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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)” |
March 6th, 2010
Wouldn’t a savvy burgler quickly close the door behind himself to avoid alerting neighbors anyway?
March 6th, 2010
Hi Scott,
I don’t think it would work well as a burglar alarm but rather a reminder to secure your open door.
March 6th, 2010
[...] via hackedgadgets Tags: alarm diy, Alarm-System, door alarm, home alarm system, how-to, make, time-delayed Categories: Circuits, DIY, Featured DIYs, HOWTO, Hack, Home, Microcontroller, Projects. [...]
March 26th, 2010
Obviously, where and when the alarm sounds is a personal preference situation, or a cats preferences, in this case but for my purposes I hacked a door alarm by placing the alarm itself remotely from the magnets as I wanted to know when sleeping, whether there was a break in or not, and the remote alarm, mounted a few feet from my head, does the job of waking me with “alarming” efficiency !
While I used a simple wired alarm, all of the standard wireless control centers will do the same thing — provide door and window contacts and sound the alarm wherever the center is located !
April 2nd, 2010
[...] Courtesy of Homebrew Tech via Hacked Gadgets [...]
June 16th, 2010
Great Project! I plan on making two for an industrial fridge and freezer to remind employees to close the door. How loud is it? And is there any way to make it wait 1 min. before going off?
thanks in advance for your answers
June 16th, 2010
Hi Shaz,
The RC time constant of the circuit could be changed to allow for any delay desired. The volume of the buzzer will depend on the one purchased. There is a link to the original project page in the article about (it was only featured here).
June 17th, 2010
Sorry for bothering you, but how would I change the RC time constant to go off after 1 minute?
June 17th, 2010
Hi Shaz,
You can look up the calculations on this page.
http://en.wikipedia.org/wiki/555_timer_IC
June 19th, 2010
This is a great idea! It just proves again that cats are notorious for being able to inspire people to do great things.
As for the 30 second delay, I think that might be a little long, cats are fast little creatures, that 30 seconds could find Fluffy up a tree.
Josh