This circuit is a temperature checker.

the right temperature to sleep for a room is between 16 and 19 deg C

the red led lights up if it is too hot in the room, green if it is the right temperature and blue if it is too cold.

here is the code

int sensorPin = A0;
int blueled = 4;
int greenled = 5;
int redled = 7;


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

void loop() {
  // put your main code here, to run repeatedly:
int reading = analogRead(sensorPin);

float voltage = reading * 5.0;
voltage/= 1024.0;

Serial.print(voltage);Serial.println("volts");

float temperatureC =  (voltage-0.5)*100;
Serial.print(temperatureC);Serial.println("degrees C");

if (temperatureC < 19 && temperatureC > 16) {
  digitalWrite(greenled,HIGH);
  digitalWrite(blueled,LOW);
  digitalWrite(redled,LOW);
}
if (temperatureC < 16) {
  digitalWrite(greenled,LOW);
  digitalWrite(blueled,HIGH);
  digitalWrite(redled,LOW);
  
}
if (temperatureC > 19 ) {
  digitalWrite(greenled,LOW);
  digitalWrite(blueled,LOW);
  digitalWrite(redled,HIGH);
  
}


delay(1000);
}

After making the baby piano I had the idea of making a baby piano that plays a short melody when a button is pressed with LEDs lighting up

It was very usefull to make the circuit on tinkercad first because I had trouble understanding how I sould do it at first. I could try different circuit without the risk of damaging my equipement

This circuit is very fun, I really like the tone() method

This could become a toy playing a different melody when each button is pressed with some colourful lights. I have seen this kinds of toys made for babies before.

I did this circuit and did a code that created a christmas light effect I really like

const int Led1pin = 2;
const int Led2pin = 3;
const int Led3pin = 4;
const int Led4pin = 5;
const int Led5pin = 6;
const int BUTTON_PIN= 12;

int ledState = LOW;     // the current state of LED
int lastButtonState;    // the previous state of button
int currentButtonState; // the current state of button
int number = 0;

void setup() {
  // put your setup code here, to run once:
  pinMode(Led1pin,OUTPUT);
  pinMode(Led2pin,OUTPUT);
  pinMode(Led3pin,OUTPUT);
  pinMode(Led4pin,OUTPUT);
  pinMode(Led5pin,OUTPUT);
  pinMode(BUTTON_PIN,INPUT_PULLUP);
  currentButtonState = digitalRead(BUTTON_PIN);
  
}

void loop() {
  // put your main code here, to run repeatedly:
  lastButtonState    = currentButtonState;      // save the last state
  currentButtonState = digitalRead(BUTTON_PIN); // read new state

  if(lastButtonState == HIGH && currentButtonState == LOW) {
    Serial.println("The button is pressed");
      if (number < 3) {
        (number) = (number) + 1;}
      else {
          (number) = 0;
          }
    }
  
  if (number == 0) {
    digitalWrite(Led1pin, HIGH); 
    digitalWrite(Led2pin, HIGH);
    digitalWrite(Led3pin, HIGH);
    digitalWrite(Led4pin, HIGH);
    digitalWrite(Led5pin, HIGH);
  }
  if (number == 1) {
    digitalWrite(Led1pin, HIGH); 
    digitalWrite(Led2pin, HIGH);
    digitalWrite(Led3pin, HIGH);
    digitalWrite(Led4pin, HIGH);
    digitalWrite(Led5pin, HIGH);
    delay(100);
    digitalWrite(Led1pin, LOW); 
    digitalWrite(Led2pin, LOW);
    digitalWrite(Led3pin, LOW);
    digitalWrite(Led4pin, LOW);
    digitalWrite(Led5pin, LOW);
    delay(100);
  }
  
  if (number == 2) {
    digitalWrite(Led1pin, HIGH); 
    digitalWrite(Led2pin, LOW);
    digitalWrite(Led3pin, HIGH);
    digitalWrite(Led4pin, LOW);
    digitalWrite(Led5pin, HIGH);
    delay(100);
    digitalWrite(Led1pin, LOW); 
    digitalWrite(Led2pin, HIGH);
    digitalWrite(Led3pin, LOW);
    digitalWrite(Led4pin, HIGH);
    digitalWrite(Led5pin, LOW);
    delay(100);
  }
  if (number == 3) {
    digitalWrite(Led1pin, HIGH); 
    delay(50);
    digitalWrite(Led2pin, HIGH);
    delay(50);
    digitalWrite(Led3pin, HIGH);
    delay(50);
    digitalWrite(Led4pin, HIGH);
    delay(50);
    digitalWrite(Led5pin, HIGH);
    delay(50);
    digitalWrite(Led1pin, LOW);
    delay(50); 
    digitalWrite(Led2pin, LOW);
    delay(50);
    digitalWrite(Led3pin, LOW);
    delay(50);
    digitalWrite(Led4pin, LOW);
    delay(50);
    digitalWrite(Led5pin, LOW);
    delay(50);
  }

}

This is the code I used, I had to try different logics to see which one worked best with this button system and this one works quite well.