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Why do the LED's not flash?

Discussion in 'LEDs and Optoelectronics' started by Wotan, Oct 10, 2011.

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  1. Wotan

    Wotan

    37
    0
    Oct 8, 2011
    Here is the circuit i've made, it's a 2 LED flasher circuit. I have followed this schematic as best as I can but the LED's just light up but don't flash.

    Here's the schematic -

    2 led-flasher-555.gif

    here's a photo of the circuit-

    2011-10-10 13-40-01.750.jpg

    And here's a drawing so of the circuit so you can see where the connections are-

    2011-10-10 14-17-47.734.jpg

    2011-10-10 14-19-15.140.jpg

    Can someone please tell me what i need to change on the circuit to make the LED's flash please?
     
    Last edited: Oct 10, 2011
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,490
    2,832
    Jan 21, 2010
    What values are the resistors (other than the 470 ohm ones)?

    Does one LED seem brighter than the other?

    The frequency may be too high for you to see the flashing.
     
  3. Wotan

    Wotan

    37
    0
    Oct 8, 2011
    The resistor going from 6 to 7 is 1k and the other resistor going from 7 to 8 is a 10k variable resistor that just dims the bulb at the top of the photo a bit at full 10k resistance and makes the bulb brighter at no resistance.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,490
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    OK, the minimum frequency is around 11Hz. That is too fast for you to see, but if you move your head quickly you may notice it (or if you use your peripheral vision.

    Try increasing the 1k resistor to 100k.
     
  5. Wotan

    Wotan

    37
    0
    Oct 8, 2011
    I replaced the 1k resistor with a 100k but it made no difference.
    And when I removed the 100k resistor all together it made no difference. I think I must have connected the circuit wrongly. Can someone please tell me where I have gone wrong?
     
    Last edited: Oct 10, 2011
  6. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    flasher circuit

    Hi there Wotan.
    You need to go over every connection and rails you have put pins in and link wire, its all to easy to double on a track by mistake, this at best will leave the circuit partially working or not at all, and at worst destroy semiconductors like transistors, you might have to pull it apart, but do it slowly and you might come across the fault, are the transistors pins in the correct orientations.

    Its happened to us all at some time, finished, power up and there is a problem, you need to find that fault or problem, some times if its happened to me i spot it at once, other times i am pulling my hair out in frustration, but found it in the end.
    Dave. :)
    PS, Sorry 555 timer, i had a two led transistor flasher on my mind.
     
    Last edited: Oct 10, 2011
  7. Wotan

    Wotan

    37
    0
    Oct 8, 2011
    Can anyone tell me from the photo I took and the picture I drew if it's wired up correctly? It's only a very basic circuit after all.
     
  8. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    I cant as i dont do bread boards and dont know which way the rails are laid out, a fellow bread board prototype member might spot some thing, when i started out on circuits i felt confident enough to solder on copper strip, ive never used a bread board, silly really i suppose.
    Dave.
    PS, If you go over it carefully you will work out whats wrong, and be able to correct it, its part of the learning curve.
     
  9. Wotan

    Wotan

    37
    0
    Oct 8, 2011
    Ok, thanks Dave, I'll keep trying and see if i can work it out.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,490
    2,832
    Jan 21, 2010
    Try taking a larger picture from directly above and ensure its focussed. If we can read the markings on the IC and identify the coloured bands on the resistors then you've got it right.

    Also mark the resistor values on your circuit, and let us know which one is the variable resistor.

    To make things simpler, remove one of the LEDs from the output (there is also a better way to drive 2 LEDs, but we'll worry about that later)
     
  11. daddles

    daddles

    443
    3
    Jun 10, 2011
    Rapidly wave your fingers in front of the LEDs -- it can help you see if they're flashing (causes a stroboscopic effect). You can also punch some holes in a disk and spin it on its axis and look at the LEDs through the holes. If you have a multimeter that can measure frequency, that will also tell you if the circuit is oscillating.
     
  12. TBennettcc

    TBennettcc

    292
    2
    Dec 4, 2010
    ...I don't know about anybody else, but I don't see a connection between ground and pin 1 of your 555 IC...

    Other than that, kudos for getting this far. Just about everything else looks right. Personally, I feel this is going to be one of those solutions that when you find it, you're gonna wanna bang your head against the wall and wonder "Why didn't I see that?"

    Your diagrams look right, save the ground on pin 1. Maybe pull everything off the breadboard and try to wire the circuit again.
     
    Last edited: Oct 11, 2011
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,490
    2,832
    Jan 21, 2010
    That would do it :)

    I found it hard to tell exactly where some of the wires were connected to, but there's clearly nothing connected to pin 1. And the same goes for his wiring diagram.
     
  14. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    LED flasher

    its going to be ahh why didant i spot that, i think thats happened to some of us at one time or another, had a mate chop the wiring out of a circuit once, before id had chance to note its configeration. Got it built back up new wiring in the end, i think he needed to have the patience to look carfully and not rush things. Placed in order and concisely would eliminate mistakes.
    Dave.
     
    Last edited: Oct 11, 2011
  15. Wotan

    Wotan

    37
    0
    Oct 8, 2011
    1 to ground, I missed that, thanks.
     
    Last edited: Oct 11, 2011
  16. Wotan

    Wotan

    37
    0
    Oct 8, 2011
    Ok, so I have put a wire from 1 to ground and put the resistance on the variable resistor from 7 to 8 at 11.8 k ohms and I put a 100 k resistor connected from pin 6 to 7. Now the LEDs flash on and off at about the same speed as a rail crossing. Thanks to everyone for their help.
     
  17. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Good on ya, pleased you got it working, onwards and upwards.
    Dave. ;)
     
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