Circuit Help

Discussion in 'Homework Help' started by cjnixon, Dec 27, 2009.

  1. cjnixon

    Thread Starter New Member

    Dec 27, 2009
    2
    0
    Hi, I am doing a product design course and do not cover any electronics so even though I think this is quite a basic circuit I cannot be sure its correct.

    I basically need to create a circuit with a tilt switch, once the tilt switch gets activated it needs to have a delay of 10 seconds before it sets off a siren. If the tilt switch gets reset it will then need to start again from 0.
    Once the circuit has be activated and the siren is on there should be no way to stop the siren apart from disconnecting the battery.

    Is this circuit correct and give me what I want?

    Using T = C x R
    To give 10 sec = 10uf x 1m ohm.
    The relay will be connected to the Siren to set it off

    [​IMG]
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Where's your resistor?

    I don't see one in your circuit.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    A resistor of 1M will not give enough current to the base of the transistor.
    The collector current is depending on the base current and the relays will not attract with a to low collector current.

    Greetings,
    Bertus
     
  4. cjnixon

    Thread Starter New Member

    Dec 27, 2009
    2
    0
    The resistor is on the left, just below the switch. I haven't drawn it very well, sorry.

    If I were to change the resistor to something lower, e.g. 100k Ω and up the capacitor to 100uf would that work? or do I have to go about sorting this problem a different way?
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    It is common practice to have a base current of Ic/10 for the transistor to be in saturation for switching.
    You will have to know the current of the relays to calculate it.
    For a timer better use a NE555 or some other timer, as the current setup will not be accurate.

    Greetings,
    Bertus
     
  6. whatsthatsmell

    Active Member

    Oct 9, 2009
    102
    4
    And, I believe, the transistor will have to be a silicon controlled rectifier (SCR) to latch the circuit and keep the siren going until power is disconnected.
     
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