# don't forget to install pyserial if not already done.
#"pip install pyserial" on command prompt :
#if python is installed on C:\ it should look like that:
# C:\users\yourname> set PATH = %PATH%;C:\python\scripts
# C:\users\yourname> pip install requests
##########################
import serial
import win32com.client

ser = serial.Serial("COM4",9600)
# COM4 might be diferent for you. choose the port where your arduino is
ser.isOpen()
shell = win32com.client.Dispatch("WScript.Shell")

while (0==0): # because why not
command = ser.read()
if (command == 'f'):
shell.SendKeys('F')

elif (command == 'd'):
shell.SendKeys('D')

elif (command == 'j'):
shell.SendKeys('J')

elif (command == 'k'):
shell.SendKeys('K')

// code inspired from: this
const char N = 4; // number of piezos
int seuil = 375; // threhold
int valeur1[N] = {0}; //first value of the N piezos
int valeur2[N] = {0}; //second value of the N piezos
unsigned long lt[N] = {0}; //time of last activation of piezos N

void setup() {
// put your setup code here, to run once:
Serial.begin(9600);
}

void loop() {
// put your main code here, to run repeatedly:

for (char i = 0; i<N; i++) { //for piezo 1 to 4 (i = 0, 1, 2, 3)
valeur1[i] = analogRead(i); //read first value
}
delay(1); // wait
for (char i = 0; i<N; i++) { //for piezo 1 to 4
valeur2[i] = analogRead(i); //read second value
}
while ((valeur1[0] < valeur2[0] || valeur1[1] < valeur2[1] || valeur1[2] < valeur2[2] ||valeur1[3] < valeur2[3]) && (valeur2[0] > seuil || valeur2[1] > seuil || valeur2[2] > seuil || valeur2[3] > seuil) ) {
//well this while can be shorter i'm sure but ... i don't know if "valeur1<valeur2" is working so ... i have to try
// while one of the piezo have an increasing signal and that the peak registered (valeur2) is better than the trehold (seuil)

for (char i = 0; i<N; i++) { //continue to read value
valeur1[i] = valeur2[i];
valeur2[i] = analogRead(i);
}

// delay(0.5);

/*
* next is looking for a decreasing value AND with 40ms of delay
* when OK, it send a signal to the computer (signal can be d, f , j or k (it depends of the piezos and how they are linked to the arduino))
* then lt is refreshed
*/
if (valeur1>valeur2) {

if (valeur1[0] >= seuil && millis()>=lt[0]+40) {
Serial.print('d');
lt[0] = millis();

}
if (valeur1[1] >= seuil && millis()>=lt[1]+40) {
Serial.print('k');
lt[1] = millis();
}
if (valeur1[2] >= seuil && millis()>=lt[2]+40) {
Serial.print('j');
lt[2] = millis();
}
if (valeur1[3] >= seuil && millis()>=lt[3]+40) {
Serial.print('f');
lt[3] = millis();
}
}
}

}

You only need .hex file, and flip. and flash it after put your Uno into DFU mode , and you can flash the hex file directly with Flip. also, if you want to modify your Arduino Sketch, you need to flash Arduino to original Firmware Serial hex file (in case you don't have you can download it). so i can imagine more like this

Upload Sketch -> DFU -> Arduino Keyboard Hex flash -> Reconnect after Flash finished -> Done. or if you want to modify sketch
DFU -> Arduino Original Firmware Serial flash with flip -> Reconnect after Flash finished -> Upload Sketch -> DFU -> Uno Hex Flash -> Reconnect -> Done.

In case there's a failure when flashing the Firmware, you can try reconnect the cable, put into DFU mode again, open USB Connection (use USB icon from top left) , and flash it again.

actually if you want to make serious project with output as Mouse or Keyboard input *cough*likeosukeyboard*cough*, you can switch over to Arduino Leonardo, Micro, or Due since they're already support HID without flashinganyotherfilethatmakesyouconfused.

const char N = 4; // number of piezos
int seuil = 350; // threhold
int valeur1[N] = {0}; //first value of the N piezos
int valeur2[N] = {0}; //second value of the N piezos
unsigned long lt[N] = {0}; //time of last activation of piezos N
uint8_t kb[8] = { 0 }; //Array that Used for Key Sending for UnoKeyboard

void setup() {
// put your setup code here, to run once:
Serial.begin(9600);
}

void loop() {
// put your main code here, to run repeatedly:

for (char i = 0; i<N; i++) { //for piezo 1 to 4 (i = 0, 1, 2, 3)
valeur1[i] = analogRead(i); //read first value
}
delay(1); // wait
for (char i = 0; i<N; i++) { //for piezo 1 to 4
valeur2[i] = analogRead(i); //read second value
}
while ((valeur1[0] < valeur2[0] || valeur1[1] < valeur2[1] || valeur1[2] < valeur2[2] ||valeur1[3] < valeur2[3]) && (valeur2[0] > seuil || valeur2[1] > seuil || valeur2[2] > seuil || valeur2[3] > seuil) ) {
//well this while can be shorter i'm sure but ... i don't know if "valeur1<valeur2" is working so ... i have to try
// while one of the piezo have an increasing signal and that the peak registered (valeur2) is better than the trehold (seuil)

for (char i = 0; i<N; i++) { //continue to read value
valeur1[i] = valeur2[i];
valeur2[i] = analogRead(i);
}

// delay(0.5);

/*
* next is looking for a decreasing value AND with 40ms of delay
* when OK, it send a signal to the computer (signal can be d, f , j or k (it depends of the piezos and how they are linked to the arduino))
* then lt is refreshed
*/

if (valeur1>valeur2) {

if (valeur1[0] >= seuil && millis()>=lt[0]+30) {
kb[2] = 7; //What's this? Read Here : https://www.arduino.cc/en/Reference/ASCIIchart
Serial.write(kb, 8); //Print the Data as Keypress output at Arduino Uno Keyboard
releaseKey(); //Release Key
//Serial.print('d');
lt[0] = millis();
}
if (valeur1[1] >= seuil && millis()>=lt[1]+30) {
kb[3] = 14;
Serial.write(kb, 8);
releaseKey();
//Serial.print('k');
lt[1] = millis();
}
if (valeur1[2] >= seuil && millis()>=lt[2]+30) {
kb[4] = 13;
Serial.write(kb, 8);
releaseKey();
//Serial.print('j');
lt[2] = millis();
}
if (valeur1[3] >= seuil && millis()>=lt[3]+30) {
kb[5] = 9;
Serial.write(kb, 8);
releaseKey();
//Serial.print('f');
lt[3] = millis();
}
}
}

}

void releaseKey(){
kb[2] = 0;
kb[3] = 0;
kb[4] = 0;
kb[5] = 0;
delay(16); //wait for osu to capture the key
Serial.write(kb, 8); //Release key after keypress
}