When you turn on the puzzle players see 7 buttons with a color illumination. From the left 3 green, with the right 3 red, and in the middle the button without illumination. When you press the button with the backlight, if next to or through one button from it there is a button without illumination, then it lights up, and just pressed the button goes out. With the help of logic, players need to swap the green and red button illumination. 

 When activated, the electromagnetic lock is turned off, but there may be another action in your escape room.