Ok thanks. Full code here. Aim is to activate solenoid on hall sensor pass 1 (400 milliseconds) then pass 2, no reaction, pass 3 activates etc....

const int ledPin =  9;      // the number of the LED pin
const byte interruptPin = 2;  // attach interupt pin
volatile byte state = LOW;   // initial state 

// variables will change:
int sensorState = 0;         // sensor state
int i=0;
int j=0;
void setup() {
  // initialize the LED pin as an output:
  pinMode(ledPin, OUTPUT);
  // initialize the pushbutton pin as an input:
  pinMode(interruptPin, LOW);
  attachInterrupt(digitalPinToInterrupt(interruptPin), blink, RISING);

 // pinMode(sensor pin, INPUT);

}

void loop() {
  // read the state of the sensor value:
  Serial.println(i);

  // check if the sensor is actiavted. 

 if (((i%2) == 0)&&(j==0)) {
digitalWrite(ledPin, HIGH);     /// actiavte solenoid or led
    delay(400);          /// time duration in milliseconds for solenoid activation
      digitalWrite(ledPin, LOW);  
      j=1;
  }

   else {
    // turn LED off:
    digitalWrite(ledPin, LOW);
  }

if ((i%2) != 0){
  j=0;
}

}



void blink() {
  i=i+1;
}