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How would you connect multiple HC-SRO4 ultrasonic sensors to one arduino uno? As of now, I have a bunch of jumper cables, 3 u.s sensors, and one arduino uno board. How would one hook up 3 sensors? A schematic would be excellent!

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It depends on what you want to do overall.

Using the HC-SR04 works roughly like this:

  1. you generate a strobe on a pin for a minimum amount of time, and this strobe is understood by the sensor as "emit the ultrasound signal"
  2. then you somehow (more on this later) measure how much time passed between your strobe and a change of level on another pin from the sensor - this is how the sensor notifies you that there was an echo and it has returned.
  3. finally, you convert the time measured into distance, using a conversion algo (typically multiplying by a certain factor) that is specific to the sensor.

Mostly the problem revolves around point #2: how to measure the time elapsed between command (strobe) and answer (change of level).

You have various options, each with pros and cons:

  • busy looping: it's easy to implement, very straightforward and works right away. The problem is that your arduino will do just that.
  • polling: it's a less aggressive version of polling, where you check the values every now and then. It lets you do more, but the drawback is that in the worst case the line changes level right after you have checked it, so it will be noticed only at the following iteration. The period used for checking is the maximum error you will get.
  • GPIO interrupt driven: this is harder but lets your arduino do many other things, while not loosing precision (I'll detail it separately)
  • HW backed: the AVR on UNO has the ability to capture events: basically there is a timer running that will stop when it gets triggered by a certain event (ex: the change of pin level cause by the echo). It is very precise, but you are limited by the number of HW blocks available on the AVR (3 iirc).

Now about the sub-cases of the GPIO interrupt-driven solution:

  • EXT based: the ATMega328p present on the UNO has what are called external interrupt sources, which is a fancy way to say that if you configure correctly a pin, the processor will notice when the pin voltage level changes and do something about it (in your case, take a time measurement) - this is quite generic solution, but unfortunately it works only on 2 pins, so you are out of lack with your 3 sensors, unless you implement some scheme where you multiplex 2 or more sensors on only one pin.
  • Pin-change-interrupt based: on AVR most of the PINS are grouped in ports. Each port contains maximum 8 pins and is labelled with a letter (A, B, C, ..). The way this interrupt works is roughly like the EXT type, but with a catch: you will get an interrupt whenever any of the pins that you have enabled on a certain port will change. In your case you could allocate only one sensor on a certain port, so that it would be obvious that if you get an interrupt for that port, it will be because of that sensor.

So now you can see why I said "it depends". If all you want to do is to get the information about proximity, you can go with busy-looping/polling.

But if you decide later on to do more on the same arduino, be aware that you might have to come back to the implementation of the ultrasonic sensor.

Finally, there is a library for this, but its implementation will fall into one of the cases i mentioned before, so be aware that the same catch applies: at some point you might want to do something that conflicts with the library.

Other points worth mentioning, that you should consider in your design (at least experiment with them):

  • orientation of the sensors: will they interfere with each other?
  • timing: do you need/want to activate them all simultaneously or do you prefer to have them activated in sequence?

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