As stated earlier in the thread, the TS wants the alarm to sound if either door is open for more than about 30 seconds. He doesn't want the buzzer to annoy him every time he gets something, only when he fails to close the door after a reasonable amount of time.Quick question: If the alarm is meant to sound when both doors are open, what happens if just one door is slightly open while the other is closed?
And this: If you have a buzzer (I have one on my freezer single door), why not use a micro switch with a Common, a Normally Open and a Normally closed set of contacts? Wire them in series (for two doors) and directly power your buzzer. No need for transistors or resistors or oscillators. I'll bang out a diagram and post it shortly.