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flip-flop question

Discussion in 'Electronic Basics' started by [email protected], Mar 24, 2008.

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  1. Guest

    I'm designing a circuit and I need something to switch between two
    inputs. I don't have very much experience, but I think a certain type
    of flip-flop will do this?

    I'm using a 555 (as a monostable vibrator), and I need the same
    momentary push button to toggle between the trigger and the reset
    lines. Basically it's like a metronome with a single start/stop
    switch.

    Can I do this with a flip-flop?
     
  2. Lord Garth

    Lord Garth Guest

    Why not build an astable multivibrator with the 555? You will then get
    a constant stream of ticks.


    Or post your schematic on alt.binaries.schematics.electronic
     
  3. Guest

    I need to be able to start/stop the ticks. From what I've seen you
    can't really do that with astable. Am I correct?
     
  4. Lord Garth

    Lord Garth Guest


    I need to be able to start/stop the ticks. From what I've seen you
    can't really do that with astable. Am I correct?

    No, you can drive the reset input and stop the oscillator with a simple
    toggle switch.
     
  5. Bob Monsen

    Bob Monsen Guest


    If I get you, I think you want to enable/disable the 555 using the same
    pushbutton, is that right?

    If so, you can use a couple of inverters (cmos 4049 perhaps) along with a
    normally open push button and and resistors/capacitors to do this.

    .-----------------------------------------.
    | |
    | |\ ___ |\ ___ |
    '----| >O-|___|--o----| >O----o---|___|---'
    |/ 100k | |/ | 100k
    | |
    4049 | 4049 .-.
    | | | 330k
    | | |
    | T '-'
    | --- |
    '-----o o----o
    |
    --- 1uF
    ---
    |
    |
    ===
    GND
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    The pushbutton is normally open. The capacitor is charged to the output of
    the right inverter. When you push the button, it changes the value of the
    input of the right inverter, causing it to change its output. This is
    independent of the starting state.

    Use the real output (not the output after the 100k resistor) to drive the
    reset of the 555.

    Regards,
    Bob Monsen
     
  6. Guest

    The only constraint I have for this circuit is the momentary switch
    for user input.
     
  7. Guest

    That looks great, but where do I put the trigger and reset lines from
    the 555?
     
  8. Lord Garth

    Lord Garth Guest

    <snip>

    The only constraint I have for this circuit is the momentary switch
    for user input.

    Okay, a T flip flop with a debounced push button switch as its clock
    input. The output goes to the reset input of the 555.

    You can form a T flip flop if you take a D flip flop and connect the Q*
    output back to the D input.
     
  9. Guest

    What do I connect the trigger input of the 555 to? The D input of the
    flip-flop?
     
  10. Bob Monsen

    Bob Monsen Guest

    That looks great, but where do I put the trigger and reset lines from
    the 555?


    You set up the 555 as an astable multivibrator, and let it free run. You
    connect this to the reset pin of the 555. When it is "high" output, the 555
    will output pulses for you. When it is "low" output, the 555 will wait for a
    "high" output, and output no pulses.

    The typical setup for an astable is to have two resistors and a capacitor.
    Connect the resistors in series with the capacitor between the voltage
    supply and ground. Connect the discharge pin between the resistors, and both
    the 'trigger' and 'threshold' to the cap:

    -----------o-----------------.
    Vcc(12v) | |
    | .-.
    | | | R1
    | | |
    | __ '-'
    '---o| |o-----. |
    .----o| |o-----)--o
    output ----)----o| |o---. | |
    reset ----)----o|__|o- | | .-.
    | | | | | R2
    | | | | |
    | | | '-'
    | | | |
    '-------------o-)--o
    | |
    | --- C1
    | ---
    | |
    GND | |
    ---------------------------o--'

    1/f = discharge time through R2 +
    charge time through R1 + R2

    Discharge through R2 is .693 * R2 * C
    Charge through R1 and R2 is .693 * (R1 + R2) * C

    So, f = 1.44/(R1*C + 2*R2*C)
    and d = 100 * R2 / (R1 + R2)

    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    Regards,
    Bob Monsen
     
  11. Lord Garth

    Lord Garth Guest

    What do I connect the trigger input of the 555 to? The D input of the
    flip-flop?

    It's an astable multivibrator...build that first, get it oscillating then
    work on holding the reset active to stop it.
     
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