555 Astable w Diminishing Frequency

Discussion in 'The Projects Forum' started by allenpitts, Apr 13, 2015.

  1. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    77
    7
    Hello AAC forum,
    Have breadboarded and PCB'd the venerable 555 for several
    different set ups. Used the 555 in mono stable mode as a
    delay on and delay off timer. And have employed it in astable
    mode as a oscilator to clock a 4017 decade counter at 1 hz and
    in the audible range, 440 hz to produce a beep.
    Found a variation of the astable that is supposed to pulse quickly
    and slow down until it stops.
    http://www.doctronics.co.uk/555.htm#astable
    [​IMG]
    Have found another version at
    http://en.wikibooks.org/wiki/Practical_Electronics/Astable/Diminishing_Frequency
    [​IMG]
    Have bread boarded both of these with little or no luck.
    So I tried reworking the schematic from Practical Electronics to make it simpler.
    [​IMG]
    This only served to reinforce that except for the 47uF capacitor between source and
    ground, called a decoupling cap, the two circuits are virtually identical.
    Have tried changing out the IC and have bread boarded both several times.
    Also have tried with and with out the grounding cap from pin 5.

    What happens is first the indicator LED comes on before the momentary
    normally off switch is engaged which was not expected. Then sometimes the LED
    stays on after the switch is pressed and sometimes it will blink once after a s
    econd or two and stays on indefinitely.

    Started over from scratch a fifth time and tried the breadboard version shown
    in the Doctronics site but is hard to read so did a fritz of the board as I
    laid it out.
    [​IMG]
    Beginning to think there is something wrong with the design.
    Any ideas?
    Thanks.
    Allen in Dallas
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Could your switch be a momentary switch?
    You could hook the top 470k to +9 volts and it should run as an astable.
     
  3. ebeowulf17

    Active Member

    Aug 12, 2014
    678
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    Any chance the polarity on the electrolytic cap is reversed? I fully admit that I don't understand the circuit well enough to know if that would cause the problem you're describing, but it seems like the sort of mistake that would be easy to make and then overlook (at least for me.)
     
  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    connect pin 4 to C1 +, this will keep the led off until it oscillates and stay off when the button is released.
     
  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The astable circuit works as drawn, but the output LED indicator polarity is drawn wrong. Connect the cathode to pin 3 and the anode to +V (with a current limiting resistor.) Note that after you push the start button, the LED will flash too fast for the human eye to detect, but give it a few seconds to slow down.
     
  6. ebeowulf17

    Active Member

    Aug 12, 2014
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    Isn't the 555 output push-pull, in which case the effect would be seen regardless of whether you used the 555's high or low pulses as the LED's on time? Granted, the duty cycle would be reversed, but you would still see cycling on the output, right?
     
  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The 555 will indeed source or sink current and will try to oscillate regardless of the polarity orientation of the LED on pin 3. However, in this circuit, pin 3 goes high as soon as power is applied, thus masking most of the dimming if the LED is connected to light on the source half of the 555 cycle. Reverse the polarity of the LED so that it lights on the sink half of the 555 cycle, and the dimming becomes visible.
     
    Last edited: Apr 14, 2015
  8. ebeowulf17

    Active Member

    Aug 12, 2014
    678
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    Interesting, thanks for the explanation.

    I'm really intrigued by this circuit, even though I don't really have a use for it. Might rummage through my leftover parts and see if I have enough to breadboard it...
     
  9. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    77
    7
    Hello ebeowulf17, dodgyDave, tracecom and the AAC forum,

    I think I figured it out. I found an application called Yenka that allows one
    to simulate a breadboard and graph the out put of the circuit.
    [​IMG]
    The graph shows the majority of the cycle is HIGH so it looks like the LED is on all the time.
    The time the LED is LOW or off is so brief that one cannot see the blink.
    So am working to get the LOW time equal to the HIGH time.
    Not having much luck after experimenting with circuit for over an hour this
    is the best I have been able to come up with.
    [​IMG]
    This combination of component values results in two pulses of .15 seconds about a half a second after the
    momentary switch is pressed and then LED stays on and is stable HIGH.
    The text at
    http://www.doctronics.co.uk/555.htm#astable
    (One has to scroll down in the article to the section marked
    "4.3 Diminishing frequency astable.")
    says if the value for R3 is greater than the value of R2 then the
    LOW time will be greater than the HIGH time. But have only been able
    to get to low pulses out of it.

    The text with the schematic says;
    "When the 'go' button is pressed, the 47 µF capacitor in parallel with the timing network, R1, R2 (Allen: my R2 and R3) and C, charges up very quickly through the 100 Ω resistor. When the button is released, the astable continues to oscillate but the charge stored slowly leaks away, with the result that it takes longer and longer to charge up the timing capacitor. To trigger the next pulse, the voltage across C (Allen: the cap between the pins 6 and 2 and ground) must increase to two thirds of the power supply voltage. After a while, the voltage across the 47 μF drops below this value and the pulses stop.
    With the values shown, the initial frequency is about 100 Hz and the output pulses coast to a stop after around 40 seconds:"

    So it would seem that decreasing the value of R1 (R2 in the black and white Yenka above) to 100k ohms should increase the
    LOW time but with exception of two initial low pulses the circuit stay stable in a HIGH state.

    Will continue to experiment but could use some ideas on why it does what it does and how to get
    the duty cycle to 50%

    Thanks.

    Allen in Dallas
     
    Last edited: Apr 14, 2015
  10. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Did you try what I posted in #6?
     
  11. ebeowulf17

    Active Member

    Aug 12, 2014
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    Yeah, everything you've found fits perfectly with tracecom's suggestion. You may still need to tweak component values for the timing you want, but switching the LED wiring seems like it would get you to where you're seeing the effect more clearly.
     
  12. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
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    R2 = R3. diode connected across R3, cathode end connected to junction of R3 and C2.
     
  13. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    With the components shown (R1 & R2 = 470k, C1 = .01uF,) the frequency calculates to just over 102Hz with pin 3 high 66.7% of each cycle. An LED connected with the anode to pin 3 will appear to be on continuously.

    To make the circuit's operation visible with the LED's anode connected to pin 3, change C1 to .1uF which will reduce the frequency to 10.2Hz. The LED will come on when power is applied, and when the button is pressed it will flash with decreasing frequency, until the flashing stops and the LED is on continuously (until the power is turned off.)

    Changing the duty cycle from 66.7% to 50% will have no significant effect on the appearance of the flashing.
     
    Last edited: Apr 14, 2015
  14. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi Allen,

    Circuit works as announced, but change C2 to eg. 220nF to slow it down to a visible flash (about 4.6Hz down to 1Hz.
    On time will get gradually longer, while off time will be fairly even at around 70ms - perhaps twinkling describes it better than flashing.

    An approximate 50% duty cycle won't be a problem, but I think you should verify that it works as is and first then change it further.

    Does it have a specific purpose, or is it just to get intimate with the little bugger?
    Do you have a preferred start and end frequency?
     
  15. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    77
    7
    Hello AAC forum,

    Finally got the system to work.
    Thanks to tracecom, ebeowulf, soren, dodgydave, and brownout for
    their advice.
    Reversing the polarity of the LED was the key.

    Thanks.

    Allen in Dallas
     
    ebeowulf17 likes this.
  16. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    It probably looses in translation, but here in DK, everyone would have shouted "screw up beer" probably in a very chanting and repeated way :D
     
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