One great thing about using an Arduino is that there are many different ready-made modules, called shields, that fit on top of the Arduino and add extra features to it without any complex electronic construction. This project uses two such shields that are stacked on top of each other.
The first shield that fits on top of the Arduino is a screwshield, sometimes called a wing shield. This shield allows you to attach wires to the Arduino using screw terminals and a screw driver. The second and topmost shield that you’ll attach to the Arduino is an LCD display shield. This shield will tell you the battery level as a measurement of voltage and as a bar graph display of the state of charge (SOC) of the battery. The project also has an option to mute the buzzer to avoid attracting zombies, if you suspect they are shuffling about nearby.
The only other electronic components in this project are a pair of resistors and a buzzer. The resistors are needed because although the Arduino has inputs to measure voltage, it can only measure voltages up to 5V. Any more than that would damage the Arduino. You’ll use the pair of resistors in an arrangement called a voltage divider. The resistors I’ve chosen for my divider reduce the voltage to the Arduino by a factor of 2.74 so that the 12V or 13V that we might find at the battery will be reduced to 4.7V or less.
VOLTAGE DIVIDERS
Using two resistors as a voltage divider (Figure 3-9) is a great way to reduce the voltage you are trying to measure to a level where it can be directly measured by, say, an Arduino.
Figure 3-9: Voltage divider
The formula to calculate Vout if you know Vin, R1, and R2 is
For example, if R1 is 470 Ω, R2 is 270 Ω, and the maximum voltage of Vin is 13V, then
In other words, even if your battery is fully charged and managing to provide 13V, only a maximum of 4.74V (below the critical 5V level) will find its way to the Arduino. If the input voltage is lower than this, then Vout will scale proportionally. For example, if the battery voltage is 6.5V (which would indicate a bit of a problem, by the way), Vout would be 2.37V.
CONSTRUCTION
Remarkably, no soldering at all is needed to make this project. The only tool you need is a screwdriver.
STEP 1: PROGRAM THE ARDUINO
Arduino programs, which are called sketches, can change whether a connection, or pin, on the Arduino is an input or an output. The Arduino remembers whether each pin was set to input or output, even after you disconnect it from the rest of the circuit. Thus, if one of your Arduino pins was an output the last time you used it, connecting the Arduino to new hardware that expects the pin to be an input could damage the Arduino or the circuit you’re connecting it to. By uploading the program to the Arduino before doing anything else, you’ll ensure that each pin functions the way your circuit expects it to.
You’ll find detailed instructions on getting started with the Arduino, connecting it to your computer, and uploading a sketch to it in Appendix B. In this case, the sketch is called Project_04_Battery_monitor and can be found with all the other program files used in this book at http://nostarch.com/zombies/.
STEP 2: BUILD THE ARDUINO SANDWICH
When used with the two shields, the Arduino Uno is on the bottom, the screwshield is plugged into that, and, finally, the LCD display shield goes on top of the screwshield (Figure 3-10). The LCD shield has to be at the top of the stack or you won’t be able to see what it says!
Figure 3-10: An Arduino “sandwich”
When pushing the pins of a shield into an Arduino or the screwshield, be careful to check that all the pins meet the holes correctly so you don’t damage them. It’s quite easy for one of the pins to splay out as you are pushing the pins in.
STEP 3: ATTACH THE RESISTORS AND BUZZER
You’ll attach the resistors and buzzer to the screw terminals of the screwshield (Figure 3-11).
Figure 3-11: Connecting components to the screwshield
The two resistors can be identified either by measuring their resistance using a multimeter (see “Using a Multimeter” on page 237) or by reading the colored stripes on the resistor body. The 470 Ω resistor will have stripes of yellow, purple, and brown; the 270 Ω resistor will have red, purple, and brown stripes. In “Resistor Color Codes” on page 225, you will find a resistor color code table and instructions on how to identify resistors by their stripes.
Some buzzers will have a positive red lead and a negative black lead. If this is the case, connect the black lead to GND (ground) and the red lead to D11 on the Arduino. Other buzzers will have identical leads; if this is the case, it doesn’t matter which way around they are connected.
SOFTWARE
The sketch for this program is mostly concerned with making sure that the right text is displayed on the LCD at the right time. I’ll walk you through it in full, though you don’t have to understand or follow how this sketch works to finish the project. You can just upload it exactly as it is into the Arduino board, following the steps explained in “Installing the Antizombie Sketches” on page 248.
If you want to learn more about Arduino programming, see Appendix C or take a look at my book Programming Arduino: Getting Started with Sketches (McGraw-Hill, 2012).
The sketch starts by importing the LiquidCrystal library, which is responsible for controlling the LCD shield. Because this library is included as a standard part of the Arduino software, there is nothing to download and install for this library.
#include <LiquidCrystal.h>
After the library command, three constants are defined for key battery voltages.
const float maxV = 12.6;
const float minV = 11.7;
const float warnV = 11.7;
These voltages are, in order, the fully charged battery voltage, the minimum voltage that you want the battery to be allowed to discharge to, and the voltage at which the buzzer should sound. These last two are both set to 11.7V. These values are pretty standard for a lead-acid car battery, but if you use a different type of battery, you can tweak them. Because they hold decimal values, the variables are of a type called float. You can find out more about Arduino data types in Appendix C.
The next few lines define constants for the Arduino pins that are used.
const int buzzerPin = 11;
const int voltagePin = A3;
const int backlightPin = 10;
const int switchPin = A0;
The Arduino’s various pins are normally identified simply by a number, so these constants give them meaningful names. You don’t need to change these pin designations unless you decide to wire up your battery monitor differently.
The final section defines constants for the values of the resistors used in the potential divider.
const float R1 = 470.0;
const float R2 = 270.0;
const float k = (R1 + R2) / R2;
The constant k is the resulting factor the input voltage will be reduced by in order to fit into the 5V measurement range of the Arduino. The next line of code initializes the LCD display, specifying which pins are used.
// RS,E,D4,D5,D6,D7