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Hello World

  1. Nov 1, 2008 #1
    I just purchased a breadboard and components because I love tinkering with things. It's not for a class or anything more of a hobby, but I just needed some advice on a simple hello world like blinking led circuit. I have gotten down the basics of lighting up leds, and my second project I wanted to create a flashing Led. However all that happens in my circuit is the Led lights up. I am using a 556 timer Ic. Could you please take a look at these videos and tell me whats wrong with my circuit.

    http://s93.photobucket.com/albums/l75/Bouzy_Bouzy/?action=view&current=P1010110.flv

    http://s93.photobucket.com/albums/l75/Bouzy_Bouzy/?action=view&current=P1010111.flv

    The top resistor is a 470 ohm .5 watt and the bottom one is a 220k ohm .5 watt. I tried 2 different capacitors. ONe is a 10uF and one is a .1uF. They both have the same effect (I don't really understand how to figure out what capacitor I need for the circuit but these seem like they should work.)
     
  2. jcsd
  3. Nov 1, 2008 #2

    Redbelly98

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    Welcome to PF!

    Mostly, I'm guessing the LED is blinking on and off too quickly for you to notice. But offhand, I would expect the 10 uF and 220k combination to work, since RC = 2 seconds in that case.

    Also, there should be 2 resistors in the circuit. Do you have a datasheet for the 556? That should show you how to set up the circuit, and how to calculate on & off times for a given choice of resistors and capacitor.
     
  4. Nov 1, 2008 #3

    dlgoff

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    Just a note here. The 556 is a dual timer (two 555s in one chip). There's also a quad timer; the 558.
     
  5. Nov 1, 2008 #4
    Thanks for the welcome. Actually, there are two resistors there. I don't know if you missed it or what, but there is a 220k ohm and a 470 ohm resistor. Actually, I followed the data sheet for the timer. I am positive the way I have it set up should work which is why it is just so frustrating. Any ideas?
     
  6. Nov 1, 2008 #5

    Redbelly98

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    Try replacing the 470 ohms with a 100k or another 220k resistor, and use the 10 uF capacaitor.

    I am guessing that with the 470 ohms, the LED turns off for only a few milliseconds before turning back on.

    From http://en.wikipedia.org/wiki/555_timer_IC#Usage :

    The timer pulse is high for a time equal to
    0.693 (R1 + R2) C

    And the timer pulse is low for a time equal to
    0.693 R2 C

    For R2 = 470 Ω and C = 10uF, the low time (off time?) is
    0.693 * 470 * 10 * 10-6 sec
    = 0.003 sec

    With a 100kΩ, it will be 0.7 seconds which is noticeable. With 220 kΩ, it's 1.5 seconds.

    Good luck! If it still doesn't work post back and we'll brainstorm some more.
     
  7. Nov 1, 2008 #6
    I pretty much put the whole idea of my calculations being wrong (with capacitor and resistors) by using a 470uF capacitor. The Led still light up like a Christmas tree. So, it has to be something wrong with how it's wired. I have gone over the circuit diagram a ton and I can't find what I did wrong.
     
  8. Nov 2, 2008 #7
    Ok, I remade the whole circuit based on this.. not quite the same function but it still should go on and off. I wired it exactly like in the picture and now when I give the board power the LED lights up for a second then goes out and won't come back on no matter how long I leave it on. But when I tap the batter on and off the led blinks once in a while every time a tap it against the leads.
     
  9. Nov 2, 2008 #8

    Redbelly98

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    Are you sure it's wired for astable (2 resistors) mode and not monostable (just 1 resistor) mode?

    If it is in astable mode, can you show how you calculate the on and off times? (Using your resistor and capacitor values)
     
  10. Nov 4, 2008 #9
    It Works!! thanks a ton! you made me figure out my calculations and now I understand it better! You rock.
     
  11. Nov 5, 2008 #10

    Redbelly98

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    Cool! There's nothing like building something and then figuring out how to make it work. That's one reason why this site is here--though it doesn't always work out with everybody all the time.
     
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