I'm using an HC-SRO4 Ultrasonic Sensor and a D10K Potentiometer. So, I was just messing around with the potentiometer in the program and then tried to change the brightness of the LED which turns on if a certain condition is met, but the potentiometer acts like an ON/OFF switch instead. I'd like to know why this happens and how can I change the LED's brightness normally in this situation.

long duration;
int distance;
int trigPin = 9;
int echoPin = 10;
int buzzer = 11;
int LED = 3;
boolean detected = false;
#define pot 0
int val = 0;

void setup() {
  pinMode(LED, OUTPUT);

void loop() {
  digitalWrite(trigPin, LOW);
digitalWrite(trigPin, HIGH);
digitalWrite(trigPin, LOW);
duration = pulseIn(echoPin, HIGH);
distance= duration*0.034/2;
val  = analogRead(pot);
val  = map(val, 0, 1023, 0, 255);
Serial.println  (distance);

if(distance < 50 && distance != 0){ // distance != 0  because sometimes the distance's first value is sometimes 0 which ruins the purpose of
 // the program
  analogWrite(LED, val);
  detected = true; 
else if(detected){ 

void alarm(){
  tone(buzzer, 450);
  tone(buzzer, 1000);
  • 1
    Write a sketch with pot and led only. – Juraj Aug 16 '18 at 17:30
  • Well, I did and it works fine. I can change the brightness of the LED. But what I don't understand is why it doesn't do so in the code above. @Juraj – Arman Samma Aug 16 '18 at 17:46
  • because the analogWrite is conditional to distance sensor reading – Juraj Aug 16 '18 at 17:47
  • Thing is can't I change the brightness while that condition is true? Well, when I make the condition true, (putting some object in front of the sensor) I turn the potentiometer and it still acts like an ON/OFF switch. – Arman Samma Aug 16 '18 at 17:52

The answer is very simple. You use pin 12 for LED. Pin 12 can't do analogWrite(). This pin doesn't support PWM on Arduino UNO (Atmega 328)

Pin 3 is not OK either. Use of tone() function disables PWM on pin 3. From reference: "Use of the tone() function will interfere with PWM output on pins 3 and 11".

  • I'm sorry. I had changed my LED pin to 12 (I'll edit it to 3 now) because I was messing around with it. It was originally a PWM pin when this problem happened – Arman Samma Aug 16 '18 at 17:20
  • @ArmanSamma: Please make sure that whatever code you post is always relevant to the question. – Edgar Bonet Aug 16 '18 at 17:32
  • I don't understand. How and why does the tone() function disable PWM on pin 3? @Juraj – Arman Samma Aug 16 '18 at 20:28
  • "tone()" in answer is a link. Read the reference of the tone function. – Juraj Aug 17 '18 at 4:28

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