555 Astable

Discussion in 'General Electronics Chat' started by Kingsparks, Mar 10, 2013.

  1. Kingsparks

    Thread Starter Member

    May 17, 2011
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    5
    Good morning everyone or whatever it is when this gets read.
    It has been a while since I put together a 555 astable for anything. I threw this together today as a clock for another part of my project and as such I guess it will work, well, I know it will since all I need is a positive going pulse.

    What has me puzzled is the waveform. I more or less expected a square wave and closer to a 50% duty cycle with the components selected for R1 and R2.

    Values are as marked on the schematic and the pot is as close to 5K as I could set it.
    Scope time base is .2us per division. Vertical gain at 0.2V per div. That is the other point. I was expecting around 6 to 8 volts output, not less than one. The scope is an older type 422 Tektronix and working fine. I have had it since I closed my business and yes, it's old but then so am I

    I forgot, scope is connected across R2.


    Picture is on the second page of the PDF.

    Would anyone be familiar enough with 555 to comment on this? I have used them before and thought I knew what to expect. Oh, also I did change the 555 just to be sure. Same thing. Suggestions please. I can use this, just not what I wanted or expected.

    Okay, I got it but it was weird. Maybe a half dozen times in my 40 years of service I have run across bad capacitors. (I mean new ones I installed.) That was it, or more then likely, just marked wrong. Tried a different capacitor and now have a nice square wave at almost 12V. I'll put that down to learning and should have tried a different cap first.
     
    Last edited: Mar 10, 2013
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,980
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    Kingsparks likes this.
  3. Kingsparks

    Thread Starter Member

    May 17, 2011
    118
    5
    Thanks Dodgydave.

    I have seen that circuit before, a much simpler circuit, fewer components. Never used it for some reason, have you? Can you adjust the frequency with a pot, say using a fixed resistor to maintain a minimum resistance between pin 3 and 2/6 connection?

    All I need is a relative stable and adjustable clock.

    Roland
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    I use it all the time , i never use pin 7 its a lot easier.
     
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  5. Kingsparks

    Thread Starter Member

    May 17, 2011
    118
    5
    I'll have to give it a shot, I'm in breadboard mode right now anyhow.

    Roland
     
  6. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    You can in series with a 1K between 5k pot and Pin 7, it will give you 50%/50% duty cycle, but it will also reduce some cycles that you can't be adjust.
     
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