555 timer

Discussion in 'The Projects Forum' started by darklord_V, Sep 17, 2011.

  1. darklord_V

    Thread Starter New Member

    Sep 7, 2011
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    i want to kmow how to configure a 555 timer correctly in multisim...
    i want this for making a digital clock project.....
    basically i havent used a 555 timer before so can someone show me how it is done in multisim.....just get me any pulse at the output.....
    i tried but failed miserably.

    Or atleast show me how a frequency counter works works???i.e how can i measure what the output is??verify the pulse?
     
    Last edited: Sep 17, 2011
  2. SgtWookie

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    Jul 17, 2007
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    You should post the circuit(s) that you have tried.

    .PNG format images are preferred, as they are compact, require no other software to view than a web browser, and are not "lossy" like .jpg format images are. Use the "Go Advanced" and "Manage Attachments" buttons to upload your image(s).

    While 555 timers are very versatile, they are woefully inadequate for timekeeping purposes; at best you will be off by several minutes per day. Even crystal oscillators are not stable enough for a clock. You need a TCXO (temperature compensated crystal oscillator) or OCXO (oven controlled crystal oscillator) to approach something resembling accurate - even then, you will still have to correct the time on a regular basis.
     
  3. Wendy

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    Mar 24, 2008
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    Actually I will disagree strongly with that last statement. Watches are crystal controlled and are finely tweaked, and even when they are not on your wrist keep excellent time. I'm assuming you are counting being on the wrist is equivalent to a oven, which hasn't been very true this summer. I'll let you know how accurate the digital clock I made per day is, and there isn't even the option to tweak the crystal frequency (though I know of one that is simple to implement). It was simply build on a protoboard with a crystal and a 4060.

    A 32.768Khz crystal can be bought for under $1, as can the 4060IC. For 1Hz you will need a second chip, the 4060 comes out with a 2Hz signal.

    Digital clocks Post #12

    I don't think Wookie uses multism, but he is the man when it comes to simulating a circuit. I tend to be a hands on type. Both of us are very good with electronics. It is possible to disagree and stay civil.
     
    Last edited: Sep 17, 2011
  4. darklord_V

    Thread Starter New Member

    Sep 7, 2011
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    thanks for the replies....
    i am quite a rookie at electronics even thou am a 1 year and a half away from graduation as its not my major:):D
    i am currently taking this as the base of this project.
    http://forum.allaboutcircuits.com/showthread.php?t=28974
    the accuracy of the clock is no prob i just have to get it working using 555
    any suggestions are welcome
    P.S:was able to generate a pulse with Ra=39k,rb=1M and c=0.1micro farad(around 50 hz.)
     
  5. Wendy

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    Mar 24, 2008
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    If you followed the link I gave I have a half designed digital clock. The minutes and seconds are working well, the hours are under construction and not yet posted (though I have a design on my computer).

    As Wookie mentioned, the 555 (which is my favorite chip) simply is not accurate enough for a clock. If you followed the link you would see a working time base that would do the job well. Since it is straight out of the data sheet for a 4060 I don't see a problem with you using it.

    If you must use a 555 then it is easy enough, it is simple algebra.

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  6. SgtWookie

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    Bill,
    Wristwatches use TCXO's, which is one of the options that I mentioned. Watch factories have the capability to tune the accuracy prior to shipping the finished watches by comparing the watch frequency to an extremely accurate time standard, which our OP won't have access to.

    Uncompensated crystals will drift far more than TCXO's will.

    OCXO's can be more stable than TCXO's depending upon how closely the temperature is maintained.

    I'm not wanting to get into a discussion on the myriad types of xtal oscillators that exist, as I don't wish to confuse our OP. However, I spent a few years working at a company that made a variety of TCXO's, OCXO's, VCXO's (voltage controlled crystal oscillators) and numerous other configurations.
     
  7. Wendy

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    Mar 24, 2008
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    For general use (especially timepieces) I think you will find they are more than accurate enough. Figure that there are 85400 seconds per day. Many watches vary about 5 seconds per day, and when I get a watch that stayed within second per day I have value it highly. A variance of 5 seconds is accurate to .006%. During my CB days (as a kid) my oscillators routinely hit ±100 Hz, out of 27Mhz. This works out to something like .004% accuracy, and was more than good enough to work with simple AM transmitters.

    I will freely admit that I have a superficial understanding of crystals in general, but I truly believe they are more accurate than you give credit for based on personal experience.

    The standard 32768 crystal is what is used in watches, and was originally developed for old NTSC televisions. They are really cheap because of this.

    Back in my metrology days I did get to see quite a few crystal ovens. I know we've both seen RB's plans for it, it is well done and simple to make.

    A quick blurb from Wikipedia using the phrase Crystal Oscillators:

    This comes pretty close to my observations.

    As I've said, I'll post results from my circuit when the data is in.
     
  8. darklord_V

    Thread Starter New Member

    Sep 7, 2011
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    here what i am trying to do is to make a divide by ten counter so that the 7 segment displays 1-9
    what am i doing wrong???
    disregard this image....
     
  9. darklord_V

    Thread Starter New Member

    Sep 7, 2011
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    this is the right one
     
  10. Wendy

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    Off the top of my head I can not judge whether the BCD counter is accurate (looks OK). You do have a minor problem with the LED display, in than you have the LEDs in parallel. This will cause a variation in brightness depending on how many are lit, which is usually not very good. Instead for each segment have a resistor per LED, it will be much more consistent. For demonstration purposes it will work well enough.

    Have you looked at the diagram for a 7490 IC? It uses a similar BCD counter, I would reference to it for a JK type BCD counter.

    Oh, and one other thing, you have all your inputs left open, instead of tying them to a high or a low. This is a major issue, all inputs must be tied to a logic level. Outputs can be left open, but you don't usually want the chip to assume a logic level, with CMOS it can cause damage in extreme circumstances, while TTL usually assumes a logic 1.
     
  11. darklord_V

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    Sep 7, 2011
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  12. Wendy

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    Yes, Set and Reset. They are major logic inputs, just because you aren't using them you still have to address them. Ground those puppies, give them a logic 0.
     
  13. Wendy

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    Something else I noticed, isn't Q outputs where your BCD count is supposed to be occurring? You are treating the J/K inputs as outputs. Remember the suggestion about looking up the internal logic diagram of the 7490.
     
  14. darklord_V

    Thread Starter New Member

    Sep 7, 2011
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    did that.....but still cant get a valid display.... can you recognize any mistake?
    here are two pics......in the schematic what i am trying to do is make the 7 segment glow a number 5.....but it wont show....
     
    Last edited: Sep 18, 2011
  15. darklord_V

    Thread Starter New Member

    Sep 7, 2011
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    i sorted these problems ....but now the problem that i face is that the & seven segment display flickers alot......and when set to 60hz the counter keeps getting stuck at each digit(i inspected and saw that the clock keeps getting stuck at either high or low after each alternate cycle)....am attaching a pic of the circuit hope that someone helps me out.....
     
  16. SgtWookie

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    Jul 17, 2007
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    You are operating 74 series TTL IC's on 12v. They are only designed to operate on 5v, ±0.5v. If you wish to operate the circuit on 12v, you must change to 4000-series CMOS ICs.

    You do not have any current limiting resistors on any of your LEDs. This is causing very high currents on your outputs.
     
  17. darklord_V

    Thread Starter New Member

    Sep 7, 2011
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    hmm but the problem is with the flip flops...i changed the v to 5volts and from a to g gave the leds 100ohm resistors...but now can barely see the numbers...they barely blink on the display...and the number 1 shows up pretty clearly.

    here is a circuit from my friend which counts alright.... the differnece is i think in the RA and R2 values .......and he is using 555_virtual while i am using the LM555CM....(but these dont matter)
    what is wrong with my circuit.....???
     
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    Last edited: Sep 19, 2011
  18. Wendy

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    You still haven't tied the logic levels on the inputs down.
     
  19. darklord_V

    Thread Starter New Member

    Sep 7, 2011
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    i grounded those two logic levels too.....now 1,4,7 are displayed properly but the 2,3,5,6 are not displayed...they just blink and vanish
     
    Last edited: Sep 21, 2011
  20. SgtWookie

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    Jul 17, 2007
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    Sounds to me like you could have a short in the 'd' segment wiring.
     
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