Building a sonar sensor array with Arduino and Python – Medium

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The HC-SR04 sensor has only four pins. Two for ground and +5v, and Echo and Trigger pins. To use the sensor we need to trigger the signal using the Trigger pin and measure the time until is received via the Echo pin. As we don’t use the Echo and Trigger pins at the same time they can share the same cable to connect to an Arduino digital pin.

For this project we are going to use an Arduino Nano which is small and broadly available. There are tons of non-official compatible clones for under $3 per unit as well.

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Breadboard setup of sonar array. Image by the author

For this breadboard setup we have connected both Trig and Echo pins to a single digital pin in Arduino. We are going to use D12, D11, D10, D9 and D8 pins for sending and receiving signals. This hardware setup is only limited by the microcontroller’s available digital pins but it can be expanded further using multiplexing where one pin can be shared by multiple sensors but only one sensor at the time.

Traditionally this will be the sequential workflow we would need to manage to poll sensors one by one:

  1. Trigger one sensor
  2. Receive the echo
  3. Calculate distance using the duration of the previous steps
  4. Communicate measurement using the serial port
  5. Process next sensor

However we are going to use a ready available Arduino library called NewPing that allows you to ping multiple sensors minimizing the delay between sensors. This will help us to measure the distance from all 5 sensors several times per second at the same time (almost). The resulting workflow would look like this:

  1. Trigger and echo all sensors async (but sequentially)
  2. When a sensor is done calculate distance
  3. When all sensors are done for the current cycle, communicate readings from all sensors using the serial port
  4. Start a new sensor reading cycle

The implementation is very straightforward and heavily commented in the code. Feel free to take a look at the full code here.

I want to put special focus on the serial communication part when all the sensors are done with the distance measurement:

void oneSensorCycle() { // Sensor ping cycle complete, do something with the results.

for (uint8_t i = 0; i < SONAR_NUM; i++) {

// Sending bytes

byte reading_high = highByte(cm[i]);

byte reading_low = lowByte(cm[i]);

byte packet[]={0x59,0x59,i,reading_high,reading_low};

Serial.write(packet, sizeof(packet));

}

}

Originally I wanted to send sensor readings via serial using strings however I realized the message will be big and harder to parse on the host side of the project. In order to improve speed and lower delay in the readings I decided to switch to a simple format using a message of 5 bytes:

Byte 1: Character ‘Y’

Byte 2: Character ‘Y’

Byte 3: Sensor index [0–255]

Byte 4: High-order byte of the measured distance (as an unsigned integer)

Byte 5: Low-order byte of the measured distance (as an unsigned integer)

Byte 1 and 2 will be just the message header to decide where a new message starts when we will be reading the incoming serial bytes. This approach is very similar to what the TF-Luna LiDar sensor is doing to communicate to the host computer.

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