555 adjustable blinking LED, dont work?

Discussion in 'The Projects Forum' started by Rendar1970, Jul 1, 2010.

  1. Rendar1970

    Thread Starter New Member

    Jul 1, 2010
    7
    0
    Im pretty new to electronics, but I tried building this circuit below.

    When I power it up the LED turns on, but is solid, no blinking.

    When I turn VR2 the LED dims to off, and back up to full brightness like its suppossed to.

    However this is suppossed to be a variable blinking LED, adjusted by VR1, however VR1 seems to have no effect. No matter how its turned or adjusted the led just stays on. No blinking.

    Is there something wrong with the schematic? Ive tried 2 different pots on VR1, and nothing.

    Really at my wits end.

    [​IMG]
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    There are a couple of mistakes in the schematic.
    1) leds may not be put parallel directly.
    2) there are no current limiting resistors.
    3) the powersupply-voltage is probably to low for white leds.
    (what are the specifications of the leds you use?)
    4) the leds are mounted up-side-down.

    Here I made some adjustments in the schematic.

    [​IMG]

    Bertus
     
  3. Dx3

    Member

    Jun 19, 2010
    87
    7
    The LM555 wants 4.5 volts minimum. (Same as an NE555)
    Pin 5 needs a capacitor to ground...about .1uf or .01uf Somewhere in there.
     
  4. Rendar1970

    Thread Starter New Member

    Jul 1, 2010
    7
    0
    Thanks Bertus,

    I already realized the problems with the LEDS, and have made the changes already. Im using RED LED's btw.


    Do you have any clue as to why the LED's are not blinking at all, and why VR1 seems to have no effect?



    BTW: This is a schematic taken from one of those "Evil Genius" books. Apparently the author is way off on most everything.

    its supposed to be a simple circuit where you have 2 clusers of 3 red leds. VR1 adjust how fast the clusters blink, and VR2 adjusts the brightness of the LED. The 2 cluster of LEDS are placed on sunglasses, to make alpha meditation glasses. I have seen another project like this using a pre-made board on make.com, but I wanted to try this
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    It's .01 on pin 5.
     
  6. Dx3

    Member

    Jun 19, 2010
    87
    7
    Beenthere..thanks. (I was having a senior moment.)

    as 4 clues...give the chip 4.5 to 15 volts.
     
  7. Rendar1970

    Thread Starter New Member

    Jul 1, 2010
    7
    0
    giving the chip 9v now.

    lucky enough to have .01uf cap from another project and made that fix.

    Everything is still the same.

    LED's stay lit, vr2 controls brighting, no blinking vr1 does nothing.....
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    What resistors do you use in series with the leds?
    Try to use 470 Ohms resistors there.
    Use a resistor of 2k2 between pin 3 of the 555 and the base of the transistor in stead of the pot.

    Bertus
     
  9. Rendar1970

    Thread Starter New Member

    Jul 1, 2010
    7
    0
    The book suggested 450 Ohm res for clusters of 3. Which is what I bought.


    Im really thank full for all the help and suggestions, but could someone explain to me what it is that is causing the LEDS to not blink.

    Are they just getting so much power that even when blinked off there still on.

    Cause all these changes and im not seeing any blinking no matter what i do with the pot that changes the blinking.
     
  10. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    They could be blinking so fast, you cannot see the blinking. Thats why it is important to know what value resistors and capacitors you are using. The 555 uses these values to determine the timing.
     
  11. smartweb

    New Member

    Jun 28, 2010
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  12. Wendy

    Moderator

    Mar 24, 2008
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    In addition to what Bertus said, the schematic has several other issues. One is the pots themselves, VR2 will short the output of the 555 if it goes all the way to ground. A CMOS 555 can resist this a bit, but a conventional 555 could be a goner (as in the magic smoke escapes). VR1 isn't as bad, if taken to 0Ω the pulse width will be so narrow as to be invisible.

    If you're interested I'll revise it with how I would do it when I have time to sketch it.

    Check this out...

    LEDs, 555s, Flashers, and Light Chasers

    My Cookbook
     
  13. Rendar1970

    Thread Starter New Member

    Jul 1, 2010
    7
    0
    Thanks for those links.

    I would really appreciate it if you would show me how you would accomplish this circuit.

    Funny thing is, I took it all apart, and remade it for like 5th time on a breadboard and it finally all worked. I was so excited I busted out the perfboard and transfered it.

    Now its sitting on perfboard and the old problem is back. Solid light, dimmable by vr2, however no blinking and vr1 does nothing.

    Im starting to get really frustrated lol.
     
  14. Rendar1970

    Thread Starter New Member

    Jul 1, 2010
    7
    0
    So, im starting to think that possibly the reason its not working is how i wired to transistor.

    That would make sense if the gate is wired wrong, the transistor would be stuck giving full power to the led right?
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    Can you use a CMOS 555? Radio Shack carries the TLC555, so they are available.

    I've come across some neat little cheats that allow you to use a CMOS 555 with 3V to light almost any number of LEDs.

    CMOS 555 Long Duration LED Flyback Flasher

    One of the other problems is making a variable rate blinker with a conventional 555 and a low voltage is it is harder than it sounds. Interestingly, the CMOS 555 doesn't have this problem, since the output goes from full power supply voltage to ground.

    Let's go with your design first...

    Vcc = 4.5VDC (this is the minimum for a conventional 555)
    1Hz rate ≈ 50% flash rate, with variable LED intensity.

    [​IMG]

    I have not built this, I threw it together. It will be variable between 20% to 80% duty cycle centered around 1Hz.

    The CMOS version will let you do this though.

    Vcc = 3V
    ½Hz-7Hz 50% Duty Cycle

    [​IMG]

    Just reread your last post, what is the transistor you are using? The type shown is a BJT (Bipolar Junction Transistor), and they are labeled so...

    [​IMG]
     
    Last edited: Jul 2, 2010
  16. SgtWookie

    Expert

    Jul 17, 2007
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    If C1 is shorted, it would cause pin 3 to stay high all the time.

    If C1 is open, it would oscillate so fast that it would look like it was on all the time.

    I prefer to use larger values for R1/R2 and smaller values for C1.
     
  17. Wendy

    Moderator

    Mar 24, 2008
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    On which one?

    I was going for easy to get values.

    The CMOS version can go much higher resistances. The conventional 555 that is pretty much middle of the road. If the power supply were 6V or more the conventional 555 could use the CMOS circuit, but R4 and R5 would have to be raised in value to compensate.
     
  18. SgtWookie

    Expert

    Jul 17, 2007
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    Bill,
    I was talking about his original schematic. I guess there was a long time from when I started writing my post to when I actually posted it.
     
  19. Wendy

    Moderator

    Mar 24, 2008
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    You know, there ought to be a 555 circuit that can operator at 4.5VDC that can do 50% and change frequency with one pot (keeping the 50% duty cycle). I'll be keeping my eyes open for it, but I don't think I've ever seen one.
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    Well then, have a look at the attached.

    The 750 Ohm resistor on the output to Vcc acts to overcome the limitations of the Darlington follower on the output. Its' value would need to be adjusted depending on what load it was driving.

    C2a and C2b cause the threshold/trigger to start up at roughly 1/3 Vcc, so that the 1st pulse is the same length as all the other pulses - within the limits of the precision of the caps, of course.

    R2 sets the maximum frequency. If omitted or two low of a value, you'd probably wind up burning out the pot.
     
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