Explain This LED Flasher Circuit

Discussion in 'General Electronics Chat' started by qbvbsite, Jul 3, 2010.

  1. qbvbsite

    Thread Starter New Member

    Jun 29, 2010
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    Hey,
    Just for kicks I wanted to make a LED flasher circuit to see If I could build one. It was a success and I built the following circuit http://wild-bohemian.com/electronics/flasher.html (Transitor LED Flasher). Now I would like to understand how it works. I know that the 470 resitors are to limit circuit to the LEDs. As for the 100K resitors, transitors, and capacitors I'm lost. I havent got to the transitor/capacitor part of the material on this site but i do know a capacitors stores and releases current and a tranistor acts as a gate (i think at least). Thanks in advance.

    --James
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    This is probably one of the oldest designs out there, they were making tube versions of it when tubes were invented.

    Short explanation (it can be a lot more complicated). Each transistor is an inverter. You have two inverters coupled with caps. When you power up the circuit one side (transistor) is a bit faster than the other and starts to conduct, sending a negative pulse to the other side, turning it off. The circuit is stable at this point until the cap being charged is fully charged, turning the transistor being feed current off. This in turn sends a negative signal to the other transistor and the whole circuit flips, and is again stable until the second cap is fully charged. Repeat ad infinitium.

    This circuit isn't used for critical applications. If the transistors are perfectly balanced neither starts conducting first and the oscillator never starts. This is a rare condition, but it happens.

    http://en.wikipedia.org/wiki/Multivibrator
     
  3. Markd77

    Senior Member

    Sep 7, 2009
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    If you go here and select astable multivibrator from the circuits, transistors menu you should see it working.
    I think you will need to read the transistor section to understand what is going on.
    http://www.falstad.com/circuit/index.html
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    And Bill, do you remember the cross-coupled neon light flashing circuit? (The so-called Tiki lights)
     
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
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  6. Wendy

    Moderator

    Mar 24, 2008
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    Now you mention, I do. I doubt I would have if you hadn't mentioned them though. Are neon bulbs older than tubes I wonder? Both are ancient by electronic standards.

    There was another discussion about just this circuit, I've spent about an hour looking for it, that also discussed this circuit in major depth.

    One of the things I remember reading is long term (years? more?) it will slowly damage the transistors because of a large back bias put on the BE junction. I wouldn't worry about it though.
     
  7. Wendy

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    Mar 24, 2008
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  8. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    And to reduce that back bias you can use diodes on the lower end that are bias to pass negative voltage currents to ground.

    Pass them through a capacitor if you want to allow a few negative volts and create a negative rail (only useful for microamps so better referred to as a psuedo rail).

    You can also use double layers of diodes to protect the transistors and create the negative rail.

    [​IMG]
     
    Last edited: Jul 3, 2010
  9. hgmjr

    Moderator

    Jan 28, 2005
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    The circuit you have linked to is a classic oscillator. There are a number of sites on the Internet that have explanations for how it operates.

    hgmjr
     
  10. qbvbsite

    Thread Starter New Member

    Jun 29, 2010
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    Thanks for all the replys. You guys are pretty awesome, I'll be reading all the links/text provided. I do think I need to read more on the material on this website to fully understand whats going on. It was pretty neat to build a circuit and have it work tho :). Can anyone recommend any good circuit simulation programs? Would make it much easier to prototype/investigate a circuit (a graphical one). Thanks again.

    --James
     
  11. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    LTSpice is free and there you can simulate circuits safely that would be a bad idea to actually build.
     
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