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Will this simple circuit work?

Discussion in 'General Electronics Discussion' started by CiaranM, Jun 13, 2012.

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  1. CiaranM

    CiaranM

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    May 19, 2012
    Correct me if I'm wrong, but by using a NOT gate to switch a transistor which will discharge a capacitor when the base voltage is high enough, can I create an oscillating voltage with a frequency of around 5RC?
    I'm thinking that since the voltage after a capacitor decreases, creating a ramped wave, a NOT gate could be placed to recognise when the voltage is near 0, and could control a transistor and discharge the cap, to restart the cycle
     

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  2. BobK

    BobK

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    Jan 5, 2010
    Nope, the inverter's input is always at ground, so it's output is always high, so the transistor is always on, effectively bypassing the capacitor and turning the LED on permanently.

    Look up astable multivibrator for a circuit that will work. It can be made either from transistors or from inverters, you should be able to find both on the web.

    Bob
     
  3. john monks

    john monks

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    Mar 9, 2012
    Have you considered using a Schmitt trigger?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
  5. CiaranM

    CiaranM

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    May 19, 2012
    what if I added a resistor between the NOT input and ground? theoretically the voltage/current would be partly directed to the NOT input, allowing it to recognise when to change states
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    No, the input is still at GND. Not even theoretically higher - unless I completely misunderstood your explanation.

    By the way: why use a gate if a simple second transistor can do the job?

    Harald
     
  7. BobK

    BobK

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    The circuit you are trying to design is called a relaxation oscillator. To make it work you need a device that stays off until a threshold voltage is reached (as the capacitor is charging) then turns on and stays on even after the voltage drops, and finally turns off again when a lower threshold is reached. A transistor and a logic gate do not operate this way. Some devices that do are:

    1. A neon light
    2. A diac
    3. A unijunction transistor.

    Google relaxation oscillator and the above devices and you will see how they work.

    Bob
     
  8. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    To expand on what Bob told you about relaxation oscillators: The Neon, Diac and Unijunction Transistor all share a common property. They all present a very high resistance until threshold voltage is exceeded. When exceeded their resistance drops dramatically and remains low until current flow is reduced below holding current threshold, where they then revert back to a high resistance.
     
  9. CiaranM

    CiaranM

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    May 19, 2012
    this is all interesting stuff, thanks!
     
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