Circuit Board
This is a photograph of the Cmos 4011 based
Fire-Door Alarm. This circuit will let you know when something that should be kept closed - has in fact been left open. Attach it to a Fire-Door and it will allow you to pass back and forth without reacting. However - if the door is open for more than 30-seconds or so - the warning buzzer will sound. The circuit has an extremely low standby current - and it's ideal for battery operation.