Monostable 555 timer problem

Discussion in 'Digital Circuit Design' started by Quark., Mar 24, 2017.

  1. Quark.

    Thread Starter New Member

    Mar 24, 2017
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    So I tried to make a monostable circuit with the 555 timer, following this guide https://www.allaboutcircuits.com/tools/555-timer-monostable-circuit/, to turn on a led.

    I used a 2200 μf capacitor and a 2.4 kΩ resistor, so the led should stay on for 6 seconds, but when I connect the power (5V), the led is on and never turns off, even if I activate the trigger.

    What is wrong?
     
  2. dl324

    AAC Fanatic!

    Mar 30, 2015
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    Welcome to AAC!

    Post a schematic so we can see how you have things connected.
     
  3. ScottWang

    Moderator

    Aug 23, 2012
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    Try 100uF and 56K or 47uF and 120K.
    If you check the circuit wiring is correct and it still doesn't works then you have to use the CMOS type as LMC555, TLC555
     
  4. Quark.

    Thread Starter New Member

    Mar 24, 2017
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    I'm already using the TLC555, I will try with other resistor and capacitor values.
    Here's the schematic I'm using:
    555mono.png
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    What are the specs of the used led?
    The TLC555 can source only 15 mA, see the maximum ratings list:

    tlc555_max_ratings.png

    When the led has a voltage drop of 2 Volts and the supply is 5 Volts, you are expecting to have 30 mA in the led.

    Bertus
     
  6. ScottWang

    Moderator

    Aug 23, 2012
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    The principle is to use the capacitor more less and the resistor more big up to 10MΩ.
     
  7. dl324

    AAC Fanatic!

    Mar 30, 2015
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    Caps larger than approximately 100uF aren't recommended because they're too leaky and tolerance is typically 20% or more. This graph shows other combinations you can use to get 6 seconds:
    upload_2017-3-24_13-50-11.png
     
    absf likes this.
  8. eetech00

    Senior Member

    Jun 8, 2013
    1,474
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    Hi

    You can use 1.1 x R x C to calculate an approximate delay

    1.1 x 820k x 6.8uF = 6.134 seconds

    See attached.

    eT
     
  9. crutschow

    Expert

    Mar 14, 2008
    21,745
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    You forgot the pullup resistor for the push-button.
     
    absf likes this.
  10. EM Fields

    Active Member

    Jun 8, 2016
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    For a CMOS 555, Pin 2 should never be allowed to float.

    Pull it up to Vcc, quiescently, when its not being pulled low.
     
  11. Quark.

    Thread Starter New Member

    Mar 24, 2017
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    Thanks all for the suggestions, I don't have time now so I can't test them.
    But after a few tests, I dicovered that capacitor 1 never charges, it stays at arround 1.5v. If I disconect pin 7, the capacitor charges to 5v and the led turns off.
     
  12. jr_73

    New Member

    Jan 25, 2017
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    Yes, that cap is the main problem. I've even had 1 uF electrolytic caps that had such high ESR that they wouldn't charge. High-quality caps help immensely.

    Moderator's Note:
    The commercial does not allowed on this forum, please don't do that, otherwise you will get ban as spammer.
     
  13. dl324

    AAC Fanatic!

    Mar 30, 2015
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    The problem is that you don't have a pull up on the trigger input (as pointed out in post #9) so it can rise above 1/3 VCC, so the output stays high.
     
  14. Quark.

    Thread Starter New Member

    Mar 24, 2017
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    I added a pull up resistor and tried different combinations of capacitors and resistors and still didn't work.
    Is possible that I have a defective TLC555 or I broke it somehow?

    Also, I made a astable circuit and didn't work, however I made Schmitt trigger and it worked fine.
     
  15. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Do you still use the 100 Ohms resistor for the led?
    It should be higher than 330 Ohms to avoid overloading the TLC555.

    Bertus
     
  16. Quark.

    Thread Starter New Member

    Mar 24, 2017
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    No, I changed the resistor.
     
  17. dl324

    AAC Fanatic!

    Mar 30, 2015
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    The device is static sensitive, and you could have broken it. But if it works in some other circuit, damage is unlikely.
    Post the complete schematic for a circuit that didn't work and give us the relevant voltages in the circuit.
     
  18. EM Fields

    Active Member

    Jun 8, 2016
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    Possibly, the leakage current through the huge cap is hogging the current and not allowing the input to rise to a detectable level.
     
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