DualSonar

Dual sonar I2C sensor for Arduino, using two HC-SR04 ultrasonic sensors and ATtiny85 MCU (or Adafruit Trinket). Created by Alexander Kirillov shurik179@gmail.com; latest version is always available at https://github.com/shurik179/DualSonar/

Rationale

HC-SR04 are cheap and reliable ultrasonic sensors, used in many robotics projects with Arduino or similar microcontrollers. However, they have two drawbacks:

1. The standard method of reading the sensor in Arduino, using PulseIn() function, ties up the microcontroller for up to 30ms and interferes with tasks which need to be run frequently such as WiFi communication using Blynk or reading input from other sensors over serial interface.

2. Each sensor requires 2 pins, so if you have several ultrasonic senors, you run out of pins quickly.

There are some ways to mitigate these probems, such as using interrupts instead of PulseIn or using NewPing library which allows one to use only one pin per sensor. This project goes one step further. Namely, we combine two HC-SR04 sensors and an ATtiny85 microcontroller to create a sensor which measures distance in two directions and communicates with Arduino or another MCU using I2C protocol, all under $10. You can combine several such dual sonar sensors, putting them on the same I2C bus (see below), so you can have virtually unlimited number of sonars using only 2 pins of Arduino.

Materials for building the sensor

• two HC-SR04 sensors (can be bought from a variety of sources on Amazon, eBay, AliExpress for as cheap as $2/piece)
• ATtiny85 MCU ($1.24 on DigiKey).
• two 3.3kOhm resistors
• jumper wires, headers, small pcb

To program the ATtiny, you can use a special programmer such as SparkFun Tiny AVR Programmer or an Arduino Uno as described here. We are using Arduino Uno as the programmer.

These materials are enough for one DualSonar sensor, which can be connected to an Arduino or any microcontroller which can act as I2C master (e.g., ESP8266-12).

Programming ATtiny85

First, burn the bootloader to ATtiny85 as described here. Next, upload the Arduino sketch DualSonar-slave.ino, included in extras folder of this repository, to ATtiny85.
This sketch requires TinyWireS library, available here; you need to install this library before you can upload the sketch.

Connections

Connect the ATtiny85 to the HC-SR04's as follows:

• Sensor1 (left): Trig pin - ATtiny85 pin 1

  Echo pin - ATtiny85 pin 3

• Sensor2 (right): Trig pin - ATtiny85 pin 1

  Echo pin -ATtiny85 pin 4

In addition, connect GND pin of each sensor to GND of ATtiny85, and Vcc pin of each sensor to Vcc pin of ATtiny85.

Finally, connect 3.3. kOhm resistors between Vcc pin of ATtiny and ATtiny85 pins 3, 4 (these act as pullup resistors for I2C bus).

The sensor is done.

Using the DualSonar sensor

To use the sensor, install DualSonar library (of which this file is a part). It provides the class DualSonar with the following functions:

• begin() - starts (activates) the sensor
• stop() - stops the sensor
• isActive() - returns true if the sensor is active, false if it is stopped or if the sensor failed to respond
• update() - updates the sensor readings
• distanceL(), distanceR() - distances measured by left and right ultrasonic sensor respectively, in mm

While sensor is active, it continually measures distances and keeps the running average of last measurements using simple low-pass filter. To avoid interference, it uses one sensor at a time, alternating between the two HC-SR04 sensors; each is pinged about 20 times a second. The maximal distance is about 3.4 meters (3444 mm); distances larger than that (or when no echo is received) are reported as 3444 mm.

Calling update() fetches these values from the sensor, after which they are available using distanceL(), distanceR() functions. Calling stop() stops the sensor, so it no longer emits pings; calling begin() again restarts it.

Note: the values returned by distanceL(), distanceR() are those fetched at latest update() call, so you must call update() frequently to make sure those values are up-to-date.

Changing I2C address

By default, the DualSonar sensor uses I2C address 17 (0x11). If this conflicts with another device on your I2C bus, or if you need to connect more than one DualSonar sensor, you can change the I2C address as follows:

1. Before uploading the DualSonar-slave.ino sketch to ATtiny85, edit line

 #define I2C_SLAVE_ADDRESS 0x11 

and change the address to an I2C address of your choice

2. When creating DualSonar object, provide the chosen I2C address as an argument:

DualSonar mySonar(MY_I2C_ADDRESS);

(replacing MY_I2C_ADDRESS by the chosen I2C address)