16-key keypad with an Arduino and a PCF8574 I/O expander module
In this video I show you a nice way to connect a 16-key keypad to an Arduino (or any i2C compatible microcontroller) with a smart device. The smart device is a PCF8574 I/O expander which allows us to have 8 I/O ports controlled via i2C. It is a great device because we do not need to sacrifice any of the native I/O ports of the Arduino since we use the i2C pins. With this additional module it is really simple to handle this 16-key keypad. This demo will show you how you can enter numbers, how you can delete characters and how you can pass the numbers to some numerical type variables (double, float, int...etc).
Arduino source code
#include <Wire.h> #include <LiquidCrystal_I2C.h> LiquidCrystal_I2C lcd(0x27, 16, 2); //16x2 LCD on the 0x27 i2C address float buttonTimer = 0; //Timer for avoiding multiple keypresses at a time bool buttonPressed = false; //status which keeps track if a key was pressed String fullString = ""; //concatenated string String pressedCharacter = ""; //recently pressed character on the keypad bool stringAdded = false; //keeps track of the concatenation double convertedNumber = 0; //final number. fullString converted into double /* !NOTICE! * There is no wiring schematics provided since I only used the i2C pins in this demonstration * SDA: A4, SCL: A5 * Both the LCD and the PCF8574 module are compatible with 5 V * The pins on the keypad are connected to the PCF8574 in an ascending order: * P0: 1st pin on the keypad, P1: 2nd pin on the keypad...etc., P7: 8th pin on the keypad * Notice that the keypad's first actual pin is the 2nd physical pin (at least on the one I used) */ void setup() { Wire.begin(); Wire.setClock(400000L); Serial.begin(9600); Serial.println("PCF8574 with a keypad"); //----------------- //LCD lcd.begin(); lcd.setCursor(0, 0); //1st line, 1st block lcd.print("Keypad 123-ABC"); lcd.setCursor(0, 1); //2nd line, 1st block lcd.print("Demonstration"); // delay(2000); //wait 2 sec // Permanent parts lcd.clear(); lcd.setCursor(0, 0); lcd.print("String: "); lcd.setCursor(0, 1); lcd.print("Double: "); //---------------------------------------------------------------------------- } void loop() { readKeyPad(); //read the keypad checkInput(); //do something if a key was pressed } void checkInput() { if (buttonPressed == true) //if a button was pressed... { if (stringAdded == true) //if a numberical button was pressed... { fullString = fullString + pressedCharacter; //concatenate //Print the string on the LCD lcd.setCursor(8, 0); lcd.print(fullString); lcd.setCursor(8, 1); //remove the values from the 2nd line lcd.print(" "); stringAdded = false; //don't let the code enter this part again } if (pressedCharacter == "A") //if we pressed the A on the keypad... { //This part does the string -> double conversion, then resets everything Serial.print("String: "); Serial.println(fullString); convertedNumber = fullString.toDouble(); //string to double (because of the decimals) //Print the converted number on the LCD lcd.setCursor(8, 1); lcd.print(convertedNumber); Serial.println(convertedNumber); fullString = ""; //reset the string convertedNumber = 0; //reset the number to 0 pressedCharacter = ""; //reset pressed character (this part will be skipped in further iterations until A is pressed again) lcd.setCursor(8, 0); //Clear the first line lcd.print(" "); } else if (pressedCharacter == "D") //D = delete { fullString = fullString.substring(0, fullString.length() - 1); // remove the last character Serial.print("String: "); Serial.println(fullString); lcd.setCursor(8 + fullString.length(), 0); //removes the last character from the LCD (abcd12|3) lcd.print(" "); lcd.setCursor(8, 0); //reprint the whole, 1 character shorter number lcd.print(fullString); pressedCharacter = ""; } } } void readKeyPad() { if (buttonPressed == true) { if (millis() - buttonTimer > 300) { //The button pressed is only set back to false after 300 ms, so we cannot press a button twice quickly buttonTimer = millis(); buttonPressed = false; } else { //do nothing } } else { //B11101111 Wire.beginTransmission(0x20); //00100000 Wire.write(B11101111); //[P7]B11101111[P0] -> [P7]1110[P4] - activates first row, [P3]1111[P0] - Sets all pins high on the MUX Wire.endTransmission(); Wire.requestFrom(0x20, 1); switch (Wire.read()) { //11101110 - P0 pin went low after pressing the button -> 1 was pressed case 238: //Button 1 Serial.print("1"); pressedCharacter = "1"; buttonPressed = true; stringAdded = true; break; //11101101 - P1 pin went low after pressing the button -> 2 was pressed case 237: //Button 2 Serial.print("2"); pressedCharacter = "2"; buttonPressed = true; stringAdded = true; break; //11101011 - P2 pin went low after pressing the button -> 3 was pressed case 235: //Button 3 Serial.print("3"); pressedCharacter = "3"; buttonPressed = true; stringAdded = true; break; //11100111 - P3 pin went low after pressing the button -> A was pressed case 231: //Button A Serial.println("A"); pressedCharacter = "A"; buttonPressed = true; break; } //------------------------------------------- //B11011111 Wire.beginTransmission(0x20); //00100000 Wire.write(B11011111); //[P7]B11011111[P0] -> [P7]1101[P4] - activates second row, [P3]1111[P0] - Sets all pins high on the MUX Wire.endTransmission(); Wire.requestFrom(0x20, 1); switch (Wire.read()) { //11011110 - P0 pin went low after pressing the button -> 2 was pressed case 222: //Button 4 Serial.print("4"); pressedCharacter = "4"; buttonPressed = true; stringAdded = true; break; case 221: //Button 5 Serial.print("5"); pressedCharacter = "5"; buttonPressed = true; stringAdded = true; break; case 219: //Button 6 Serial.print("6"); pressedCharacter = "6"; buttonPressed = true; stringAdded = true; break; case 215: //Button B Serial.println("B"); pressedCharacter = "B"; buttonPressed = true; break; } //------------------------------------------- //B10111111 Wire.beginTransmission(0x20); //00100000 Wire.write(B10111111); Wire.endTransmission(); Wire.requestFrom(0x20, 1); switch (Wire.read()) { case 190: //Button 7 Serial.print("7"); pressedCharacter = "7"; buttonPressed = true; stringAdded = true; break; case 189: //Button 8 Serial.print("8"); pressedCharacter = "8"; buttonPressed = true; stringAdded = true; break; case 187: //Button 9 Serial.print("9"); pressedCharacter = "9"; buttonPressed = true; stringAdded = true; break; case 183: //Button C Serial.println("C"); pressedCharacter = "C"; buttonPressed = true; break; } //------------------------------------------- //B01111111 Wire.beginTransmission(0x20); //00100000 Wire.write(B01111111); Wire.endTransmission(); Wire.requestFrom(0x20, 1); switch (Wire.read()) { case 126: //Button * Serial.print("."); pressedCharacter = "."; buttonPressed = true; stringAdded = true; break; case 125: //Button 0 Serial.print("0"); pressedCharacter = "0"; buttonPressed = true; stringAdded = true; break; case 123: //Button # Serial.print("#"); pressedCharacter = "#"; buttonPressed = true; break; case 119: //Button D Serial.println("D"); pressedCharacter = "D"; buttonPressed = true; break; } buttonTimer = millis(); } }
