555 timer problem

Discussion in 'The Projects Forum' started by the kidson, Jul 18, 2011.

  1. the kidson

    Thread Starter Member

    May 15, 2011
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    hi all first post in a while...finally got around to some tinkering...my problem is as so...i built a simple 555 alternating led circuit last night and it was working fine...today i attempted to build it again..only to no avail...i cannot get the led to flash...i have a 10k resistor from power to pin7...a100k resistor from pin7 to pin6....i have a capacitor from pin6 to 0v...i have a jumper from pin2 to pin6...and the positive side of the led in the output pin3 to 0v....i think this is how i had it last nite...any ideas??...thanks for your time
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,127
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    One idea: post a schematic. I know you named the pins and maybe the 555 gurus will be able to go from that, but a picture sure is easier. You'll get much better attention here. ;)
     
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Are pin 4 and 8 tied to Vcc?
    Is pin 1 connected to GND?
    You connected output 3 via the LED directly to GND?
    Shouldn't there be a resistor in series? Maybe you blew up your LED/555? :D

    What's your power supply voltage?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Without a current limiting resistor from pin 3 to the LED's anode, you will burn up the LED due to excessive current.

    If you connected the LED backwards, it will become damaged due to exceeding the maximum reverse breakdown voltage, which is usually 5v.

    A standard transistorized 555 timer needs somewhere between 5v and 16v to work properly.
     
  5. the kidson

    Thread Starter Member

    May 15, 2011
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    thanks you all for your responses...i appreciate it...i do have a resistor on the led from the output...i have the circuit laid out on breadboard...i have included a drawing of the circuit to the best of my ability of how it is laid out...sorry noob :)...the values for R1 is 10k...R2 is 100k...R3 is 300 ohms..and the capacitor is 470 uf....my power source is 9v....again thanks for looking....http://imageshack.us/photo/my-images/819/sany3173.jpg/[​IMG]
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Your schematic shows the LED connected backwards, and pin 4 is not connected to +V like it should be. If pin 4 is low, pin 7 will also be low, preventing the capacitor from charging.

    With those values, on power-up the LED will stay on for nearly a minute, then turn off for ~30 seconds, then back on for ~35 seconds, and then repeat off for 30, on for 35, etc.

    It may be that you have exhausted your 9v battery. If you're using a red LED, you'll have about 20mA flowing through it, which will drain a typical alkaline 9v PP3 battery in about 25 hours, and a rechargeable battery in much less.
     
  7. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Connect pin 4 to + as well.

    Your LED has to be inverted.
     
  8. magnet18

    Senior Member

    Dec 22, 2010
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    Also, though it's probably not an issue with this circuit, it's generally a good idea to put a small cap from pin 5 to ground in case of noise.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Try replacing the 470uF cap with a much smaller one; say 10uF to 47uF.

    Also, you really need a capacitor across pins 1 (ground) and 8 (+V). The datasheet calls for one 0.1uF and one 1uF or larger.
     
  10. Wendy

    Moderator

    Mar 24, 2008
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  11. the kidson

    Thread Starter Member

    May 15, 2011
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    thank you Bill...these are appreciated...will make for some interesting reading after this post...to everyone...i placed a jumper from pin 4 to the (+)...nothing...still no flashing...i dont understand...i had it working perfect yesterday with two alternating leds...i took a picture of the circuit as it is on the breadboard...perhaps that will offer more insight...please forgive my lack of knowledge in terminology and schematics...im reading more and more learning...here is the pic of the circuit...again i thank you for your time and patience...the kid

    http://imageshack.us/g/691/sany3175i.jpg/
     
  12. Sparky49

    Active Member

    Jul 16, 2011
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    It looks okay to me - but I think it's quite confusing.

    Try laying it out as per this diagram.

    It's far easier - in this case - to have one positive and one negative rail.

    Also, some people might wrap LED and resistor legs together, but that's something I really hate. Why bother with the breadboard if you're going to wrap components together? Just use the holes.
     
    Last edited: Jul 19, 2011
  13. the kidson

    Thread Starter Member

    May 15, 2011
    32
    0
    thanks for the response Sparky....i built this as specified to your layout with the exception of a couple different resistor values because i didnt have those exact ones..the led flashes once upon power up and thats it..it doesnt do anything after that...i also built the 555 LONG DURATION MINIMUM PARTS RED LED FLASHER too...the led would just slowly grow a bit more dim...but not go out...is it possible that i may have somehow ruined the 555?....thanks
     
  14. Sparky49

    Active Member

    Jul 16, 2011
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    It's possible...

    Try this first.
     
  15. the kidson

    Thread Starter Member

    May 15, 2011
    32
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    thanks sparky...again forgive me...is the drawing you suggest adding a transistor to pin3 then hooking the led to the transistor?...as always...thanks...
     
  16. Sparky49

    Active Member

    Jul 16, 2011
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    Yes, it is.:D

    No problem, remember the only stupid question is the one which isn't asked.
     
  17. the kidson

    Thread Starter Member

    May 15, 2011
    32
    0
    thanks again...im about to hook up the transistor..but i have one more question...do i use the resistors that are already in use?? or do i use new ones and hook to power...output and led..hope im making sense...thanks
     
  18. Sparky49

    Active Member

    Jul 16, 2011
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    R3 on your schematic is the same as the one on my schematic. However, you will need two other resistors for R1 and R2 on my schematic.

    Consider them R4 and R5 respectively from now on.
     
  19. the kidson

    Thread Starter Member

    May 15, 2011
    32
    0
    i hooked it all up and the same result applied...it flashed once then nada...is it most likely the timer is shot??..or maybe even a capacitor?...
     
  20. Sparky49

    Active Member

    Jul 16, 2011
    834
    417
    Try another timer...

    I have a very strong hate/hate relationship with 555s, they hate me and I hate them.

    Haters gonna hate!:p
     
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