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555 flasher isnt flashing

Discussion in 'LEDs and Optoelectronics' started by messy, Aug 12, 2018.

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  1. messy

    messy

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    Jan 12, 2017
    I made a post a long time ago about 555 timers and I considered resurrecting that post as this question is basically the next step along but as the questions are different i thought it'd be tidyer to make a new post.

    I'm trying to teach myself about 555 timers and have read through all the basic tutorials I can found. Having done that I created the attached circuit on a breadboard to check I understood what I had read (apologies for the mess, I couldn't rearrange the chip pins on the drawing tool).


    I was expecting the LED to flash at a frequency of about 7Hz but what I actually get is a steady on LED. I thought maybe it was just blinking too fast too see so I swapped R2 from a 10k to 100k hoping this would reduce the freq to a little under 1Hz but still a steady on LED.

    How I thought this would word - when the output is high C1 would charge and once it hit its threshold voltage the output would switch to low and connect the discharge pin to ground, discharging C1 until it hits the trigger voltage - setting output high, disconnecting discharge pin and charging C1 again.

    Since the LED is contestant on that must mean that for some reason either C1 never reaches its threshold or for some reason the chip doesn't realize its reached it?.....right?


    Have I done something wrong or missed something?
     

    Attached Files:

  2. messy

    messy

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    Jan 12, 2017
    also just noticed the info by the led - mine isnt a LTL-307ee, its just the only LED icon it seemed to have
     
  3. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Connect the supply -ve to ground
     
  4. messy

    messy

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    Jan 12, 2017
    sorry, not sure what you mean....connect the supply pin on the chip to ground?
     
  5. WHONOES

    WHONOES

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    May 20, 2017
    If the diagram is straight from your simulator, then the negative end of V1, the 9V supply, needs to be connected to ground as is the cathode of D1 and the end of C1, otherwise it simply won't work.
     
  6. messy

    messy

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    Jan 12, 2017
    aah, apologies for my crappy drawing - thats just meant to be the +V, the ground line is at the bottom (Ill just draw the diagram in paint next time).

    Grasping at straws here but could it be an issue with the polarity of the LED? (I'm at work and cant test till I get home).
     
  7. WHONOES

    WHONOES

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    May 20, 2017
    Yes, it could easily be a back to front LED.
     
  8. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Show us your physical layout.

    Most beginners invariably make a simple error with component placement.......
     
  9. messy

    messy

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    Jan 12, 2017
    I turned the LED around and it didn't light at all (as id expect tbh)

    I wont lie, I pulled it apart and re-did it a bunch of times when making it, particularly around R2 (100k).
     

    Attached Files:

  10. kellys_eye

    kellys_eye

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    Jun 25, 2010
    The 555 output is pin 3 - your images show pin 3 connected to +Vcc and pin 4 as the output.......
     
  11. messy

    messy

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    Jan 12, 2017
    I think that must just be the picture angle, pin 4 is a short black wire and is connecting to +V
    Pin 3 is the very long green wire connecting to the beige resister which in turn connects to led - ground
     
  12. WHONOES

    WHONOES

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    May 20, 2017
    Are the outer power line strips continuous? The breaks in the red and blue line seems to suggest they are not. If not, that could be you problem.
     
    (*steve*) likes this.
  13. messy

    messy

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    Jan 12, 2017
    the power lines are indeed continuous, the led is getting power and is steady on (about 2.5 volts i think? will check again when i get home).

    The power lines are separate from the central component section and run the length of the board.
    The central tracks run in lines across the width of the board with a break down the very center. - You can see the chip straddles the break.
     
  14. WHONOES

    WHONOES

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    May 20, 2017
    Tried you circuit in my simulator and it works fine at a shade under 7Hz.
    What voltage level is pin 3 on the 555 stuck at? Also check that you have the correct voltages on the supply, ground and reset pins, particularly the reset as if it is floating or connected to ground, it will not work.
     
  15. messy

    messy

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    Jan 12, 2017
    when i measure across the led legs i get 2.5

    2.55 on pin 3 output
    4.29 on pin 4 reset
    2.57 on pin 1 ground
    2.55 on pin 8 +V
    4.29 on pin 2 trigger
    4.29 on pin 6 thresh
    9 on pin 7 dischrge

    0.11 across the led resitor
    0.11 across R1
    4.68 across r2
    4.3 across the cap

    all values are steady
     
    Last edited: Aug 14, 2018
  16. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Check your timing component values. You are using a 100k pot where your schematic shows 10k and the 1k resistor value may be suspect (I can't see the colours properly). What about the timing capacitor? Correct value? Correct polarity?
     
  17. Alec_t

    Alec_t

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    Jul 7, 2015
    Pin 1should be 0V and pins 4 and 8 should be 9V (assuming you are using a 9V supply).
     
  18. messy

    messy

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    Jan 12, 2017
    the 10k was switched to 100k to lower the frequency. I believe with a 100k it should now be about 1Hz?
    I am using a 9v battery
    The capacitor is def 10uF and polarity is correct

    I re-checked the values and they are the same however I may be using the multimeter wrong.
    When reading the value of the pins im holding the red (V) probe to the chip pin and the black gnd probe to the negative terminal of the battery - is that correct?

    The multimeter dial is set to DCV 20
     
  19. messy

    messy

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    Jan 12, 2017
    i removed the beige led resistor since it didn't seem to be doing much, v over the led went up .03 as you'd expect,

    i read the resistance of the 100k at max resistance and with the MM set to 200k i was getting a reading of 44 for some reason.

    swapped it back for the original 10k which is reading 10 on 20k setting which seems better. Hasnt changed anything tho, output is the same.





    it must be something im doing (or not doing) as I have another circuit on breadboard for a basic atari punk console (synth generator) and thats doing exactly the same - its giving a continual tone rather than an on/off square wave.

    I actually did the led one in an attempt to figure out what the problem was :p
     
    Last edited: Aug 14, 2018
  20. Alec_t

    Alec_t

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    Jul 7, 2015
    Yes, providing battery negative is actually connected to your circuit correctly.
     
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