Noise from 556?

Discussion in 'General Electronics Chat' started by rougie, Apr 24, 2008.

  1. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    Hello,

    I have been fiddling around with the 556 timer. As you know, a 556 timer simply incorporates two 555 timers in one chip.

    Timer #1 generates a 180HZ square wave (I believe its called "astable"!) and timer #2 generates a 38 KHZ square wave. The ouputs of timer #1 and #2 connect to some other circuits. But there's one problem. When I connect the outputs of these timers to the rest of my circuit, my LCD screen flickers.

    I am not an electronic engineer or anything (I only have college) and therefore, all I tried to do to resolve this is to try to put an integrator (a Resistor using a 5K pot, with a parallel cap (0.01)) and I have tried to vary the resistor and I see that the wave attenuates. But still, the flickering persists. The only time that the flickering stops is when I disconnect the timer #1, 180HZ output signal from the rest of my circuit.

    What can one do to get rid of flikering caused by the output of the 555 or 556 timer chips.

    I did try to put 0.01, 0.001, 0.1 and 4.7 mico farad caps in parrallel with my DC power supply and no luck.

    Any suggestions?

    With regards
    Bob
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    do you have the schematic of the circuit? if yes post it
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Did you put a cap across the 556's power/ground pins?
    Usually, a 0.1uF/100nF ceramic or tantalum bypass cap is used there to avoid supply fluctuations.

    But post your schematic anyway.
     
  4. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hello Mik3,

    Unfortunately, I have a 100 oin CPLD, 1 40 Pin PIC, 2 flash chips, 2-DC/DC convertors, and a couple of IR LED's and 6 transistors (2n2222) and more. Therefore, unfortunately, it would be just too much to post.

    I was just wondering if anyone can give me an insight on what is usually done when noise is caused by 555 timers?

    Sorry. Please get back with any suggestions!

    With regards Bobby
     
  5. mik3

    Senior Member

    Feb 4, 2008
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    Its not easy to say. Many factors can affect the operation of a circuit but as SGTWookie said bypass the 556 timer with capacitors connected close to its power supply terminals to provide it the rapid changes in current when it pulses and minimize the supply line noise.
    Have a try.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    If the output of your 180kHz side is connected to a largely reactive (capacitive or inductive) load, you could be getting a great deal of noise on your supply. A capacitive load will look like a brief series of shorts occurring at a 180kHz rate. An inductive load (even long signal runs) can be generating high reverse EMF spikes.
     
  7. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hello Mik3,

    No luck, I just tried to put two 0.1 mf ceramic (The ones that are flat and round and the size of a dime)

    Its not that it flickers, what it really does is that I see faint horizontal lines slowly scrolling from the top of the screen down to the bottom.

    I don't know what esle to try... must admit I feel discouraged!

    If you have any other ideas I would really appreciate it.

    PS, I am powering up the 556 with 3.3VDC, that shouldn't be the problem, the specs say it is operable from 1.8 to 18VDC (if I recall... I don't have the specs in front of me right now!!!!)

    With regards
    Robby!
     
  8. mik3

    Senior Member

    Feb 4, 2008
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    I am not experienced with LCD's but these horizontal lines scrolling down slowly are due to bad controlling of the display by the microcontroller.

    Do you know what the 556 does in the circuit? is it a clock signal for something or it does something else?
     
  9. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
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    Hello SgtWookie,

    I am feeding these outputs in a 3.3VDC CPLD! (MAXII from Altera).

    In the CPLD these signals are processed and the processed signal is redirected to an output of my CPLD.

    But get this, the output of the CPLD containing the processed signal is connected to a 2N2222 transistor and when I disconnected the 2N2222 wire going from the transistor to the output of the CPLD, the flickering stopped!!!!!!!

    Now, I cam probably show just that portion of my schematic. Please bear in mind I have not used much of semies since my Cegep days and I am probably doing something really wrong here.

    Should I still post?

    With regards
    Robby!
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    A 555 creates current spikes of 400mA each time its output switches. Two supply bypass capacitors are recommended on its datasheet to arrest the spikes.

    Doesn't anybody look at datasheets anymore??
     
  11. mik3

    Senior Member

    Feb 4, 2008
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    I guess the 180Hz of the timer is for the refresh rate of the LCD. I think it is not working at the right frequency (180Hz as you said) and it works at a lower one so it causes flickering to the display. I have never used a LCD display but i know that if the refresh rate is low a flickering is caused, so i made a guess. if you want check it
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Which manufacturer's datasheet are you looking at? Of the three NE/SE556 datasheets (Phillips, ST Microelectronics, Fairchild) and a Phillips application note that I have, none of them specifically state a recommended bypass capacitor size or count; the Phillips application note suggests that "bypass capacitance in the range of 0.01uF to 10uF is not uncommon", but no specifics.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Robby,
    Which 556 are you using?
    Bipolar 556's have a minimum Vcc requirement of 4.5v
    Some CMOS versions can run with a lower Vcc.

    If you attempt to run the timer at less than the minimum Vcc, you will likely see some very high current spikes and tendencies to overheat.
     
  14. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    To avoid confusion I will re-post my question in detail.

    Thankyou all for answering this one.

    Robby.
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    Yep, right after the smoke escapes.

    I don't remember what it is for the 556, but the 555 uses pin 5 as a filtering pin. If you're designing this from scratch you might put an amplifier between your circuit and the 556 to remove the loading.
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    In his new post he is using a Cmos 556. It has a minor amount of switching current.

    He is straining the power supply voltage by trying to drive an IR LED from a transistor without any current-limiting resistor. So the power supply voltage is bouncing up and down and it flickers his display.
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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