The SR04 is a very powerful and commonly used distance/proximity sensor. And that is what we are going to look at first. The SR04 sensor emits ultrasonic waves which are sound waves at such a high frequency (40 kHz) that they are inaudible to humans. When these waves come across an object, some of them get reflected. These reflected waves get picked up by the sensor and it calculates how much time it took for the wave to return. It then converts this time into distance.

We are firstly going to use this sensor to make a simple proximity switch. Basically, when you bring an object closer than the set threshold distance, an LED is going to light up.

This is the circuit that we need to construct. Again, be very careful about where everything goes and make sure there are no mistakes. It is very easy to make a mistake, no matter how much experience you've had with Arduinos.

In reality it is going to look something like this, much messier than the Fritzing circuit depicted...

The LCD screen that we will be using is an I2C LCD1602.

This display screen can be programmed to display whatever you want in a 16x2 matrix. This means that the screen (as you will soon find out) has two rows capable of fitting 16 characters in each row.

Before setting up the complete circuit, look at the back of the LCD. Plug in four wires, as follows:

And then set up the circuit, as follows:

Now you will have to trust me on this next step. We are going to manually install a library that the LCD requires to run. You will be using this same method in future, so be patient and try to understand what we are doing here.

Download the LiquidCrystal_I2C.zip file from http://www.wentztech.com/filevault/Electronics/Arduino/.

Now, in the Arduino IDE, go to Sketch | Include Library | Add ZIP library and browse to the downloaded ZIP file. You are good to go.

If this doesn't work, you can manually extract the contents to: C:\Users\<Username>\Documents\Arduino\libraries...

Now comes the part where we combine what we have learnt so far in this chapter into one project. We are going use the sensor to calculate the distance between an object and relay this information to be displayed on the LCD screen in real time.

You are going to combine both the circuits from the previous two sections and create something like this:

Note that, at the bottom of the setup, you can see two power VCC/5V/red wires merging into one. We did not use a breadboard because we wanted to save space. A simple way to go about this is to use a male splitter. A crude way is to cut a male to male in two and cut one female to female wire in two, strip off a bit of their plastic insulation and twist the copper ends (two males and one female) together and simply tape the joint.

Open up a new sketch and load the Digital_Ruler.ino file.

Save it as Digital_Ruler.ino in your Chapter 2 directory and upload the code to the Arduino.

The result, if everything has gone right, will be exactly...

That was fun, right? And a bit challenging, correct? Good! That's when you know you are learning. So let's just summarize what we achieved in this chapter. We first programmed a HC-SR04 Ultrasound sensor and used it to measure a distance which was then displayed on the Arduino UNO Serial Monitor. Next, we played around with the I2C LCD1602 screen, and then, we combined what we learned from the two sections into one project called the Digital Ruler (Scale). You successfully created a digital measuring tape, which made it compact and less cumbersome to use. But since it can measure between 0 to 2 meters, it can only be used indoors. Higher ranges can be achieved using better (and more expensive) sensors.

In the next chapter, we will learn about touch sensors, which will along with a powerful processing software allow us to convert finger gestures to text.