How do I install a monostable 555 timer IC on this circuit?

Discussion in 'General Electronics Chat' started by seanbein, Mar 16, 2010.

  1. seanbein

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  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    This only works as a SHADOW detector, not as a motion detector. So you just have to put your finger (or a card) on the photo-diode to see it works. As long as your finger is there, it will be active, when it sees enough light, inactive.
     
  3. seanbein

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    Mar 16, 2010
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    I would like to know whether it is possible to use a 555 timer because when there is difference in light intensity of the photodiodes (i wave my hand across horizontally) the buzzer sounds but only for a short period.What i am trying to do is to trigger the buzzer to sound longer.Do you think it is possible?I need help to connect the 555 to the given circuit
     
  4. KMoffett

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    Dec 19, 2007
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    You might try increasing the values of C3 and/or R4, since this is supposed to function somewhat as a monostable to prolong the buzzer.

    Ken
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    I like KMoffets idea, but if you feel you have to add a 555 Monostable try this...

    The 555 Projects

    555 Monostable

    I really like this little chippie, but sometimes I think it is a situation where if the only tool you have is a hammer everything starts looking like a nail. :D
     
  6. seanbein

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    Mar 16, 2010
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    Thank you all..Kmoffet do you have the calculation for the c3 and r4 to calculate the duration of time where the buzzer turns on?
    I find this circuit interesting too=D
     
  7. KMoffett

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    Dec 19, 2007
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    Benching your beeper circuit: D3 and everything to the right, using a 2N7000, and triggering with a 50mS/9V pulse to D3, I get a ~1.5 second beep, with a 1/4 second tone trail-off. With a 2.2uF cap for C3 I get a ~3 second beep with a 1/2 second tone trail-off. So doubling C3 doubles the beep length. Doubling R4 would also double the beep length. Increasing R4 would also lengthen the trail-off. Doubling C3 and halving R4 would keep the same beep length but reduce the tone trail-off. If you don't want the trail-off, a 555 monostable would be a better solution. How long do you want the beep to last?

    Ken
     
  8. seanbein

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    Mar 16, 2010
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    I want it to last about 15 seconds but I'd be grateful if I can find out about the calculations and formulas.
     
  9. KMoffett

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    Google RC Discharge caluclation

    First you would need to know know the voltage that the capacitor charges to when pulsed. In that schematic it would be ~9V (Vdd).
    Next you would need to know the exact gate voltage at which the MOSFET just stops conducting, Vgs(th)...0.8v to 3v for a 2N7000. Quite a wide range spec.
    The time from turn-on to turn-off is the time for the capacitor to discharge from 9v to Vgs(th).

    The formula to discharge the capacitor from Vo (9V) to Vgs(th) is:
    Vgs(th) = Vo*e^[-t/RC]...where e is 2.718, t is in seconds, R is in Ohms and C is in Farads


    http://www.welwyn-tt.com/products/resistors/calculation-tools.asp Click in Capacitor Discharge Calculator.

    With all the wide tolerances in Vgs(th), capacitors, and resistors you will wind up experimenting with component values. With 1M, 10μF, and a 2N7000 I get ~15s

    Ken
     
  10. seanbein

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    Mar 16, 2010
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    Thank you Ken,you have helped a lot. Since you have already assembled the circuit, I would like to ask if you know how to install a basic dark detector circuit whereby during night an led or a bulb turns on in the circuit?Iwant to add on that feature
     
  11. KMoffett

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    Dec 19, 2007
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    All I bench tested was the the circuit past D3. What's your over all goal...in detail?

    Ken
     
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