555 Blink circuit

Discussion in 'The Projects Forum' started by electronicsnewbie, Aug 9, 2012.

  1. electronicsnewbie

    Thread Starter New Member

    Dec 7, 2009
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    I built this 555 LED blinker circuit shown here , which is nothing out of the ordinary. The circuit appears to work almost as it should except that the LED never goes off properly. Instead of blinking on off it goes bright them dim. The LED is quite bright even in the "off" state it just get brighter on the "on" stage. Any suggestions why the LED won't go off completely ? What could I have done wrong ?

    Thanks,
     
  2. #12

    Expert

    Nov 30, 2010
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    After examining the datasheet, I have to conclude that you made an error in assembling it because the schematic is correct.
     
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  3. electronicsnewbie

    Thread Starter New Member

    Dec 7, 2009
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    That's what I thought, and what's the most likeley cuase. I can't see any errors. It does oscillate as it should, it's just pin 3 never goes low. The only thing connect to pin 3 is a resistor and led which run from V+ to pin 3. BTW I'm using NE555 if that makes any difference ?
     
  4. #12

    Expert

    Nov 30, 2010
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    Odd, the schematic you showed has the resistor and LED from pin 3 to ground...not V+


    Change the end of your LED circuit to ground and see what happens.
     
    Last edited: Aug 10, 2012
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  5. electronicsnewbie

    Thread Starter New Member

    Dec 7, 2009
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    Good point! I forgot I changed it round (to see if it worked the other way) . It still has the same effect even that way. Thanks,
     
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I agree with #12 that the schematic looks ok, but you might want to compare your assembly to one that I built that works.
     
    Last edited: Aug 10, 2012
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  7. wayneh

    Expert

    Sep 9, 2010
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    Use a DMM to watch the voltage on pin 3 when no LED and resistor are attached. You'll see it does not swing fully from rail to rail.

    But it's unlikely to be "leaking" enough current to light an LED when it should be off. You've got something wrong. Maybe time for a picture?
     
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  8. absf

    Senior Member

    Dec 29, 2010
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    I breadboard the exact circuit as pointed by the OP and it switches off clean and solid. The duty cycle is about 63% and about 0.5 Hz (1 cycle every 2 seconds).

    I tried to reverse the polarity of the 1uF cap at pin 2 but the LED would just stayed on and never turn OFF. So my conclusion is the same as #12 and tracecom that either the connection is wrong OR it could be the NE555 is bad. Could be due to one side of the output transistors in 555 being leaky as suspected by wayneh :rolleyes:

    Allen
     
    Last edited: Aug 11, 2012
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  9. #12

    Expert

    Nov 30, 2010
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    Yes, electrolytic capacitors leak like crazy if you put them in backwards.
     
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  10. electronicsnewbie

    Thread Starter New Member

    Dec 7, 2009
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    I finally got it to work properly by changing the 555 out for another one! Woo hoo :)
    I'm never buying IC's off ebay again !

    Thank you all for your help,
     
  11. #12

    Expert

    Nov 30, 2010
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    blankety-blank counterfit chips, etc. :D
     
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