Door Open Too Long

Discussion in 'The Projects Forum' started by mws7497, Apr 17, 2015.

  1. mws7497

    Thread Starter New Member

    Apr 17, 2015
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    Hi all,

    I'm a complete newbie to electronics. I have recently taken over running a community centre, which daily has children and vulnerable adults in the centre daily. We have a problem with people either leaving the main door open or nursery staff standing with the door open whilst they talk to parents.

    I built a simple battery operated buzzer with a NC magnetic switch. People are ignoring it so I would like to create a timer circuit that would, after a period of time, say 15 seconds, set off a more annoying alarm.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    NC implies that the switch is closed when the door is closed????

    How does that actuate the buzzer?

    What powers the buzzer?
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    If NC means that the switch is normally closed when there is no magnet and therefore the switch is open when the door is closed then that is preferred.
    In this case, you can use the switch to power the circuit which will conserve battery power.

    In either case, if you use a microcontroller, the code can be designed to consume minimum power (less than 1μA) when the alarm is not active.
     
  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    you would be better off educating your staff and parents, not to stand in the doorway and to keep the doors closed, as well as the annoying siren.
     
    Last edited: Apr 17, 2015
  5. Reloadron

    Active Member

    Jan 15, 2015
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    For the less than savvy electronics type I would suggest a "Delay on Make" type timer. As can be seen in the link there are literally dozens to choose from. The timer could be triggered by a magnetic reed basic off the shelf door switch and powered by a simple wall wart power supply. The audible alarm can be any number of common buzzers or pizeo audible devices. Once triggered by an open door the timer begins a time out cycle, if the door is not closed before the preset timeout the alarm sounds. Everything is pretty much simple turnkey requiring very little electronics knowledge. Door switches can be had as SPDT (Single Pole Double Throw) types offering normally open and normally closed contacts with a common.

    I also agree the first step is educating the staff that a doorway is not a meeting place for discussion with clients, as my mother would tell us as kids... "In or Out"!

    Ron
     
  6. MrChips

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    Oct 2, 2009
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    I built an alarm for the exact same purpose (to keep a basement door closed when small children are around).
    It uses a microcontroller driving a piezoelectric transducer and powered by a rechargeable battery. It lasted for years without recharge.

    You can do the same with any simple micropowered mcu available today.
     
  7. mws7497

    Thread Starter New Member

    Apr 17, 2015
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    Thanks for the pointers guys. Unfortunately Dodgy dave, they're not my staff. The relationships at the centre are complicated.
     
  8. MikeML

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    You didn't answer the questions in post #2?
     
  9. mws7497

    Thread Starter New Member

    Apr 17, 2015
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    Post 3 answered it
     
  10. MikeML

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    Didn't answer what powers the buzzer, or a new question: How much current does the buzzer require?
     
  11. mws7497

    Thread Starter New Member

    Apr 17, 2015
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    Sorry. I didn't specify 9v when I originally said battery. The buzzer operates between 6 and 12v
     
  12. MikeML

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    Is it a two-wire piezo-buzzer that sounds with no additional electronics?
     
  13. mws7497

    Thread Starter New Member

    Apr 17, 2015
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    Yes, it's either on or off. As we have a number of vulnerable children using the centre, I need an buzzer that sounds on opening whilst people enter or leave, but I was hoping to make something that would, after a short delay, become more annoying. My mum also used to say, "in or out, not both" as well. I might make a bright sign for the door as well.
     
  14. MikeML

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    Oct 2, 2009
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    Here is a simple circuit that delays the turn-on of the sounder, and resets quickly for the next cycle. Almost any NFET will work for M1.


    The blue trace represents the current through the sounder, showing it turns on at about 22 sec after the switch closes...

    30c.gif

    The delay is proportional to R1*C1. It is dependent on the threshold voltage of the NFET, so you might have to change C1 +- 50% to get your desired delay.
     
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  15. MikeML

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    Here is an alternate version using a 555 chip in lieu of the NFET. Similar principle as before. Timing is more predictable.

    30s.gif
     
  16. Joel Vasquez

    New Member

    Oct 17, 2014
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    I would say a 555 timer cased set up. You could even sub the resistor with a variable resistor to adjust the time.
     
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