Turn off Relay in X seconds

/*
 *  Find ButtonOne Library at https://github.com/MrAlvin/ButtonOne
 *
 * Test circuit:
 *
 *  Connect a pushbutton to pin A1 (ButtonPin), connect other side of button to ground.
 *
 *  Pin 13 (StatusPin) is used for output
 *    many Arduino versions already have a LED connected to this pin,
 *    otherwise connect: pin13 - LED - resistor - ground
 *
 *  Potentiometer connected to <+ - A3 - ground> see diagram example https://labitat.dk/wiki/Arduino_beginners_workshop#CIRC08_-_Twisting_-_.28Potentiometers.29
 *
 */

#include "ButtonOne.h"

int sensorPin = A3;    // select the input pin for the potentiometer
int ledPin = 13;      // select the pin for the LED
int buttonPin = 7;
unsigned long sensorValue = 0;  // variable to store the value coming from the sensor
unsigned long relay_millis = 0;


// Setup a ButtonOne instance on pin 7.
ButtonOne button(buttonPin);


//*********************************************
// setup function - to run once:
//*********************************************
void setup() {
  // enable the standard led on pin 13.
  pinMode(ledPin, OUTPUT);      // sets the digital pin as output

  //initiate internal button management values
  button.begin();

  // link the Press function to be called on a button Press event.
  button.attachPress(btnPress);

    // initialize serial communications at 9600 bps:
  Serial.begin(9600);
  Serial.println( F("Hello") );

} // setup


//*********************************************
// main code -  to run repeatedly:
//*********************************************
void loop() {
  button.check(); // keep watching the push button:

  sensorValue = analogRead(sensorPin);

  updateDisplay();

  time_out_relay();

} // loop

//*********************************************
void time_out_relay() {
  if(millis() - relay_millis >=  0)  {
     turn_off_relay();
  }
}

//*********************************************
void turn_on_relay(){
  digitalWrite(ledPin, HIGH);
  Serial.println( F("Relay ON") );
}

//*********************************************
void turn_off_relay(){
  digitalWrite(ledPin, LOW);
  Serial.println( F("Relay OFF") );
}

//*********************************************
// this function will be called when the button is pressed
//*********************************************

void btnPress() {
  Serial.print("Off in ");
  Serial.print(sensorValue);
  Serial.print(" seconds");

  relay_millis = millis() + (sensorValue * 1000);
  turn_on_relay();
} // btnPress


//*********************************************
// update display
//*********************************************
void updateDisplay() {
 static unsigned long last_millis = 0;

 if(millis() - last_millis >  1000)  { //update once every second
     last_millis = millis();            //ready for next update
     Serial.println(sensorValue);
 }
}

// End