BlackCow from Homebrew Tech has a curious cat who likes to open the front door if it wasn’t properly latched. This Time Delay Door Alarm Project solves that problem by monitoring the door and producing a tone if it has been left open for more than around 30 seconds. This way it isn’t buzzing every time you use the door normally. The schematic is available on his site if you are interested in making your own.
“When the door opens and the magnetic switch is disconnected the pull up resistor (R2) sets the trigger, on the first 555, high and allows the 33uF capacitor (C1) to charge through the 1 meg-ohm resistor (R1). When the charge reaches 2/3 of the supply voltage the output pin on the 555 is set low. The time it takes for the capacitor (C1) to reach 2/3 of the supply voltage is found by this equation, Time(sec) = (1.1) x (Resistance) x (Capacitance)”
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