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zero-crossing square wave generator schematic

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by donniegunz, Jan 9, 2012.

  1. donniegunz

    donniegunz New Member

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    Hi, I have been trying to find a good schematic online for a zero-crossing square wave signal generator but I haven't had any luck. I would like to power it with 18v DC, if that matters. Does anyone have something I can go off of?
     
  2. Boncuk

    Boncuk New Member

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    A zero-crossing unit and a square wave generator are two different things.

    The zero-crossing unit has its input mains frequency derived via an optocoupler, whereas a square wave generator (oscillator) has its frequency decisive R and C.

    Here is a typical zero-crossing unit with Schmitt-Trigger output. The Schmitt-Trigger takes care of clean square wave output wave shape.

    R1 is calculated for a mains voltage of 230VAC.

    Boncuk
     

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    Last edited: Jan 10, 2012
  3. donniegunz

    donniegunz New Member

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    Hmmmm...well I am just going to show my ignorance/lazyness here. But when you say 230VAC Mains Voltage it kinda scares me cuz all I really need is like 18v p-p. The project I am trying to finish is a portable frequency source using 18vdc (i.e. 2-9v batteries)with a variable frequency output (i.e. 1hz, 10hz, 100hz....). Now here is where I am trying to be lazy...I know I need the batteries and I know I need the variable output so I am looking for a god amoungst men, A knight in shining armor to come and tell me everything I need after the batteries. A schematic and parts list would be the bees knees but I will take any help I can get.
     
  4. dave

    Dave New Member

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  5. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    ONLINE

    Just put a capacitor in the output to remove DC bias. Then your square wave will cross zero.

    John
     
  6. donniegunz

    donniegunz New Member

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    Forgive me for being naive but it seems like you would need more than 1 capacitor and a battery to make a zero-crossing (+/-) squarewave. Are you for real or are you just messin with me?
     
  7. Roff

    Roff Well-Known Member

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    You need to provide more info if you are going to get design help. You need to tell us what sort of load you want to drive, the frequency range, etc.
    Why don't you tell us your application?
     
  8. RCinFLA

    RCinFLA Well-Known Member

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    You don't need battery. A resistor to ground after the coupling cap will ensure quicker settling time. The capacitor value will depend on the load you wnat to support and how low a frequency squarewave you need to pass. It will have a low frequency rolloff that will result in slumping of the top flat portion if the coupling cap is not large enough at the lowest frequency
    You also need to have a 50-50 duty cycle or the peaks will be unsymetrical around ground.
     
    Last edited: Jan 10, 2012
  9. donniegunz

    donniegunz New Member

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    Okay. Sorry for not being specific enough. I will try to explain:

    I want to construct a portable 18vdc frequency calibrator. It will be used to simulate output from a variable reluctance sensor on turbine flow meter. The ideal output is a squarewave of 15 volt peak-to-peak amplitude which can be set to any lower value with a built-in attenuator (e.g. sliding potentiometer). I would love a switch to select zero based or zero center outputs for standard or zero crossing inputs. A frequency range from 1hz to 6khz would be the cats pajamas.

    This may seem like a far cry from my original post but I wanted to start with the basics and work my way up from there. So there you have it in a nutshell. Any ideas?
     
  10. ronv

    ronv Well-Known Member Most Helpful Member

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    OK lets check the specs..
    Frequency 1 HZ to 100 HZ? In steps or continuously variable? Steps would be easier to calibrate.
    Amplitude 0 to 15 volts or minus 7.5 to plus 7.5 volts. Both adjustable or only 0 to 15?
    Sorry missed the 0 - 6 KHZ.
     
    Last edited: Jan 10, 2012
  11. donniegunz

    donniegunz New Member

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    Actually that was supposed to read 60Khz. So, yep I would like to do it in steps 1Hz - 60Khz. An amplitude of 0 to 15 volts and minus 7.5 to plus 7.5 volts is what I am shooting for. what do you think about using a 15V operated 555 in astable mode. If pin 3 is fed back to pins 2 and 6, through a resistor, I can get a 50% duty ratio square wave. Can I use a combination of pot and fixed resistor, selected by a switch to get various fixed frequencies from 1hz to 60kHz, say 1,10,100,1000 ... 60000 hz.
     
  12. ronv

    ronv Well-Known Member Most Helpful Member

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    Sure, Here is a 555 that will get you almost there. The pot can be replaced with a switch with selected resistors for the frequencies you need. The part in the box represents a switch to give the +/- pulse or the 0 to 15 volt pulse.
     

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  13. donniegunz

    donniegunz New Member

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    You Rock! I'm gonna give it a shot later this week and let you know how it turns out. Thanks Again!!!
     

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