How to replace the delay() function in my code?

Question

I'm new to Arduino and I currently have a project that involves the control of an LED via ultrasonic sensor. Basically, if an object passes through the ultrasonic sensor, the LED will be ON for 5 seconds but adding another set of LED and ultrasonic sensor with the same function gives a problem because the other LED won't light up if the other one is ON and vice versa because of the delay whereas what I want is to be able to make them both light up and stay lit for 5 seconds after their respective ultrasonic sensors detect an object. I've tried using the "Blink without delay" example but I can't understand it nor make it work. Please help me guys and thanks for reading. Here is my current code:

int trigPin1=2;
int echoPin1=3;
int trigPin2=4;
int echoPin2=5;
int led1=10;
int led2=11;
void setup() {
 Serial.begin(9600);
 pinMode(trigPin1, OUTPUT);
 pinMode(echoPin1, INPUT);
 pinMode(trigPin2, OUTPUT);
 pinMode(echoPin2, INPUT);
 pinMode(led1,OUTPUT);
 pinMode(led2,OUTPUT);
}
void loop() {
 long duration1, distance1;
 digitalWrite(trigPin1, LOW); // Added this line
 delayMicroseconds(2); // Added this line
 digitalWrite(trigPin1, HIGH);
 delayMicroseconds(10); // Added this line
 digitalWrite(trigPin1, LOW);
 duration1 = pulseIn(echoPin1, HIGH);
 distance1 = (duration1/2) / 29.1;
 if (distance1 >= 500 || distance1 <= 0){
 Serial.println("Out of range");
 }
 else {
 Serial.print ( "Sensor1 ");
 Serial.print ( distance1);
 Serial.println("cm");
 }
 long duration2, distance2;
 digitalWrite(trigPin2, LOW); // Added this line
 delayMicroseconds(2); // Added this line
 digitalWrite(trigPin2, HIGH);
 delayMicroseconds(10); // Added this line
 digitalWrite(trigPin2, LOW);
 duration2 = pulseIn(echoPin2, HIGH);
 distance2= (duration2/2) / 29.1;
 if (distance2 >= 500 || distance2 <= 0){
 Serial.println("Out of range");
 }
 else {
 Serial.print("Sensor2 ");
 Serial.print(distance2);
 Serial.println("cm"); 
 }
 if(distance1>20 || distance2>20){
 digitalWrite(led1,LOW);
 digitalWrite(led2,LOW);
 }
 else if(distance1<20){
 digitalWrite(led1,HIGH);
 delay(5000);
 }
 else if(distance2<20){
 digitalWrite(led2,HIGH);
 delay(5000);
 }
}

Answer 1

The first set of delaymicroseconds for measuring is fine.

you'll want to replace the delay(5000) when switching the leds.

 if(distance1>20 || distance2>20){
 digitalWrite(led1,LOW);
 digitalWrite(led2,LOW);
 }
 else if(distance1<20){
 digitalWrite(led1,HIGH);
 delay(5000);
 }
 else if(distance2<20){
 digitalWrite(led2,HIGH);
 delay(5000);
 }

you can do that by using a timestamp of when the distance was last high:

 if(distance1<20){
 digitalWrite(led1,HIGH);
 timestamp1 = millis();
 }
 if(distance2<20){
 digitalWrite(led2,HIGH);
 timestamp2 = millis();
 }
if(millis() - timestamp1 > 5000){
 digitalWrite(led1,LOW);
}
if(millis() - timestamp2 > 5000){
 digitalWrite(led2,LOW);
}

timestamp1 and timestamp2 should be globals (or static locals). This code will delay turning off the led to 5 seconds after the sensor last detected something close.

• Lelouch Lamperouge: Thank you very much! I managed to do it using your hints but I believe the code after if(millis()-timestamp1>5000) should be digitalWrite(led1,LOW)
• ratchet freak: @LelouchLamperouge indeed they should be set to low there. copy paste errors...