Need Help on a Simple Time Delay Circuit

Discussion in 'The Projects Forum' started by dcodner, Nov 21, 2007.

  1. dcodner

    Thread Starter New Member

    Nov 21, 2007
    3
    0
    Hey guys I just found this site. I am having trouble with my Project for school. I have to design a burglar alarm circuit. The requirements are: must not exceed 9volts, alarm must sound 10secs after it is triggered and go off for 10 sec. It must be cheap like under $20-$30 to construct. And no 555 timers may be used.
    So far I have a 9 volt battery, 1 - 1M Ohm Resistor in series with a 10-micro farad Capacitor. My siren is connected across my capacitor. and i have a switch at the positive terminal of the battery ran in series with the resistor. but every time i hit the switch i get no delay. Please help.
    Thanks in adv.,
    D
     
  2. dcodner

    Thread Starter New Member

    Nov 21, 2007
    3
    0
    any ideas would really be appreciated
     
  3. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    Dayum! ;)

    Well the 555 just contains a couple of comparators and a couple of other things which you can replicate. You don't even need all of it. Just replicate what you need.
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Try a Google search on the word "monostable." You will find many circuits which do not require a 555.
     
  5. dcodner

    Thread Starter New Member

    Nov 21, 2007
    3
    0
    How do I replicate a 555 timer? What components do I need and how do i put them together? I have no idea how to out everything together. In class we only learn about theory and impractical applications.

    Also i did a Google search on "monostable" and I got almost all hits about monostable vibrators (have no idea what that is).
     
  6. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    You can find out about monostables at http://en.wikipedia.org/wiki/Multivibrator
     
  7. Voltboy

    Active Member

    Jan 10, 2007
    197
    0
    You could use a high capacitance capacitor and connect it to the negative of the comparator and when it charge it keep the comparator on and keep discharging, remember you must use a voltage divider in the comparator positive input of the comparator. Connect the output of the comparator to a flip flop.
    I'll recommend a 1000uf capacitor with 10k resistor.
    I'm not sure if this will work but you could try
    Click here for the time constant calculator
     
  8. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    OK, let us see. Here's one possible way of doing it:

    Trigger switch closes and triggers a monostable M1 of T=20 sec and another one M2 of T=10 sec. Alarm sounds when M1 is on and M2 is off.

    A monostable can be easily built with a capacitor and a resistor and a comparator.

    Another way of doing it could be with a single capacitor charging and two comparators so that the output is true only between two voltage levels:

    Capacitor starts charging from 0V
    Between 0 and X volts alarm is silent.
    Between X and Y volts alarm sounds
    When voltage reaches Y volts system resets to 0V
     
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