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Reed switch door alarm help

Discussion in 'General Electronics Discussion' started by student786786, Nov 9, 2017.

  1. student786786

    student786786

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    Nov 9, 2017
    i am currently doing an electronics project which has a reed switch which activates a door alarm for 4 minutes. However, i would like the owner using the alarm 30 seconds to leave the room. i have used a 555 timer monostable to do this and have used a 555 timer monostable to create a 4 minute alarm when the reed switch is broken.

    Can anybody help me link the two systems together using a not gate etc.

    when the first switch is pressed in my first 555 timer to enable the 30 second turn on delay,onlt then do i want the second 555 timer, which has the reed switch to work. when the reed switch is broken a buzzer or led will sound or light up.

    i am using 9v supply.
     
  2. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Use the button triggered 555' output to hold the reset pin low on the reed switch 555. You'll need an NPN transistor to invert the logic to the reset pin though.

    Chris
     
  3. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Something like this. The resistor values are not critical.

    Chris
    upload_2017-11-9_17-3-50.png
     
  4. 73's de Edd

    73's de Edd

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    Sir student786786 . . . . . (Class of 2025)

    With due initial consideration that you just might already be utilizing the umbiquitous 555 timer in a chip.

    Why not use this initially timed power disruption for accomplishing that feat.
    Adjust with a lower value of either Rx or Cx to shorten the time to your ~30 second off time need.

    CONSIDERING . . . . . . the simpler solution might actually just be his shouting out . . . . .
    " IT'S ME LEAVING "
    ( as ole Daddy Warbucks is leaving for the day . . . . . . at . . . . . . . 2PM.)

    TaaaaaaDaaaaaaa . . . . . ( pencil sticks drum roll )

    alarm delay ckt.png


    ooooooooooopppppsie #1 . . . . . . looks like, after framing in I forgot to pull the ground symbol back down and place on extreme bottom of the line extending down from 555 pin 1.


    73's de Edd
     
    Last edited: Nov 9, 2017
  5. ChosunOne

    ChosunOne

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    Jun 20, 2010
    So the owner/user has 30 seconds to leave the room after arming/turning on the alarm.

    Out of curiosity, how is the owner/user going to come back into the room without triggering the alarm?
     
  6. 73's de Edd

    73's de Edd

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    I was interpreting the situation as that merely being a front door alarm that would let people be working in the back catacombs and accomplishing great and wonderful things.
    If a customer opens the front door, it thereby alerts the need of the holder of the short straw to go up front to wait on that incoming customer.
     
  7. student786786

    student786786

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    Nov 9, 2017
    I have tried using the diagram that Chris sent, which required using the Npn resistor to the rest on the reed switch 555. However, no power goes goes too the second 555 timer. Please can you help me. Is that the only link from the first 555 timer to the second 555 timer. I have attached the two circuits, any help would be vital thank you. Would they both have to connect to the 9v rail and the 0v rail?

    The values of the resistors and capictors for the delay are wrong and I know the correct values, All I want to know is how the two systems can connect together.
     

    Attached Files:

  8. student786786

    student786786

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    Nov 9, 2017
    Thank you Ed I will keep that in mind next Time, it has got me thinking.
     
  9. student786786

    student786786

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    Nov 9, 2017
    Reed Switch alarm
     

    Attached Files:

  10. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    For this discussion:
    Exit 555 = U1
    Entrance 555 = U2

    When I drew that circuit I assumed that U1 & U2 share the same Battery. IE: Vcc (9V rail) and GND (0V rail).

    I suspect that you neglected to disconnect U2's Reset pin from the 9V rail. This must be done before tying the 2N2222's (Q1) Collector to U2's Reset pin. If you didn't bad things will happen. When Q1 turns on it will short the 9V rail to GND! You very well may have fried Q1!

    Chris
     
  11. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Here's a complete schematic. The component ID numbers won't match yours but the values should be correct. To save a bit of power I placed the exit delay LED in series with Q1's base. I also increased some R values from what you had to save some more battery life. I kept your RC time constant values the same though.

    Chris

    upload_2017-11-10_18-49-23.png
     
  12. student786786

    student786786

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    Nov 9, 2017
    Thank you so much, I really appreciate it.
     
  13. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    You're welcome. Any questions just ask.

    Chris
     
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