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#define TRANSISTOR_GATE_PIN 2
#define LIGHT_LEVEL_INPUT_PIN A0
#define FLASH_DURATION_MS 5
 
int lightLevel;
int lastLightLevel = 1023;
 
void setup() {
  pinMode(TRANSISTOR_GATE_PIN, OUTPUT);
}
 
void loop() {
  
  lightLevel = analogRead(LIGHT_LEVEL_INPUT_PIN);
  
  if (lightLevel > ((lastLightLevel + 1023) / 2)) {
    digitalWrite(TRANSISTOR_GATE_PIN, HIGH);
    delay(FLASH_DURATION_MS);
    digitalWrite(TRANSISTOR_GATE_PIN, LOW);    
  } 
  
  lastLightLevel = lightLevel;
  
}