In this tech demo, seven LEDs arranged in a row light up consecutively and then turn off in the reverse order. The output is activated when the photosensor senses near-darkness.

//LED "ladder" activated by a photosensor
//Adapted from https://courses.ideate.cmu.edu/16-223/f2017/text/lib/SoundDemo.html (sound demo)

// ================================================================================

const int LED2 = 2;
const int LED3 = 3;
const int LED4 = 4;
const int LED5 = 5;
const int LED6 = 6;
const int LED7 = 7;
const int LED8 = 8;

//adapted from https://learn.adafruit.com/photocells/using-a-photocell
int photocellPin = A0;
int photocellReading;

// ================================================================================

void setup()
{

// Initialize the outputs.
pinMode(LED2, OUTPUT);
digitalWrite(LED2, LOW);
pinMode(LED3, OUTPUT);
digitalWrite(LED3, LOW);
pinMode(LED4, OUTPUT);
digitalWrite(LED4, LOW);
pinMode(LED5, OUTPUT);
digitalWrite(LED5, LOW);
pinMode(LED6, OUTPUT);
digitalWrite(LED6, LOW);
pinMode(LED7, OUTPUT);
digitalWrite(LED7, LOW);
pinMode(LED8, OUTPUT);
digitalWrite(LED8, LOW);

}
// ================================================================================

void loop()
{
photocellReading = analogRead(photocellPin);

if (photocellReading < 10) {
//if it is very dark, enter the loop

for(int i = 0; i < 4; i++) {
//Start going up the ladder with all LEDs in off position
digitalWrite(LED2, HIGH);
delay(50);
digitalWrite(LED3, HIGH);
delay(250);
digitalWrite(LED4, HIGH);
delay(450);
digitalWrite(LED5, HIGH);
delay(650);
digitalWrite(LED6, HIGH);
delay(850);
digitalWrite(LED7, HIGH);
delay(1050);
digitalWrite(LED8, HIGH);

//Now we go down the ladder
delay(1250);
digitalWrite(LED8, LOW);
delay(1300);
digitalWrite(LED7, LOW);
delay(1350);
digitalWrite(LED6, LOW);
delay(1400);
digitalWrite(LED5, LOW);
delay(1450);
digitalWrite(LED4, LOW);
delay(1500);
digitalWrite(LED3, LOW);
delay(1550);
digitalWrite(LED2, LOW);
delay(1600);
}
}
}