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74HC14 issues please HELP!

Discussion in 'General Electronics Discussion' started by sawula, Nov 26, 2010.

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  1. sawula

    sawula

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    Nov 26, 2010
    I have two built circuits and I am trying to integrate them. In one ciruit there is 74HC14 IC and there is a 2.2k resister between two gates (pin2 and pin 3) and in other circuit they are directly connected (pin1 and 2). I want to know why is 2.2k resister used? I know the purpose is to invert the output of first gate but why we can't directly connect those pins in both cases?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,271
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    Jan 21, 2010
    Directly connecting the output of a Schmitt trigger to it's input will cause it to oscillate at a very high frequency. With the addition of a resistor you can then add a capacitor between the input and ground to control the frequency.

    Check page 5 of this.

    This talks about CMOS oscillators in general and how a ring of inverters will oscillate.

    edit: Naturally, without the rest of the circuit it is impossible to tell if something else is going on.

    Frankly, I think the short across pins 2 and 3 would be an error.
     
  3. sawula

    sawula

    2
    0
    Nov 26, 2010
    Sorry for the mistake. I wanted to say pin2 and pin3. The two circuits are x10 transmitter and a x10 receiver. 74HC14 is used to detect the zero crossings of the AC power lines. (x10 is a protocol that is used to communicate over power lines) And I want to combine both transmitter and the receiver together and create a transceicver. In the transmitter ciruit pin2 and pin3 of 74HC14 is directly connected and in the receiver circuit they are connected through a 2.2k resister. I want know the difference inbetween so that I can use either directly connection or the connection through 2.2k resister for my combined transceicver circuit.
     

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  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,271
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    Jan 21, 2010
    As shown on that diagram, the 2k2 resistor does nothing significant.

    However, I'd wager that there are power and ground connections (and a resistor) that connect to the 4N35.

    Did you arrive at this circuit by tracing connections on a PCB?
     
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