555 Astable help Please

Discussion in 'The Projects Forum' started by liteace, Mar 25, 2013.

  1. liteace

    Thread Starter Member

    Mar 7, 2012
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    Hi All, Im bagging my head on the wall here and need help. What I would like is to set up a 555 in astable mode, I have looked at 555 Astable Circuit Calculator but just cant get my head round it.
    These dont have to be exact but what I would like is:

    Frequency 900Mhz
    Duty cycle 90%
    Cycle time 1.25 second
    Time high 1 s
    Time low 1.25 ms

    Can this be done, If so would someone please help me adding the correct value of Cap & the 2 resistors

    Thanks in advance
     
  2. w2aew

    Member

    Jan 3, 2012
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    You'll have to be clearer about what you want. The numbers you posted don't make any sense. High time plus low time should equal cycle time, and yours doesn't. High time divided by cycle time would give you duty cycle, but your numbers don't jive. And none of these numbers relates at all to 900MHz frequency. By the way, the 555 can't get anywhere close to 900MHz.
     
  3. tubeguy

    Well-Known Member

    Nov 3, 2012
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    You can't get 900MHZ from a 555 timer.

    Do you really need 900MHZ and what is it for?
     
  4. liteace

    Thread Starter Member

    Mar 7, 2012
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    What I need to do is heat a nichrome wire to keep it at a constant temperature and stop it burning out, Im just starting out at this so maybe Ive got my figures wrong
     
  5. liteace

    Thread Starter Member

    Mar 7, 2012
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    Sorry exact times are:

    High time 980.0ms
    Low time 120.0ms
    Cycle time 1.10 s

    does that make any sense
     
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    This will get you close.

    NE555N Astable Multivibrator

    R1 = 2700 Ohms
    R2 = 330 Ohms
    C1 = 470 microFarads
    Frequency = 0.91191 Hz
    Duty cycle (% on) = 90.18
     
    Last edited: Mar 25, 2013
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  7. liteace

    Thread Starter Member

    Mar 7, 2012
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    Thanks that worked, I dont think I can get it any better than that. I have here a HCF4541 as advised by member Antonv, what I need to do with this is have a 1 shot 5 min timer, any advise on how I go about that

    Thanks

    [​IMG]
     
  8. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    He was asking for 900MHz, not mHz, wasn't he?

    EDIT: Sorry, you're right. mHz--1/cycle time = 0.909 Hz.
     
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    You are welcome.

    I have never used an HCF4541, but the datsheet has some application information. You can download it from the ST web site.

    Unless I needed very precise time, I would just use another 555 for a 5 minute one-shot.
     
  10. liteace

    Thread Starter Member

    Mar 7, 2012
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    Thanks, So Im now looking for the 555 in Monostable mode, have I done this right:

    Capacitor C, 47f
    Resistor R , 6ohm

    = 310 secs /60 = 5.1 mis ?

    Thanks again
     
    Last edited: Mar 26, 2013
  11. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    [​IMG]

    Basic one-shot (monostable) 555 timer setup with trigger and reset switches.
     
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  12. liteace

    Thread Starter Member

    Mar 7, 2012
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    Thanks, so the re-set stops it untill the trigger is pressed again or does it restart it.
    Pin 3, output goes where on the astable circuit ?

    Thanks again
     
  13. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    That's right mathematically, but I don't know where you are going to get a 47F capacitor. :D

    A more realistic selection would be 270k for R1 and 1000μF for C1, which calculates to 297 seconds. Of course, component tolerances will change the actual time. You can adjust the time by using a 220k resistor and a 100k pot in series.
     
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  14. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    If you have it set for a long output pulse, the reset would put it back to the starting state. But if you let the pulse time out on its own, pressing the trigger button again will re-activate the output.

    Pin 3 is the output. It's where your signal is emitted from.
     
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  15. liteace

    Thread Starter Member

    Mar 7, 2012
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    Thanks, I dont need it to reset. Sorry Im a bit lost on pin 3, is this going before, like 1st in line before the astable circuit, is pin 3 power ??

    Thanks
     
  16. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    If you don't need the reset just connect it to +. This will hold it high, so it won't reset while in the floating state.

    In your astable circuit you had an output. That's where your pulse came from. That's where you would measure the output frequency and duty cycle. It doesn't change for the monostable. Your output (pin 3) is where you measure the output pulse. When pin 3 is high, it is putting out a positive output (the positive part of the pulse). When it's low, the pulse is low. It's where your output signal appears. I don't think I can make that any clearer.
     
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  17. liteace

    Thread Starter Member

    Mar 7, 2012
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    Yes sorry long day, as Im new to all this it takes its time to sink in, so when i scope Pin 3 Im going to have 12v for the set time & that is my 12v switch feed for the astable circuit :confused:

    Thanks
     
  18. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    I'm not sure what the output voltage would be, I imagine if you have a 12v input voltage (power to the chip), the output would be closer to 8 or 9 v (was thinking 2/3 the source voltage, but I could be thinking something else). Not quite sure what you mean about "12v switch feed", but yes--connect your o-scope to the output (pin 3) of the 555 timer and you'll see the output pulse "ON" for a set amount of time, then go "OFF".
     
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  19. antonv

    Member

    Nov 27, 2012
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    Here is a circuit with the 14541 in it set up for about 5 minutes.
     
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  20. liteace

    Thread Starter Member

    Mar 7, 2012
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    Thank you very much for the circuit, I have ordered the other parts I need. Question R15 between Q1 & U3 is that 15 ohm or 15 K &
    also I dont understand R3 & R4 "0805" do I use the resitors that I already have in the 555 curcuit & just 1 more thing if i replaced SW1 with a SPST would that damage anything ?

    Thanks Again
     
    Last edited: Mar 28, 2013
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