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CD4027B: The otput Q won't change

ZeroD
Expert 1470 points
Part Number: CD4027B
Other Parts Discussed in Thread: CD4050B, CD40106B

Dear *,

I attached my schematic with CD4027B. I have connected toggle button on P2 and when press the button the 14V comes to K1, J1 and through R10 10Ohm C8 100n to CLK1 but the Q1 is always 12V and Q1_n always 0V

please can you help?

BR.

TOGGLE-BUTTON.pdf 

  • 0
    Guru 283350 points

    What is the purpose of the circuit? What do you expect to happen?

  • 0 in reply to Clemens Ladisch
    Expert 1470 points

    When I press the button I want to toggle the output Q on the rising edge of the CLK.

    Upon press the J and K are 1 and the CLK is rising.

    is it possible that the RC circuit are the problem?

  • 0 in reply to ZeroD
    Guru 283350 points

    Just connect J and K directly to VDD; their state is relevant only during a rising edge.

  • 0 in reply to Clemens Ladisch
    Expert 1470 points

    Dear Clemens,

    i connected the J and K direct to VDD=12V, and now when i press the button the Q output changes but when i release the button then it changes back to previous state?

    J=1, K=1 -> button press CLK from 0->1 -> Q from 1->0, release the button Q from 0->1

    i also removed C7 and C8 but i get the same thing.

    ( one observation when i unplug the board, the capacitors have some charge left, and then when i press the button the Q changes and stays in the state to another button press ??)

    Br.

  • 0 in reply to ZeroD
    Guru 283350 points

    I'd guess that the RC filter is too weak, and the button is still bouncing. Check with an oscilloscope.

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 44765 points

    Hey Br,

    Could you provide some scope shots of the operation with the issue you are experiencing. Also make sure to zoom in on the button press to verify if the button is still bouncing. The RC may be too small.

  • 0 in reply to Dylan Hubbard
    Expert 1470 points

    Dear Dylan,

    for some season after the CLK input goes from high to low the Q,Q_N changes again

    a) regarding button press denouncing RC

    I have first RC then the buffer CD4050BE and then the button press signal goes to CD4027BE JK bistabil CLK input.

    I don't see any bouncing of the button:

    Blue button signal

    Yellow after the RC

    Green on the CD4050BE = CLK in of the CD4027BE 

    b)  when i release the button the clk goes from high to low and then the output changes ??

    Br,

    David.

  • 0 in reply to ZeroD
    TI__Mastermind 44765 points

    Hi David,

    Could you zoom in on the falling edge green signal. As much as you can while keeping the whole edge on screen. I think I might know whats going on here.

  • 0 in reply to Dylan Hubbard
    Expert 1470 points

    Dear Dylan,

    i will do that but currently i don't have the oscilloscope by hand. i will do it ASAP

    please can you explain what do you think that is going on?

    Br.

  • 0 in reply to Dylan Hubbard
    Expert 1470 points

    Dear Dylan,

    here are the oscilloscope shots:

    yellow: button

    blue: after RC button -> buffer input

    purple: buffer output -> J-K CLK input

    green: j-k output Q and QN

    whole ***:

    zoomed rising edge

    zoomed falling edge

    Br.

  • 0 in reply to ZeroD
    TI__Guru* 96862 points

    Hi ZeroD,

    I believe what Dylan is looking for is oscillation on the CD4050B. This is a standard CMOS device and it will oscillate at the output when presented with a very slow input transition (which is what is happening in your circuit).

    You probably haven't seen the oscillations since they are probably happening at ~10ns time scale, and all the scope shots here are at ~2us/pt (about 2000x to large to see the problem).

    Here's an FAQ that talks about what's happening:

     

    If you swap the CD4050B for a CD40106B (hex Schmitt-trigger inverter) it will remove this problem, however you will need to invert your button logic values (or use 2 stages of the inverter).  Here's more info on Schmitt-triggers and why you need them: