555 frequency issues

Discussion in 'General Electronics Chat' started by nzemke, May 12, 2012.

  1. nzemke

    Thread Starter New Member

    May 12, 2012
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    I have been looking around everywhere trying to figure out what I did wrong with my calculations and I'm hoping that you can help me.

    I'm trying to create a square wave oscillator at a frequency around 31,000 hz. I put together a little excel spread sheet to do the calculations for me. I took the calculations from the http://www.kpsec.freeuk.com/555timer.htm website. The calculations are as follows:
    F = (1.4/((R1 + (2 * R2)) * C1))
    R1 = 10000
    R2 = 17700
    C1 = 0.001 uf (102 cap)
    I have this setup on a breadboard and it works but it gives me a frequency of 11.8Khz.
    One question I have is if the C1 has to be an Electrolytic or if you can use a ceramic?
    Really confused and hoping that someone can point out my error.
     
  2. Wendy

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    Mar 24, 2008
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  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I don't know what your problem is, but it is probably either a measurement error or a component value error.
    I don't think you will find a 1nF electrolytic capacitor. 100nF is the lowest value i have seen.
    What kind of cap are you using, i.e., ceramic, mica, polyester, etc.?
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The circuit below works for me.

    [​IMG]


    Change C1, R1, and R2 to the values shown in the following table.


    [​IMG]
     
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  5. MrChips

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    Oct 2, 2009
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    The 555 formulas are guidelines only. I would not be too concerned if you do not get exact results. Remember, your capacitance can be off by as much as 20%.

    You should not use electrolytics for the timing capacitor, C1, if timing is critical.

    Try to reduce R2 to around 10K.
     
  6. Ron H

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    11.8kHz vs 31kHz is not just a component tolerance problem.
     
  7. MrChips

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    Geez Ron, somehow I suspected you will notice this.
     
  8. nzemke

    Thread Starter New Member

    May 12, 2012
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    Below is the circuit that I'm using.
    [​IMG][​IMG]
     
  9. nzemke

    Thread Starter New Member

    May 12, 2012
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    Tracecom, I'll put together that circuit this afternoon and see if I'm getting a better result.
    Thanks everyone for the assistance!
     
  10. MrChips

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    Oct 2, 2009
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    What are the colors on those resistors? They don't seem right.
     
  11. nzemke

    Thread Starter New Member

    May 12, 2012
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    Red-Purple-Red
    brown-green-orange
    brown-black-orange
    [​IMG]
     
  12. nzemke

    Thread Starter New Member

    May 12, 2012
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    In case anyone was interested I just attached the excel spreadsheet I was using to calculate everything.
     
    • 555.zip
      File size:
      22.6 KB
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      19
  13. tracecom

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    Here's a photo of my layout.
     
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  14. Wendy

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    Mar 24, 2008
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    [​IMG]

    [​IMG]

    R1 is a 10KΩ, R2 is a 2.7KΩ (red violet red). Is it in parallel with the 15KΩ? I see no 17.7KΩ resistor. C2 is for noise control, you can remove it in a small circuit with no effect what so ever.
     
  15. nzemke

    Thread Starter New Member

    May 12, 2012
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    I have them running in series.
     
  16. nzemke

    Thread Starter New Member

    May 12, 2012
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    tracecom, I noticed that you are using the ne555n chip and I'm using the ne555p, will this cause issues?
     
  17. tracecom

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    Apr 16, 2010
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    No, the two are functionally identical.

    BTW, if you don't need a reset function, you can eliminate SW1 and R3 in my schematic and tie pin 4 directly to Vcc.
     
  18. Wendy

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    Mar 24, 2008
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    I find it interesting the OP is getting 11.8Khz, and should be getting 5.04Khz. It is a X2 difference. I am really suspicious of instrumentation error, the frequency counter. Have you looked at the waveforms with an oscope? I'm thinking it is that or a hard to spot wiring error. 555's are extremely predictable.
     
  19. Ron H

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    Bill, I think you have combined two posts. In post #1, he says he is expecting around 31kHz, and is getting 11.8kHz. There is nothing wrong with his equation, and if you plug in the component values, you get 30.8kHz. A sim gave me 31.5kHz.
     
  20. takao21203

    Distinguished Member

    Apr 28, 2012
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    w/o transistor this circuit will readily blink.

    I changed components:

    2 diodes from a television PCB, don't know 1n4148 or whatever
    68K/1uF -> fast blinking, 10Hz
    5K adjustable resistor not 2x 220 Ohms

    Is there any reason why you need to calculate frequency?

    Just any kind of capacitor will do.
     
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