Questions about RS flip flop using transistors

Discussion in 'General Electronics Chat' started by tpny, Nov 8, 2012.

  1. tpny

    Thread Starter Member

    May 6, 2012
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    Hi attached is my RS (reset/set) flip flop circuit made by transistors (I got it off google image search based on related keywords). Some questions:

    1. Are my resistor values sized appropriately?
    2. Why are R2 and R4 necessary or are they not?
    3. Are R6 and R7 necessary? I don't have them in my circuit right now and they work but I'm wondering if I should?

    Thank you so much!!
     
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  2. Austin Clark

    Member

    Dec 28, 2011
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    From what I can tell, R6 and R7 aren't necessary, and you shouldn't use them because they could prevent the bases of the RS transistors from being fully pulled down.

    The resistor sizes are fine.

    R2 and R4 are necessary. If they were omitted, the outputs could never go over the forward voltage of the BE junction, and it would be difficult to turn the LED transistor on, as the output of one wouldn't adequately saturate it.

    EDIT: Upon second evaluation, it seems you could use higher values for R6 and R7, and that may give you a stronger output.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    R2 is not necessary but R4 is so that the two transistors share the base current properly. I'd note which transistors but they are unlabeled. Always label transistors on a schematic.

    R6 and R7 are unneeded and undesirable.
     
  4. tpny

    Thread Starter Member

    May 6, 2012
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    the npn's are 2n4401.

    I notice when I touch the (+)end of my voltmeter probe at the base leads of the flip flop transistors, it lights up or shuts off the led sometimes. Leads me to feel less confident about this setup - vulnerable to external disturbance/noise? How do I make it more (noise) proof to this? Thank!
     
  5. Austin Clark

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    Dec 28, 2011
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    I believe you can achieve this by increase the values of R2 and R4.
    You can also use pull-up or pull-down resistors, to make sure it only goes up or down when actually connected to a "strong" HIGH or LOW signal (By strong I mean relatively low resistance to ground or supply).
     
  6. tpny

    Thread Starter Member

    May 6, 2012
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    That's interesting! Why is that? - making output stronger.. Thanks so much!!
     
  7. Austin Clark

    Member

    Dec 28, 2011
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    Sorry, I meant R2 and R4 here.
     
  8. tpny

    Thread Starter Member

    May 6, 2012
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    Could you explain how making R2, R4 makes output stronger? And what size should I try?

    And where should I be putting the pull-up/down resistors you are suggesting? Thank you!
     
  9. Austin Clark

    Member

    Dec 28, 2011
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    Very little current is needed to saturate the transistors in this case, so increasing their value won't hurt that, but at the same time, if you increase the value of R4, more current coming from that 33k resistor will go to the base of the transistor with the LED. The transistor with the LED doesn't need much to fully saturate (Turn on), so it likely won't make a difference in this case, but if you wanted to use that output on several transistors directly, or if you wanted have a greater load, it would help.

    EDIT: Oh, and the pull-up or pull-down resistors (I recommend pull-down resistors in this case) should be placed from the base of the transistors acting as your SET and RESET pins directly to ground. These resistors should be of a high value, 100k or greater. This keeps the transistor from switching on with weak noise.
     
  10. tpny

    Thread Starter Member

    May 6, 2012
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    I was thinking of also doing this (see attached) to drive the base of the ff transistor to low.. Is this better or worse? Thanks!
     
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  11. Austin Clark

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    Dec 28, 2011
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    Lots worse. Remember that diodes have a forward voltage. If this forward voltage is greater than the forward voltage of the transistor, it will never fully be pulled down. The resistors R8 and R9 serve no purpose, and make it more difficult to drop the voltage at the transistors collectors (the top parts).

    EDIT: I do see what you were going for though.
     
  12. tpny

    Thread Starter Member

    May 6, 2012
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    R8 and R9 make the transistors below them inverting, so when the base of one of them is high, the collector is pulled to ground which causes the ff transistor base to see low. When the base of R8 goes low, the collector is Vcc and diode blocks this from getting seen by ff transistor base.. That was my logic...
     
  13. Austin Clark

    Member

    Dec 28, 2011
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    Yeah, I could see that. The only thing is, the resistors aren't necessary to make it inverting in the way you want. That is, you don't ever need their outputs to be HIGH. Just either LOW or High impedance (essentially not connected to anything).
     
  14. Ron H

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    Apr 14, 2005
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