3.4 Sketching Things Out

Before we start writing code, we first need to make sure we can communicate with the Arduino. Then we will learn how to collect and act upon data sent by the flex sensor with a program (what the Arduino community prefers to call a sketch).

The first sketch we write will detect when the flex resistor values have changed. If the change is large enough (in other words, if water is making the resistor bend), we will transmit a request to a PHP server that will process the request. That server will then send out an email notifying us of the change.

We will build the sketch incrementally, first by connecting the flex sensor to the Arduino and collecting values when the sensor is straight and then when it bends in both directions. Once these values have been identified, we will write conditional statements that will call functions to send HTTP GET statements containing data we will include in the email alert.

Configuring an Arduino

We will use the Arduino IDE to write, compile, and download our code into the Arduino. For those who would like a more comprehensive introduction to Arduino programming, read Maik Schmidt’s excellent Arduino: A Quick Start Guide [Sch11].

If you are already familiar with the Arduino or are willing to hang on for the ride, let’s get started by launching the Arduino IDE. Check to ensure that your Arduino is connected via USB cable and recognized and selected accordingly on one of the serial ports identified by the Arduino IDE’s Tools→Serial Port menu. You can perform a quick test of your configuration with the LED Blink example program located in the Arduino IDE’s File→Examples→1.Basics→Blink menu. Upload it to the attached Arduino and check to see that it executes correctly.

If it fails to do so, first verify that the Arduino is correctly plugged into the computer and powered by the USB. If it is, check next to be sure you’ve selected the correct serial port in the Arduino IDE and highlighted the right type of Arduino board in the Tools→Board. A few correctly placed mouse clicks on either of these settings usually fixes the problem.

The Flex Sensor Sketch

Now that the Arduino is connected and tested, we can write a sketch that will validate and interpret the bending of the flex sensor. We will begin by defining a few constants that we will refer to in the program.

Since we have to account for the sensor bending in either direction, we will define two named constants that will be used to set the upper and lower threshold limits.

We place these constants at the beginning of the sketch so they’re easier to locate in case we need to edit these values later on. By convention, defined constants are also all uppercase so that they are easier to identify in the code. Let’s call them FLEX_TOO_HI and FLEX_TOO_LOW. The range between these upper and lower limits will depend on the degree of flex that is optimal for your own scenario. I prefer a variance of plus or minus five units to allow a minor amount of bend before the notification event is triggered. Having such a range will allow us to account for minor environmental effects like a light breeze or a low-grade vibration.

We also need to account for the Arduino’s onboard LED and the analog pin that the flex sensor is attached to.

• FLEX_TOO_HIGH is the value of the assigned analog pin when the flex sensor is bent forward past this threshold.

• FLEX_TOO_LOW is the value of the assigned analog pin when the flex sensor is bent backward past this threshold.

• ONBOARD_LED is assigned to the Arduino’s onboard LED located at pin 13. We will use it provide us with a visual indicator when the flex resistor has deviated far enough to send an alert. This allows us to use the Arduino’s onboard LED as a kind of visual debugger so that we can visually confirm that the flex events are being detected.

• FLEX_SENSOR is connected to the analog pin on the Arduino that the flex resistor is connected to. In this case, that value is 0 because the resistor is connected to pin 0.

These constants will be defined at the beginning of the sketch.

Now we will create two variables to capture the changing value and state of the flex resistor and set their initial values to zero.

• bend_value will store the changing analog values of the flex resistor as it bends.

• bend_state is the binary condition of the flex sensor. If it’s straight, its value is equal to zero. If the flex resistor deviates either direction, we will set its state to one.

These variables will follow after the define statements we wrote earlier.

With the constants defined and the variables initialized, we need to set up the serial port to monitor the continuous stream of values being polled in the main program’s loop. The onboard LED also has to be configured so we can see it turn on and off based on the bend_state of the flex resistor.

With the upper and lower flex bending limits defined, we need a routine that will check to see if these limits have been exceeded. If they have, we will turn on the Arduino’s onboard LED. When the flex resistor returns to its resting straight position, we will turn the LED off.

Note the first line of this code block: digitalWrite(ONBOARD_LED, bend_state ? HIGH : LOW);. This ternary operation polls the current state of the flex resistor based on the value (0 or 1) that we passed to the function. The conditional statement that follows prints out an appropriate message to the Arduino IDE’s serial window. If the bend_state is true (HIGH), the flex resistor has been bent past the limits we defined. In other words, water has exceeded the threshold. If it’s false (LOW), the flex resistor is straight (i.e., the water level is not rising).

All that is left to write is the program’s main loop. Poll the FLEX_SENSOR pin (currently defined as analog pin 0) every second for any increase or decrease in value. When a flex event is detected, print the bend_value to the serial port so we can see it displayed in the Arduino IDE’s serial window.

The main loop of the sketch will poll the value of the flex resistor every second. A switch statement tests the condition of the flex resistor. If its last status was straight (case 0:), check to see if it has since bent beyond the upper and lower threshold limits. If so, set the bend_state accordingly and call the SendWaterAlert function. Conversely, if the resistor’s last status was bent (case 1:), check to see if it’s now straight. If it is, set the bend_state variable to zero and pass that new state to the SendWaterAlert function.

Depending on the type of flex sensor and Ethernet shield used along with the voltage pin selected, your baseline value may be different from the baseline one I recorded. My flex sensor reported a value of 470.

Note the use of semicolons to mark the end of a line of instruction and brackets to identify conditional blocks. Save the file. It’s also a good idea to place this and all other code you write under your preferred choice of version control before proceeding. I recommend Git,[25] but others like Mercurial and Subversion are certainly better than any non--version controlled alternative.

Later on, we will ask the SendWaterAlert function to call another function that will connect to a designated PHP server. This in turn will send an email alert that will contain the appropriate alert and the bend_value being monitored. But before we do, we will verify that our threshold test is working by monitoring the messages sent to the Arduino IDE’s serial window.

Run the Sketch

Save and click the Verify button in the Arduino IDE’s toolbar. This will compile the sketch to check for any syntax errors. After confirming that there are none, send the sketch to the Arduino by clicking the Upload button on the toolbar. You should see the Arduino’s onboard LED flash a few times, indicating that it is receiving the sketch. When the rapid flashing stops, the sketch should be running.

Open up the Arduino IDE’s Serial Monitor window. Assuming you haven’t yet commented out the Serial.print("bend_value="); statement in the main loop of the sketch, observe the numbers that are continuously scrolling upward at a rate of roughly once a second on the serial monitor’s display. If the characters being displayed in the window look like gibberish, make sure to select the correct baud rate (in this case, 9600) in the serial monitor’s drop-down list located in the lower right corner of the serial monitor window. Take note of the values of the flex resistor when it is straight, bent forward, and backward.

Depending on the amount of electrical resistance and the type of hardware being used, update the FLEX_TOO_HIGH and FLEX_TOO_LOW constants to better calibrate them to the changing values you are seeing in the serial window. Once these defined amounts have been entered, save the program and upload again to the Arduino, performing the same procedure as before. It may take two or three tries to narrow in on the high and low values that help determine the bend state of the flex resistor.

With the modified upper and lower limits set to best suit your particular configuration, observe the Arduino’s onboard LED to ensure that it lights up when the flex resistor bends far enough forward or backward and turns off when the resistor is straightened back to its original position.

Testing the Sketch

When you are confident that the hardware setup and the uploaded Arduino sketch are behaving correctly, it’s time to try a simple water test by filling up a bowl with water and dipping the bobber into the water while holding the base of the flex resistor between your thumb and forefinger. As an extra precaution, wrap any exposed solder connecting the two wires to the flex resistor in waterproof electrical tape. I suggest wrapping the tape several layers thick, both to have a solid base to hold the resistor as well as to protect it from any errant drops of water that may accidentally splash or spill.

After properly and safely setting up the test, verify that as the buoyancy of the water deflects the bobber attached to the flex resistor, the resistor bends far enough in either direction to turn the LED light on.

Be careful not to submerge the exposed flex resistor. While the amount of current flowing through the Arduino is relatively low, water and electricity can make for a deadly combination. Place any electronics, including the flex resistor and attached bobber, in a sealed plastic bag with enough room to allow the flex resistor to bend. Use a high degree of caution to make absolutely sure to not get any of the exposed wiring or electrical connections wet. Doing so could damage your equipment or, even worse, you.

The base functionality of the water level notifier is complete. However, its method of communicating a rise in water height is limited to a tiny LED on the Arduino board. While that may be fine for science projects and people who work right next to the Arduino monitoring the water source in question, it needs to broadcast its alert beyond simple light illumination.

Receiving an email notification makes more sense, especially when the location of water measurement is somewhere in the home that is not frequently visited. Perhaps the detector will even operate at a remote location, such as when monitoring the status of a sump pit at a vacation home after a heavy rain.

To do so, we will need to clip on an Ethernet shield to the Arduino and write some code to send an email when the bend threshold is crossed. But before we add more hardware to this project, we first need to set up a web-based email notification application that our Arduino sketch can call upon when it needs to send out an alert.