TLC555 2 Led Flasher issue

Discussion in 'General Electronics Chat' started by ShockBoy, Oct 27, 2009.

  1. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    Hi All. This is my First post here, I Love this site. Very full of great information. Anyway, I am using a tlc555 ic to flash 2 LED's 5mm. Here is the issue. One led works great, the other flashes, though does not go off.
    When I remove one LED the other (that was continuously on and kinda blinking) blinks normally. To me this seems like..well I don't know.. Thanks All for any info.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    It would help a great deal to post a schematic (.png format preferred) of the exact circuit you're using.

    The TLC555 is a CMOS 555 timer. It can sink about 10 times as much current as it can source. This may be why you are noticing such a difference in LED intensity.
     
  3. ELECTRONERD

    Senior Member

    May 26, 2009
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    Welcome to AAC ShockBoy!

    Could you please post a schematic (preferably in PNG format) so we know what circuit you have?

    Austin
     
  4. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    I'm Having issues with finding an image hosting site, would be nice just to upload here.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    You can!
    Click the "Go Advanced" button below the reply text box, then click the "Manage Attachments" button, navigate to where you have the image saved...
     
  6. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    Cool, uploaded image, but still unsure if it will post... ...
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    It sure did post. Good job!

    [​IMG]

    What are the ratings for the LEDs that you are using? (Typical Vf @ current specification)

    Is it LED2 that is looking dim?

    What values are you using for R1 and R2?
     
  8. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    10 times the current? Do I need to up the resistor value for the positive out? (so to speak).
     
  9. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    Thanks, R1 and R2 are 1/4w 10K and the LED's are straight out of an old pc case. I had 100K resistors in but the flash rate was really slow.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    OK, the 470 Ohm resistors you're using for the LED current limiting resistors are too low in value for the TLC555 timer's source output.

    The basic formula for LED current limiting resistors (Rlimit) is:
    Rlimit >= (Vcc - Vf_LED) / Desired_Current
    where Vf_LED is the forward voltage of the LED at the desired current.
    Right now, we don't have a clue what your LEDs are rated for, but probably somewhere around 25mA or less. However, you're limited by the 10mA that the TLC555 is capable of sourcing when Vcc is 15V. If your Vcc is less, it won't be able to source that much current.

    What voltage are you using for Vcc?
     
  11. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    12v starter battery from my boat. I've got it hooked into a L7812 vr circuit on a breadboard. I can easally put 3, possibly 4 led's into this and have them still light up.
    Led 1 is the one that stay on when led2 is inserted. If led 2 is removed, then led 1 blinks normally.
    Thank You.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Well, if you're using a 12.7v boat battery, then the output from your 7812 regulator will be around 10.7v. At that voltage, your TLC555 will be lucky to be able to source around 8mA current.

    I'm assuming you bought the TLC555 at a local Radio Shack store; they sell them.

    They also sell the LM555 timer. The difference is that the TLC555 is CMOS, meant for low-power circuits and is much more efficient. The LM555 is a bjt (standard bipolar junction transistor) design; it can source/sink much more current than the TLC555 design.

    You could opt to add an NPN transistor and a base resistor to drive D2, or go with an LM555 timer.

    Here's how you could add the NPN transistor and a base resistor to drive D2:

    [​IMG]

    Oh, I changed R1 to 2.2k to make the on/off times of the LEDs more even. You shouldn't go that low with the TLC555. Better to reduce C1 to 1uF, and increase R1 to 10k, R2 to 100k.
     
  13. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    Thanks for all the help. I'll give back if I can.
     
  14. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    That clear's things up, Sgt.Wookie, I'll keep you updated. I will give your solution a shot. I'm new to this, There are 2n's all over the place (net) I've got salvaged parts and none of them are marked 2n...anything... I know npn and pnp but..I don't know.. I got the tlc from an old tape deck. never giving up... I love this bleep.
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    You may be finding a lot of Asian transistors.
    Lots of times they don't put the first two characters of the part number on the part itself.
    For example, a "2SC1815" transistor might just be marked "C1815".
    Try adding a "2S" on the front of the part number and searching on sites like http://Alldatasheet.com or http://datasheetcatalog.com
     
  16. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    Ya, I've got plenty of 1815's. Thank You!
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    The 2SC1815 would work just fine in this application.

    Either of the datasheet search sites I mentioned above should have a datasheet available.
     
  18. Wendy

    Moderator

    Mar 24, 2008
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    Last edited: Oct 28, 2009
  19. ShockBoy

    Thread Starter Active Member

    Oct 27, 2009
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    I will try both ways. Can someone explain to me what happened in my schematic? Why did one LED stay lit when the other LED was inserted? Something to do with the discharge terminal and the low output?
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    Bill,
    ShockBoy is using a TI TLC555 LinMOS timer. When Vcc=15v, it can sink up to 100mA, but source only 10mA. With his Vcc=10.7v (12.7v battery with a 7812 regulator dropping 2v), I estimate that it can only source between 5mA-7mA; not very much.

    ShockBoy,
    You might try swapping the LEDs around. I have an idea that one of them may be shorted.
    When only L1 is in the circuit, it flashes. When L2 is added, L1 glows steady.
    So, remove both L1 and L2, and try using the LED you're using for L2 in L1's position. If it doesn't flash, the LED is shorted.
     
    Last edited: Oct 28, 2009
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