Hacked Gadgets Forum

March 5, 2010

Time Delay Door Alarm Project

at 1:57 pm. Filed under DIY Hacks, Electronic Hacks

door_alarm_project


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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11 Responses to “Time Delay Door Alarm Project”

  1. MadScott Says:

    Wouldn’t a savvy burgler quickly close the door behind himself to avoid alerting neighbors anyway?

  2. Alan Parekh Says:

    Hi Scott,

    I don’t think it would work well as a burglar alarm but rather a reminder to secure your open door.

  3. Alarm DIY – How to Make a Time-Delayed Door Alarm! Says:

    […] 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. […]

  4. Michael Parker Says:

    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 !

  5. SCADA, Telematics & GPS Technologies » Blog Archive » Time delay door alarm Says:

    […] Courtesy of Homebrew Tech via Hacked Gadgets […]

  6. Shaz Says:

    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

  7. Alan Parekh Says:

    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).

  8. Shaz Says:

    Sorry for bothering you, but how would I change the RC time constant to go off after 1 minute?

  9. Alan Parekh Says:

    Hi Shaz,

    You can look up the calculations on this page.
    http://en.wikipedia.org/wiki/555_timer_IC

  10. Josh Says:

    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

  11. John Lam Says:

    neat idea… would it be possible to have a LED light go on (possibly flashing)after 30 seconds..THEN after another 30 seconds have the alarm go off?

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