555 timer problem a bit different...

Discussion in 'The Projects Forum' started by docscott999, Mar 26, 2013.

  1. docscott999

    Thread Starter New Member

    Jan 28, 2013
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    I have built a 555 timer circuit and it worked great on the experimenter board.
    I had soldered all the components however I forgot to connect lead 1 of the 555 timer to ground before I applied power. When I did this my LED lit up then faded away (I didn't have the motor hooked up at the time, was just testing the circuit with the LED). Now, nothing seems to work and my question is which component is the one that likely failed in this scenario? I really don't want to have to take each component out and test them individually. Do you think the 555 IC would have failed without the ground hooked up or maybe the capacitor?

    Attached is the circuit.
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I don't think that leaving pin 1 floating would damage the 555.
    Repost your schematic, with all component values shown.
     
  3. tracecom

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    Apr 16, 2010
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    Are you aware that you have the LED connected to pin 3 in such a way that pin 3 is sinking current? In other words, the LED will light when pin 3 is low.
     
  4. docscott999

    Thread Starter New Member

    Jan 28, 2013
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    I am by no means an electronics expert but are you referring to the LED being hooked up to the positive side of the circuit? If so, I had it originally hooked up to the negative side but I could not get the on/off times the way I needed them. ( I needed a 1s on and about 12 minoff. With the help of this forum this configuration was suggested and it worked. But do you not feel it is connected correctly?
     
  5. tracecom

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    It will work either using the 555 to source current or to sink current (which is the way you have it.) I just wanted to be sure you had drawn it the way it was built.
     
  6. tracecom

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    I don't think any of the components were damaged. I think the most likely problem is a wiring error. If you post a labeled schematic and pictures of your assembly (front and back), someone will likely spot the problem.
     
    Last edited: Mar 27, 2013
  7. docscott999

    Thread Starter New Member

    Jan 28, 2013
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    Ok I will post the schematic with all the values when I get off work.
     
  8. docscott999

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    Jan 28, 2013
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    ok i have attached photos of the front and back of the circuit as well as the labeled schematic. See anything wired wrong?
     
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  9. Ron H

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    Apr 14, 2005
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    Will it oscillate if you crank the pot down to minimum resistance?
    Even if the wiring is correct, it may not oscillate with 4700uF/220K, because the capacitor leakage may be yoo high.
     
  10. tracecom

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    It's hard to see all the details, but I don't see any obvious wiring errors. It looks like R2 is 330 Ω instead of 390, but that doesn't matter. I can't see where the 4700 μF cap's negative pin is tied to ground, but it may be. It looks like there could be some solder bridges on the bottom; check those. Check the polarity on the LED again; I can't tell from the pictures if it is correct.

    The worst potential problem that I see is that the pads on the IC pins look like they have been really hot. The IC probably should have been socketed; it may be fried. It's going to be a pain to change if you don't find the problem elsewhere.
     
  11. docscott999

    Thread Starter New Member

    Jan 28, 2013
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    Ron, It worked well on my experimenter board so I'm sure the the capacitor is ok.
     
  12. docscott999

    Thread Starter New Member

    Jan 28, 2013
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    I checked for solder bridges with a magnifying glass and dont see any, however I can see how it may look like there is in the picture. The LED polarity is correct i believe (flat side connected to pin 3).
    I have removed the IC and there is some continuity between pins 1 and 8 when tested with my ohm meter. I am assuming, like you mentioned that the IC is bad. I will replace it but socket it this time.
    Is it true there should be no continuity between pins 1 and 8? I get a reading of about 750 ohms between these two pins (out of circuit).
     
  13. tracecom

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    There should be about 13kΩ between pins 1 and 8, at least on the two that I checked.
     
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