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Could someone explain this simple circuit to me? (beginner)

  1. Dec 11, 2016 #1
    Hello all,

    I am attempting to teach myself some basic electrical knowledge, but I am slow to start. I understand what resistors do and how capacitors store a static electric charge ect...

    The question is, can someone explain to me the flow of electricity from beginning to start of this circuit and how it causes the 2 leds to alternate? I assembled the circuit from the diagram on a breadboard, and it does work, I am just not understanding why. What is the purpose of the 10k resistors and the capacitors in this circuit, how do they work together to cause the two leds to alternate?

    Thank you!

    Attached Files:

  2. jcsd
  3. Dec 12, 2016 #2


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    Science Advisor

    It is an example of a (digital) oscillator. Short explanation:
    • Assume Q1 is in saturation (collector voltage is lower than the base voltage). Then LED1 is "on".
    • While Q1 is "on", the 100μ capacitor charges through the 10k resistor connected to the base of Q2
    • When the voltage at the base of Q2 has become high enough, Q2 starts conducting, pulling the collector down. This change is transmitted to the base of Q1 though the 100μ capacitor, driving the base down, reducing the collector current.
    • When the collector current of Q1 is reduced, the voltage across the collector load is reduced and thus the collector voltage increases.LED1 is starting to turn "off".
    • The increase in Q1 collector voltage is transmitted to the base of Q2, increasing the collector current of Q2 which again drives the collector voltage of Q2 lower.
    • This continues until Q2 goes into saturation (the collector current no longer depends on the base voltage). LED2 is "on".
    • Now the cycle starts again with Q2 and Q1 exchanged.
  4. Dec 12, 2016 #3


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    Staff: Mentor

    Hi levydee. hygetx.gif

    You will find no end of web sites where basic electronics is well explained, together with superb circuit waveforms. A google search on "transistor astable multivibrator" will turn up a plethora of helpful sites.

    If you have the circuit operating, try changing some R or C and see what happens. You could easily place a capacitor or resistor in parallel with one you have, to double or halve its current value.

    Good luck with your self-study!
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