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  1. #1
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    Suggest a debouncing circuit for push buttons

    Hi I need help for debouncing circuit for push bottons which will serve as clock to the up and down pin of my up down counter (74192). any recommnedations?

    •   Alt8th April 2005, 18:59

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  2. #2
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    debouncing circuit

    The traditional approach is RS flip flops. Your switch is single pole double throw. Both inputs to the RS flip flop have pull up resistors. Your switch grounds one or the other.

    The RS flip flop can be made from two each two input NAND gates. One of the two input on each gate goes to the output of the other gate. The other of the two inputs is the R or S input of the flip flop.



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    debounce flip flop

    if you are using a single pole double throw switch then flatulent's solution will work. but you said that you are using push "buttons". so that means there are two of them and they are single pole single throw switches (correct me if im wrong). well in that case you will have to include an RC delay for both the switches. that delay could be made with a resistor and capacitor, or if you are using a microcontroller you can program the delay with software.



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    rc debounce

    Pass the button signal through a D flip-flop, and then clock the flop at a rate that is slower than the button's worst-case bounce duration. For example, a 50Hz clock would clean up a button that has up to 20ms of bounce. If you are doing this in software, simply read the button at a 50Hz rate. If you have metastability phobia, use two flip-flops in series.

    I've seen many incorrectly designed RC debouncers that respond sluggishly, output double pulses, or both. If you use an RC debouncer, be sure that the time constant is slow enough so that the voltage takes longer to rise or fall between the two hysteresis thresholds than the worst-case bounce duration. Also, wider thresholds improve overall performance.



  5. #5
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    rc debounce circuit

    u can use ic74922 or 74923

    This will avoid this problem
    no other ic of low cost won't be able to avoid like
    74147



    •   Alt9th April 2005, 10:59

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    switch debounce circuit

    Despite the negative feedback regarding debounce circuit built on RC basis,

    The versatile circuit shown can be used to realize several different circuit functions: an astable multivibrator, a monostable multivibrator, a switch debouncer, or a frequency discriminator.



    Full details and advices regarding components value :
    http://www.elecdesign.com/Articles/A...6212/6212.html

    An effective switch debounce function can be achieved by making T longer than the switch bounce time.
    Connecting VIN to VDD will cause the circuit to function as an astable multivibrator with an output period of T.
    This is suitable for his counter 74192. Just keep the finger on switch and counter will advance.
    The number of pulses generated will depend on how long VIN is high compared to T.



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  8. #8
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    d flip flop debounce

    u can also use a monostable multivibrator connected
    adjust the time period of it T=20msec that is the debouncing time



  9. #9
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    rc debouncing circuit

    use a simple RC filter ,see this one :



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    counter switch debouncer

    That's a perfect example of an unreliable RC debouncer. See my message above.



  11. #11
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    debounce ic

    Quote Originally Posted by echo47
    That's a perfect example of an unreliable RC debouncer. See my message above.
    OK ...
    but if button press takes more than RC timeconstant it can work without any problem ,what do you think ?
    using flipflops is expensive method because of using flipflop and spreading or PCB dimension.



  12. #12
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    switch debounce circuits

    That circuit needs a schmitt trigger at the inverter's input. Then select R and C so the bounce duration is shorter than the time it takes for the voltage to rise from the trigger's lower threshold to its upper threshold. That's a somewhat different calculation than the RC time constant.

    Typical schmitt-input inverter chips have relatively closely-spaced thresholds, so you would need relatively large R C values. That makes the button response sluggish. Wider schmitt thresholds would let you reduce the R C values, and thereby improve the button response speed.

    If you plan to press that button frequently, you should add a series resistor to avoid burning the contacts with the capacitor discarge.



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