The relay as switch
In this example we use our relay as a switch. The center contact, connects either to the upper or to the lower side. The upper LED is on, when the button is NOT pressed, and the LED at the lower side (you can use a yellow one) is on, when the button is pressed. In all security relevant circuits NO-contacts are used to signal a circuit break in the event of a fault. This can easily be imagined for the red LED. The push button serves as security contact, and when normally ON (by a closed door), the LED downside is ON, for a normale condition. If the door is opened, and the button released, then the upper LED (red) gets ON and indicates a security problem.