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Playing with transistors

  1. Jul 6, 2011 #1
    Hello engineers,

    I learned the theory of transistors and that is all fine and dandy but in order to learn how to drive a car, you must drive a car, you cannot learn how to drive from books.

    So i googled some small transistor beginner projects and I stumbled onto a fun LED flasher.

    [PLAIN]http://pokit.org/get/4b011e2f5e0a6490eef4bf0e13e8edd3.jpg [Broken]

    Now, I assembled this and wouldn't you know it flashes ^^. But, yea I am not a kid with legos, I want to understand this circuit. Original site provided a slim explanation.

    I am getting confused here, why is capacitor's cathode connected to positive voltage supply?

    Can anybody help me understand what is going on, in detail? I think this will help me understand in general dynamic RC circuits like this. I think this is a good start. I want to see those holes, electrons in action, what charges first, what conducts etc.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jul 6, 2011 #2
  4. Jul 6, 2011 #3
    You really need to come to my college, seriously. Thanks, will come back with impressions^^
  5. Jul 6, 2011 #4
    Hello ,
    From the link You provided me, multivibrator, I really don't understand this sentence. I understood everything up to it. How can collector current fall, if the transistor is turning on. Does it mean like, fall down from Vcc, as in rise by magnitude? It doesn't make sense to me.

    And another:


    Is there any formula that I can calculate speed of these flashes by knowing circuit parameters(flashes per sec)? Or is there any manual way of doing this, like deriving such formula.

    I made this circuit, with 10 uF. I speculated that bigger capacitors take longer to discharge, so I mounted 100 uF and the flashes slowed down dramatically, I could even count them. Whereas with 10 uF they were really fast.
    Last edited: Jul 6, 2011
  6. Jul 7, 2011 #5
    That's an interesting issue!

    As the capacitor slowly charges through the 100K resistor, the voltage across it's terminals rises to maybe -0.5Vdc. The polarity is backwards. But then the NPN transistor 2N3904 starts to turn on. Its collector is connected to the base of the PNP, so when the lower NPN turns on, it will turn the upper PNP transistor on. The PNP turns on hard. It lights the LED. The LED voltage (about 1.8V) dumps a big charge flow through the capacitor, through the base of NPN, and into ground.

    What does the capacitor see? It sees a slow tiny voltage which rises to -0.5Vdc. Then it sees a big +1.8V spike.

    What if the capacitor polarity was reversed from the schematic depiction? Well, a small 0.5V reverse voltage probably won't hurt an electrolytic capacitor. But the bigger spikes might hurt it over time. So, the capacitor must be oriented so that the bigger positive spikes will hit the capacitor positive terminal.

    In reality these voltages are so low that probably you could install the capacitor in either direction without harm.
    Last edited: Jul 7, 2011
  7. Jul 7, 2011 #6
    Nah, they meant that the collector voltage falls toward zero. They could have said "as TR2 turns on, the right hand end of C2 gets connected to ground."

    Yes, and in theory if you make the capacitor 10X larger, the time between flashes will be exactly 10X longer.
  8. Jul 7, 2011 #7
    Yea, transistor CE voltage did come cross my mind. Thank you !
  9. Jul 7, 2011 #8

    Is there any source on the web where I can study more closely electric potential? Any exercises or so?

    And does capacitor has a fixed cathode/anode, or can it be reversed? Like positive voltage to be on cathode and so.

    I am freshman of EE and I am still trying to grasp that idea of electric potential.
    Last edited: Jul 7, 2011
  10. Jul 7, 2011 #9
  11. Jul 7, 2011 #10
  12. Jul 8, 2011 #11
    Ok I understood everything that is related to flasher circuit.

    Can anybody suggest me some circuits that involve: diodes, capacitors, transistors and resistors ofc. Some circuits that aren't so basic but not as complicated, and will help me further increase knowledge of circuits.
  13. Jul 8, 2011 #12
    You could build something useful.
    Since I have a particular interest in test gear I will suggest something here.

    This is an old design, but based on easy transistorised modules etc that can easily be modified to suit but ends up with a useful gadget.

    Thre are seven pages in all let me have an email address that can receive scans if you want the full article - there are 7 pages in all.

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  14. Jul 8, 2011 #13
    I will PM you with my personal email.
  15. Jul 8, 2011 #14
    Yes sorry I meant by pm
  16. Jul 8, 2011 #15
    OK check your Email.
  17. Jul 8, 2011 #16
    Ok I got it, Thank you!! I will get right on it.
  18. Jul 8, 2011 #17
    Oh my God this is so cool!! Whole gadget is smashed into small pieces! Do I have to use exact same circuit elements? Or can I improvise with what I have?
  19. Jul 8, 2011 #18
    The whole idea is to improvise and experiment.
  20. Jul 8, 2011 #19
    Challenge is what I like. I've got almost everything from the list of components apart from some capacitors and 2W resistors. Thank you very much Studiot.
  21. Jul 8, 2011 #20
    2W = 4 x 0.5 watts, 2 series/parallel combinations.
  22. Jul 8, 2011 #21
    Yea thought of that too. Some basic calculations. Tomorrow gonna order some potentiometers and capacitors(Got only large capacitors, and no so many, so i can't combine to get smaller ones), because I am missing those and I think I got some old electronics that will provide me with switches. But anyway this is totally in my budget and doable.
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