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Flashing LED schematic

Discussion in 'General Electronics Chat' started by ssherwood, Oct 24, 2008.

  1. ssherwood

    Thread Starter New Member

    Aug 25, 2008
    The flashing LED schematic is puzzling me, I have built it and conducted many tests to figure out exactly how the lights turn on and off. I am thinking that when the capacitors begin discharging, they interrupt the BE junction current at the transistor, turning the LED off (C1 discharging turns Q2 and LED2 OFF) Is that right? If so, then I have another question...when the circuit first starts, both LED's are on, then they start switching. How do the capacitors charge when their associated collector is on, isn't this a negative polarity when the CAP needs a positive? C2 for example, gets a negative from Q1 base, but if Q2 is on at first, then isn't the positive side of the C2 negative because of the current flow from the Q2 collector? Maybe I am seeing this whole thing wrong, please help me! Hopefully this is enough information. Thanks so much, I am struggling here! :confused: More info --the LED's alternately blink...when one is off, the other is on, and the switching on/off times can be changed by the POT's or by capacitor values.
    Last edited: Oct 24, 2008
  2. SgtWookie


    Jul 17, 2007
    Have a read through this page:

    It explains your circuit, plus many variations of it, and will save me lots of typing ;)

    Note that when the author refers to "valves", he means "electronic vacuum tubes".
  3. boff1


    Oct 15, 2008
    This circuit used to be called a 'multivibrator' many years ago!
  4. ssherwood

    Thread Starter New Member

    Aug 25, 2008
    SgtWookie, Boff1, thank you for your help. The weblink is perfect and is precisely what I needed!!!:)