The system contains a door sensor, a window sensor, and a key activation switch. If the switch is on, opening the window will set off the alarm immediately. Opening the door will start a 30 second timer. If the key switch is not turned off within the 30 seconds, the alarm is set off. The door sensor and window sensor output a low when they are opened. The 30 second timer requires a high to activate and outputs a high when the timer expires. The key switch outputs a high when it is activated and a low when it is de-activated. In your circuit schematic, represent the timer as a box labeled "30 s timer"