin should be used only to drive high-impedance loads, such as potentiometers. As a consequence of this you will get a voltage drop as you draw current from the pin. According to Ohm's law $V = R I$, so you will have a voltage drop $V$ for a current $I$ going through a resistance $R$.

The datasheet of the sensors reports a working current of $I_s = 0.015A$ per sensor, which gives a voltage drop of $ V_d = 100 \Omega \cdot 0.015 A = 1.5V $, so for each sensor you connect to these 5V, your "5V" power supply would be effectively reduced by 1.5V. I am actually surprised it works at all with one sensor!

So if you are using the 5V from the analog out header, then you can probably fix the problem by using P9_07 instead. This is connected directly to the power supply and should provide enough current for your application.

Yes I was using the 5V from the Analog out. It's working until I plug in 4 sensors.
Maybe when I switch to P9_07 it will run more reliably and stable.
Just to make sure I dont break anything I will attach 2 pictures to make sure I use the right pinout (because P7_09 is not marked in any picture like this one: http://bela.io/belaDiagram/). (and btw. I hope using the Audio Expander Capelet doesn't make any difference)
AND to make sure the power supply is compatible. I'm pretty sure from what I've read and what the label of the power supply shows that it is compatible, ...but I could still be wrong...

PSU-Label

P9_07

Again, thank you very much for your help. I appreciate it very much.

We did not mark P9_07 on the diagram, to make sure that anyone finding out about it would have to understand the consequences of using it. In hindsight, we should probably add it.

The power supply looks ok. The important bit is
alt text

that tells you the voltage and that the positive is on the inside of the barrel connector, and negative is outside ("center positive").
Now, as long as the size of the barrel matches (or "almost" matches), then you should be fine.

https://www.modmypi.com/blog/how-do-i-power-my-beaglebone-black

Yes, this was exactly what I was focusing at to make sure it is compatible. And yes the barrel fits perfectly.

Last question. Can I plug the bela to my computer AND simultaneously to the power supply?
I want to be able to read printed messages.

    SteffenE Last question. Can I plug the bela to my computer AND simultaneously to the power supply?

    Yes you can, no problems at all. Even better, you can disconnect either at any time and it will still run off the other one.

      2 years later

      Hi,

      I am trying to create a virtual piano. For this I am planing to using an ultrasonic module for getting the input. I have followed the same voltage divider circuit as mentioned in this discussion. Unfortunately, when I tried to run the provided example (examples/06-Sensors/ultrasonic-distance) it is not giving me an output as expected. But I can see some fluctuations in the scope though the rt_printf("pulseLength: %d, distance: %fcm\n", pulseLength, distance); is not giving me any output.
      It would be a great help If someone can help me this.

      Thank you in advance,
      Jonath John

      Can you tell us how you wired up the device?
      What does the scope show exactly? Does it show any signal at all on the first channel? that is where the digital input should come in: scope.log(digitalRead(context, n, gEchoDigitalInPin), distance/100);

      Thank you for replaying.
      I have connected vcc to 5v, Trig pin to analog out - 0, Echo pin to digital 1 which is taken out from the voltage divider and the rest is grounded.

      echo-/\/\/\--|--/\/\/--gnd
      2.2k   |   4.2k
             |
       Bela digital -1

      Regarding the scope, it is showing signal on the first channel, but it is not printing the distance or the pulse length.

        Where did you get 5V from on Bela? You should get them from P9.07, not from the analog out header.

        Jonath Regarding the scope, it is showing signal on the first channel, but it is not printing the distance or the pulse length.

        The scope is not supposed to print anything, the print should happen in the console in the Bela IDE. If there is no printing in the Bela IDE, then show a screenshot of the scope, please.

          Ok, I was taking 5V from analog out. Let me just ry it from p9.07. Regarding the distance, I mean it was not printing in the Bela IDE. probably changing to p9.07 should solve the problem.

          Thank you so much

          Even after changing to p9.07 it wasn't printing the output. After making some changes in the code it started to work fine. I tried both using p9.07 and and from analog out. Taking out from p9.07 is giving much more accurate and stable output.

          I appreciate your time
          Kind regards

          To print the value, I just put the print statement out of the "if(pulseLength >= gMinPulseLength)" statement.

           if(gcount > 50000){
                rt_printf(" Distance: %fcm\n", gdistance);
                gcount = 0;
            }
            ++gcount;
          2 months later

          Hi, I hope it's ok to jump in on this thread (please let me know if I should start a new one!)

          I've been following this and another ultrasonic bela/Pure Data thread to get two sensors working with my Bela and audio capelet.

          So far each sensor is setup for a pair of digital triggers/echoes (P8_07/P8_08 & P8_09/P8_10) with voltage dividers. Each sensor works individually and sending fine to Pure Data when powered by the analog out 5V pin (but the sensors don't work when both connected, I'm assuming because the voltage drop is too low).

          When I swap the voltage supply from the analog 5V pin to P9_07 to try and power both sensors, both sensors stop working/receiving/sending data, even when tested individually. This is with using barrel jack & USB power.

          Is there something I'm missing to use the P9_07 as 5V supply with two sensors - e.g. is this pin still usable with the audio capelet and is there anything I need to add to the code? Or could I have made a mistake with the voltage division or sensor setup?

          Any help/thoughts much appreciated. Forgive me if I'm misunderstanding anything obvious/crucial too!

          Cheers

          There shouldn't be any problem from the power supply: these sensors do not require much current.

          Can you try to connect both sensors but only enable the code to use one at a time and verify that they both work fine (individually)? It may well be something in your code or in the library code I wrote 3 years ago that breaks the software when using two at a time ...

            giuliomoro

            Thanks a lot for getting back to me!

            This is great, I hadn't even thought to check the code side of things! I'll check that out... I followed the detailed advice on the other large ultrasonic sensor thread too which was really helpful, so the render is mostly that too. Can you advise what's the best way to enable each sensor in the code one at a time - is there a line I can write in...? (I'm sure that's a dumb question, sorry - very new to the scripting side of things. Though absolutely loving Bela and what it can do).

              you probably are using some code based on this. If you have two sensors connected, I would assume that you will have duplicated some lines of code. Comment out the duplicated lines.
              Hint: posting your code would help in helping you.

                giuliomoro

                Thanks a lot - I do believe to my best of my memory and knowledge that this is what the code is based on!

                And you're very right, a top tip haha - here is the code... I imagine there's probably redundant stuff in there too from myself not knowing exactly what to chop out etc

                Thanks a lot for your help, I really appreciate having some support with this

                EDIT by giuliomoro: I deleted the 600+ lines cpp code that was making it hard for me to browse the thread. The diff in the post below highlights the important parts of it.