square

Discussion in 'The Projects Forum' started by rakeshm55, Jan 3, 2012.

  1. rakeshm55

    Thread Starter Member

    Oct 19, 2010
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    Hi
    i am driving a buzzer (4khz+/500hz) 5Vp-p. using a square wave generator.

    The circuit diagram and datasheet of the NAND gate used is attached.

    Is this a correct circuit???
    I have implemented a control input tooo.....
    I also need a duty cycle of 50%.

    I tried simulating in pspice but the input to NAND gate U6D climbs near 6V and fallls below -3V. Though the input volt is tolerant to 7V I am not sure whether this is a standard technique.... Do i need to add a resistance to U6D feed back input.....

    Also the NAND gate input has ESD protection diode implemented, so will this not clip the -ve rail run (-3V) trigger.

    Please help me to develop a proper circuit. If the circuit designed is functional please tell me whether the 50 duty cycle be met.
     
  2. rakeshm55

    Thread Starter Member

    Oct 19, 2010
    68
    0
    Also Is it correct to apply a slow rising signal as the input of a CMOS circuit...
    I mean to say that the input voltage to U6D varies at a slow rate....
     
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    488
    You would actually use a schmitt trigger input for that.

    I don't know if any RC CMOS oscillator will have EXACTLY 50% duty cycle.

    You could use any oscillator at f x 2 and pass it through a toggle flipflop.
    Do you need to use the 3.3V? Since you have 5V available and they have the same reference why not use them instead?

    The following circuit gives you an idea. Single inverter ICs are available (I used the NAND so you have an enable/disable input available but this can be done at the FF too). Or use a 555 oscillator. I guess the D-FF can be made with gates too if necessary.

    [​IMG]
     
  4. rakeshm55

    Thread Starter Member

    Oct 19, 2010
    68
    0
    Hi thanks,
    My input control voltage from microcontroller is at 3.3V level. We actually have 5.2V rather than a 5V level.
    This was the reason for choosing 3.3V. Thanks for your circuit but wont your circuit give 50% duty cycle.

    Say VH (hysteriesis) = .8V

    In this case Frequency wont it be (1/.8RC) with 50% duty cycle
     
  5. rakeshm55

    Thread Starter Member

    Oct 19, 2010
    68
    0
    Is it mandatory that I need to drive buzzer with 50% duty cycle. Please find the attached datasheet of the device.
    In the datasheet as typical operating condition they 50% duty cycle.

    By the way I changed the schematics as per your suggestion pls find the updated schematics
     
  6. andre@electrocomp.co.za

    New Member

    Jun 22, 2011
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    the circuit praondevou presented will give precisely 50% duty cycle
     
  7. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    As Andre said, the circuit will give EXACTLY 50% duty cycle. The input frequency of the FF needs to be 8kHz for a 4kHz output.

    Low voltage flipflops are available too http://search.digikey.com/us/en/products/74LVC1G80GV,125/568-7772-1-ND/2753916

    I do not say there is no way to make an RC CMOS oscillator with exactly 50% duty cycle, but the easiest is to simple divide a non-50% by 2.

    Since you are enabling the circuit with a uC, why not program the oscillator itself into the chip?
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    You should never specify anything to be exactly a given value. Of course, you cannot be responsible for what a teacher may have required of you, but in real life a tolerance range always exists.

    Thus an RC oscillator may have timing errors of some percent, depending on just how it is designed. Even the output of a divide-by-two circuit will show some assymetry if we can measure it closely enough, possibly in parts per million rather than percent.

    But who is bothered with parts per million when they are driving a buzzer? Does even a few percent error make any effect - and if so, what?
     
  9. rakeshm55

    Thread Starter Member

    Oct 19, 2010
    68
    0
    Hi guys thanks for your support...

    Now i am using 74LV132 for the RC oscillator.... Its V hysterisis varies from .4V to 1.2V ( with respect to temperature) though the typical value is .65V ..... i am not sure about how it will vary with chip to chip.....

    Given the resonant frequency range for buzzer (4khz +/-500Hz) 12% approx ..... With the above variation in Hysterisis voltage will the circuit provide proper frequency to the input of buzzer...... Lets assume , i design the RC oscillator for 4000Hz with a chip of Vh= 0.65V.

    Another chipset has A Vh = .4V now the change in frequency of oscillation be 38% which will be outside the range specified by the buzzer...

    is my conclusion right??? Or should I modify using some more stable RC oscillators like ones using comparator....
     
  10. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    What's wrong with the suggestions we made?

    Is component count the reason, do you need to build an oscillator with only one chip?

    Hysteresis can change a lot from chip to chip.

    Best and simplest option for 50% duty cycle IMO is a tiny uC or the solution I posted above, no adjustments necessary (as for other oscillators).
     
  11. rakeshm55

    Thread Starter Member

    Oct 19, 2010
    68
    0
    Hi,
    I thank all of you for the support and guidance.... i have learned a lot by interacting with u all in this forum......

    Initially I was refering to data sheet of buzzer where its mentioned for 50% duty cycle....
    Datasheet attached...

    But As Adjuster suggested 50% duty cycle may not be stringent criteria....

    Reviewers of the design were of the notion that it is better design an RC Timer using comparator for better stable frequency than 74LV132....







     
  12. rakeshm55

    Thread Starter Member

    Oct 19, 2010
    68
    0
    Hi
    i modified the circuit with an RC timer using comparator.
    Please find the simulation model attached....
    For the purpose of simulation I have used 5V operation.....

    I have only added RC timer for 4kHz oscillation and low frequency oscillation is still maintained by the 74LV132 RC oscillator which I showed in the previous schematics..

    Please review and give suggestions .....
     
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