Pasted

#include <SPI.h>
#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>
 #define OLED_RESET 4
Adafruit_SSD1306 display(OLED_RESET);
 // these constants won't change:
const int ledPin = 13;      // LED connected to digital pin 13
const int knockSensor = A0; // the piezo is connected to analog pin 0
const int threshold = 100;  // threshold value to decide when the detected sound is a knock or not
  // these variables will change:
int sensorReading = 0;      // variable to store the value read from the sensor pin
int ledState = LOW;         // variable used to store the last LED status, to toggle the light
 int buttonPin = 12;
int counter = 0;
int systemState;
 byte objA;
byte objB;
byte answer;
  void setup() {
  pinMode(ledPin, OUTPUT); // declare the ledPin as as OUTPUT
  pinMode(buttonPin, INPUT_PULLUP);
  Serial.begin(115200);       // use the serial port
  randomSeed(analogRead(0) + analogRead(1) + analogRead(2) + analogRead(3) + analogRead(4) + analogRead(5));
  // by default, we'll generate the high voltage from the 3.3v line internally! (neat!)
  display.begin(SSD1306_SWITCHCAPVCC, 0x3C);  // initialize with the I2C addr 0x3C (for the 128x32)
  // init done
   // Show image buffer on the display hardware.
  // Since the buffer is intialized with an Adafruit splashscreen
  // internally, this will display the splashscreen.
  display.display();
  delay(2000);
   // Clear the buffer.
  display.clearDisplay();
  display.setTextSize(3);
  display.setTextColor(WHITE);
  setSystemState(0);
}
  void loop() {
   switch (systemState) {
    case 0:
      if (!digitalRead(buttonPin)) {
        setSystemState(1);
      } //end if
      break;
    case 1:
      // read the sensor and store it in the variable sensorReading:
      sensorReading = analogRead(knockSensor);
       // if the sensor reading is greater than the threshold:
      if (sensorReading >= threshold) {
        // toggle the status of the ledPin:
        ledState = !ledState;
        // update the LED pin itself:
        digitalWrite(ledPin, ledState);
        // send the string "Knock!" back to the computer, followed by newline
        Serial.println("Knock!");
        counter++;
         display.setCursor(0, 0);
        display.println(counter);
        display.display();
        delay(100);  // delay to avoid overloading the serial port buffer
        display.clearDisplay();
      } // end if
        if (!digitalRead(buttonPin)) {
        setSystemState(2);
      } //end if
      break;
      case 2:
       if (!digitalRead(buttonPin)) {
        setSystemState(1);
       } //end if
      break;
    default:
      break;
    } //end switch
   }//endloop
 void setSystemState(int newState) {
  switch (newState) {
    case 0:
      display.setCursor(0, 0);
      display.println("Press");
      display.display();
      delay(100);  // delay to avoid overloading the serial port buffer
      display.clearDisplay();
      break;
    case 1:
      objA = random(1, 9);
      objB = random(1, 9);
      answer = objA * objB;
      counter = 0;
      display.setCursor(0, 0);
      display.println(String(objA) + "X" + String(objB));
      display.display();
      delay(100);  // delay to avoid overloading the serial port buffer
      display.clearDisplay();
      delay(2000);
      break;
    case 2:
      display.setCursor(0, 0);
      if (counter == answer) {
        display.setCursor(0, 0);
        display.println(String(counter) + " V");
      } else  {
        display.println(String(counter) + " W " + String(answer));
      }//end if
       display.display();
      delay(100);  // delay to avoid overloading the serial port buffer
      display.clearDisplay();
      delay(2000);
    default:
      break;
   }// end switch
  systemState = newState;
}//end setSystemState