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555 continuous timer

Discussion in 'General Electronics Discussion' started by bigkim100, Jun 12, 2021.

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  1. bigkim100

    bigkim100

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    Apr 17, 2013
    I have built this continuous cycle timer, (schematic attached) and it works, but it doesnt seem to have much range.

    I need the 2 cycles (ON/OFF) to be around 1 to about 5 seconds, (ON/OFF Cycles to be adjusted individually) but as it is, it seems to only have timing cycles of a second, or less, or so..

    Whatcha think, and thanks in advance
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Change the value of this capacitor for other times:
    upload_2021-6-12_17-35-19.png
    Use any of the many "555 online calculator" tools to find which values are suitable. as the 100 µF give you around 1s, use e.g. 1 470 µF capacitor to get at ~ 5 s.
     
  3. bigkim100

    bigkim100

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    Apr 17, 2013
    For better control of the ON/OFF cycles should I use a circuit that uses 2-555's?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Not necessarily. It makes the circuit more complex. If the one you have performs satisfactorily, there is no need for a more complex circuit.
     
  5. bigkim100

    bigkim100

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    Apr 17, 2013
    Im finding that the Led attached to ground, does not turn off, and that the circuit is very unstable, any ideas?
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Are the 3.3 kΩ resistors for the LEDs correctly assembled? Does the relay turn off when the red LED is on?
    Please specify what you mean by 'unstable'.
     
  7. bertus

    bertus Moderator

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    Nov 8, 2019
    Hello,

    I would connect the ends of the pots too:
    MTM timer schematic comment.png
    Bertus
     
  8. AnalogKid

    AnalogKid

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    Jun 10, 2015
    The problem is that there are three errors in the schematic, and one of them is critical.

    As with all standard 555 astable circuits, the two timing resistors interact; without adding diodes, you cannot have one be on time only and the other be off time only. Without diodes, the on time always will be equal to or greater than the off time, never less. There is a circuit in the CMOS 555 datasheet that can be modified with two diodes for much better performance.

    Note: the two pots are labeled incorrectly. That's two small errors.

    The big error is that the 1N4001 is backwards. As shown, it bypasses the lower pot curing the discharge part of a cycle, so the lower pot has almost no effect on the output frequency.

    If you reverse the 1N4001 diode, you will be close. With the diode reversed, the cap will charge up through the upper pot while the lower pot is mostly bypassed by the diode. This is the ON time adjustment. As is normal for a 555 astable, the lower pot will adjust the OFF time without interference from the upper pot.

    Increase the two 100 ohm resistors to 1 K each. This will protect the 555 Discharge output from excessive current if the pots are set to 0 ohms accidentally.

    ak
     
    Last edited: Jun 12, 2021
    Harald Kapp likes this.
  9. AnalogKid

    AnalogKid

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    Here is a circuit similar to yours, with the diode positioned correctly. I'm searching for a schematic with a variation of the 555 astable, with two timing resistors and two diodes for complete isolation between the adjustments. There is a common variation with two diodes feeding one pot, which makes the pot a duty cycle adjustment with a fixed frequency, but that's not what you have described.

    ak

    [​IMG]
     
    Last edited: Jun 12, 2021
  10. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Here is a fully adjustable version. R3 limits the minimum waveform period to approx 1 s. Two diodes make the two output phase width adjustments completely independent.

    ak
    555-Hyst-Adj-1-c.gif
     
    bertus likes this.
  11. bigkim100

    bigkim100

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    Apr 17, 2013
    Thank nyou
    Thank you so much Analogue, I do appreciate it
     
  12. bigkim100

    bigkim100

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    Apr 17, 2013
    I cannot get this one to work either...Im sorry
     
  13. bertus

    bertus Moderator

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    Nov 8, 2019
    Hello,

    Is pin 8 also connected to the powersupply.
    That connection seems to be missing in the schematic @AnalogKid posted.

    Bertus
     
  14. AnalogKid

    AnalogKid

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    Jun 10, 2015
    My schematic software does not show power and ground connections for common components. This removes a lot of clutter from the drawing, but can be confusing until you are used to it.

    C2 is the power supply decoupling capacitor, and connects to pins 1 and 8 with the shortest possible leads. It is shown connected to pin 8 only because that is a convenient tie point in the drawing.

    The connection from Vcc to pin 4 is a signal connection to disable the Reset function. The Vcc power pin is pin 8.

    Pin 7 (Discharge) is left floating on purpose. It is not used, and is terminated internally so it does not need a pull up/down resistor.

    Something not mentioned before (and now I cannot edit/update my original answer) concerns the equations for the high and low output pulse widths. The data sheet equations are not accurate for the circuit in post #1. This is because the diode - when connected correctly as in posts #8 and #9, does not complete short out the lower timing resistor (Rb on the datasheet) during the charge portion of the cycle. The diode's forward voltage (Vf) affects how long it takes for the capacitor to charge up to the Threshold input transition voltage, and there is a small current through the resistor in parallel with the diode that varies with the resistor value. This means the two adjustments are not completely independent of each other.

    In my circuit (post #10), the two diodes mean that both the charge and discharge portion of a cycle use the discharge equation in the datasheet. Also, that equation will be slightly incorrect because of the diode Vf. However, the value shift will be identical for both half-cycles of the output waveform, AND they will be truly independent of each other.

    ak
     
  15. bigkim100

    bigkim100

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    Apr 17, 2013
    Ok, so lets say that the on cycles and oss cycles are to be between 1 and 5 seconds?
    and is 1 ground and 8 positive??
    sorry about this
     
  16. bigkim100

    bigkim100

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    Apr 17, 2013
    sorry, I meant 1 and 5 to ground 8 and 4 positive????
     
  17. bertus

    bertus Moderator

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    Nov 8, 2019
    Hello,

    Pin 1 should be ground.
    Pin 5 can have a capacitor to ground, but can not be grounded directly.
    (see also post #9).

    Bertus
     
  18. AnalogKid

    AnalogKid

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    Jun 10, 2015
    What does the datasheet say?

    ak
     
  19. bigkim100

    bigkim100

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    0
    Apr 17, 2013
    Ok, got it working
     
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