NOT gate

Discussion in 'The Projects Forum' started by Bluerocket, Dec 15, 2013.

  1. Bluerocket

    Thread Starter New Member

    Dec 10, 2013
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    Hello. I'm trying to use a NOT gate to power a LED and an Oscillator when the transistor's base doesn't recieve voltage. I attached an image with the circuit. The circle with T is a thermostat. The problems are:

    1) The whole thing is powered at 12v. A NOT gate needs a resistance at the collector. The oscillator is made for 12v drawing 14mA. But because of the resistance the voltage drops and the oscillator doesn't recieve 12v. That means the resistance has to be small.
    2) A small resistance means that a great amount of current will flow through the transistor in the saturation mode. But as I understood, in saturation mode common-emitter resistance is very small so why do my transistors heat up?

    Please suggest me some type of transistor and the values of base and collector resistors to maximixe the oscillators voltage and to make the transistor stable. Or if there's another method to turn on the Oscillator and LED when the thermostat gets off then please suggest it.

    Thank you :)

    NOT gate.jpg
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Just make the transistor switch active when you need it:

    [​IMG]

    Since I have no idea what your unspecified drive signal is I could not account for that; I imagine it needs to be inverted.
     
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  3. Bluerocket

    Thread Starter New Member

    Dec 10, 2013
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    That's not OK! If the base is not powered then the oscillator won't work. I need the oscillator to work when the base doesn't recieve votlage.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,014
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    Just add a transistor inverter.
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Here is how I would do it: I cant see why the extra diode or the capacitor is there. The capacitor will damage the thermostat contacts.
     
  6. Bluerocket

    Thread Starter New Member

    Dec 10, 2013
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    Thank you very much for your effort. The oscillator makes a speaker to buzz when the thermo goes off but I want it to beep just for 5 seconds. The oscillator was at the relay's contacts which had an off delay when the thermostat went off (that's why the capacitor was there) and the other diode was to don't allow the capacitor to discharge trough the green LED.
    What are the two circles in the top right corner?
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    One is the 12V power source; the other is a voltage source which open/closes the switch for the simulation. LTSpice uses a voltage-controlled switch, so to run a simulation where the switch opens and closes, you specify that by using a pulsed voltage source.
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    So the relay is there just to make a delay?
     
  9. Bluerocket

    Thread Starter New Member

    Dec 10, 2013
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    Yes, the relay is there just for the delay.
     
  10. Bluerocket

    Thread Starter New Member

    Dec 10, 2013
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    Mister MikeML your circuit is surely good? I ask this becuase when the thermo is off I see that both LED's will light up.
     
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Ok, here is a circuit that does what you asked for without using the relay. The delay is proportional to R4*C1. The red and green traces correspond to the red and green LEDs I placed on the schematic. The PFET is almost any power device rated at Vds < -20V
     
    absf likes this.
  12. Bluerocket

    Thread Starter New Member

    Dec 10, 2013
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    Thank you but in your circuit the Oscillator will work together with the green LED. That's not what I want. The green works when thermo is on and the red and osc when the thermo is off but the osc has to work just 5 seconds after the thermo is off.
     
  13. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Ok, try this:
     
  14. Bluerocket

    Thread Starter New Member

    Dec 10, 2013
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    Thank you very much :)
     
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