Help with making a 555 timer circuit

Discussion in 'The Projects Forum' started by paulareno, Sep 6, 2009.

  1. paulareno

    Thread Starter New Member

    Sep 6, 2009
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    Hi, my name is paul and i'm just new here and looking for some help :)

    I need a circuit that when light is detected it will trigger a switch and hold the switch on for about 5 seconds than release it and when the light goes out it must trigger the switch once more.

    So like this

    Light = Switch for 5 seconds than release
    NoLight = switch on than off.

    i figured i'd need a 555 timer but not sure.

    Thanks, Paul
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    How will you detect the light (comparator?)?
     
  3. paulareno

    Thread Starter New Member

    Sep 6, 2009
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    sorry i forgot, it willbe using photocell
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    Which one?
     
  5. paulareno

    Thread Starter New Member

    Sep 6, 2009
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    Sorry i don't follow, arn't photocells all the same?

    Can you suggest what i would need?
     
    Last edited: Sep 6, 2009
  6. mik3

    Senior Member

    Feb 4, 2008
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    No, do you have the part number?
     
  7. logans-electronics

    Member

    Sep 1, 2009
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    The Radio Shack Engineer's 555 Timer Mini-Notebook help me out a lot. i see them on ebay all the time for a few dollars.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    If you're talking a simple photocell (CDS cell) you can use a simple pot to create a voltage that moves as the light level changes, measured with a DVM (voltmeter).

    The catch is you actually have two different functions, if I read your post correctly, which will require two different circuits. One triggers the time when it goes dark, the other triggers the time when it goes light. In other words, want to trigger this circuit if there is a change, correct?

    You can do this by trial and error, it may work or not, if you don't have any datasheets. I think most folks have done this at one time or another.

    Start simple, use a meter to check the voltage swing, which should be very wide, then get back with us. You may be able to use a 555, or if the swing isn't wide enough a comparitor. Either way the circuit is relatively simple.

    [​IMG]

    Figure using a large pot, say 100KΩ or 1MΩ, to do this. Once you have a resistance you can replace it with a fixed resistor, or leave it as is.

    We'll need the numbers, power supply voltage, maximum voltage, minimum voltage, to go from here.

    For what it's worth, I started using the Radio Shack series made by Forrest Mimms III, who has been known to visit once or twice many years ago. I like them myself, and this is pretty simple.
     
  9. paulareno

    Thread Starter New Member

    Sep 6, 2009
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    Hi again.

    I will be working with a 3.7v battery at 280mah, sorry but this is all i will have to work with atm.

    The max voltage would be 4v and the min would proberlly be arround 2v, in the future i will most likely replacing this battery with a more powerfull battery.

    I do not have a pot here with me so i'm not going to use one for now, when there is light i'm getting 1.8v through the photcell and close to dark is 1.5v
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    Hmmm, if you can't buy parts I guess you're going to be out of luck. Resistors are pretty basic stuff not to buy if needed. You didn't give us a parts list at the beginning of this post. No problem though, let us know when you have more parts than a 555 and photocell (but you did realize you were going to need resistors and capacitor for it too, right?).

    Just kidding, but a standard 555 requires a minimum of 4.5V. You might be able to use a CMOS 555, but you're going to have trouble lighting a white LED, unless you want to go for one of my other designs. Qty 4 AAA batteries are pretty small too.

    You're going to have to define what you want a little more carefully.

    Here is some background info I recommend you read...

    LEDs, 555s, Flashers, and Light Chasers

    The 555 Projects

    555 Monostable

    CMOS 555 Long Duration LED Flyback Flasher

    You could try a 100KΩ resistor for the variable and see what it does in terms of the max and minimum voltage.

    With such a small battery, your LED and circuit is rated for hours, not days. A CMOS 555 will extend that, but I don't expect this will be very practical.
     
    Last edited: Sep 6, 2009
  11. paulareno

    Thread Starter New Member

    Sep 6, 2009
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    Hi,

    I have resistors ranging from 1k to 10m and also alot of caps i just don't have a pot lols :)

    also the 555 timer i have is 7555cn i'm pretty sure it's cmos???


    Will post back in a while
     
  12. Wendy

    Moderator

    Mar 24, 2008
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    Yes, you can use this circuit to light several LEDs with such a low voltage reliably. Truth, power supply voltage isn't something you change without a complete redesign though.

    [​IMG]
     
  13. paulareno

    Thread Starter New Member

    Sep 6, 2009
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    0
    Ok,

    I have thought about it and changed things, what i need to do now is very simply (i think)

    I have made the light detector and it sends a signal of 3v when it detects light.

    Because the signal coming from the light detector will be constant while there is light i need a setup where when it detects the signal it will conect two contacts on another circuit for one second and than release.

    I'm not sure if i would use a 555 timer to do this but yeah.

    Thank you for sticking arround and helping, lols i bet your sick of me already :p
     
  14. Wendy

    Moderator

    Mar 24, 2008
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    Naw, it is part of the fun. You learn by doing, but also by asking questions.
     
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