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Timer circuit

Discussion in 'General Electronics Discussion' started by Karthik rajagopal, May 20, 2018.

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  1. Karthik rajagopal

    Karthik rajagopal

    238
    9
    May 9, 2016
    Hi all ,
    I have been building many projects where I face a common problem while using a 555 delay circuit. The thing that I have noticed is that every time I switch the circuit off and switch it on again I get a different time delay than expected I.e I get the time delay as per calculations only for the first time . So in order to do that , I had a reset switch which I used to control a relay to short the + end of the capacitor to ground via a 10 ohm resistor. As it thought this might be a crude way of doing this, Is there any other way I can get this stored charge to back to zero?


    Thanks in advance.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    If you are generating a pulse-train (i.e. the 555 is wired up in an astable circuit) then this is a feature.

    You could try increasing the frequency and using a divider to divide the output by 256. This will cause the output to have a 50% duty cycle which may be a problem.

    I've seen this used for long delays in an astable circuit too (the 55 is held reset when the divided output changes.

    If you can give us some more details we may be able to suggest something.
     
  3. WHONOES

    WHONOES

    1,154
    313
    May 20, 2017
    You could try using a CD4538. It is to monostables in one package is very simple to use and is reliable.
     
  4. Karthik rajagopal

    Karthik rajagopal

    238
    9
    May 9, 2016
    Thanks for the reply.
    If there is a lot to be changed, please suggest me a alternate circuit /IC that will replace that without that defect. By the way I don't know to program.
    Thanks
     
  5. Karthik rajagopal

    Karthik rajagopal

    238
    9
    May 9, 2016
    Thank you so much for the response.
    A humble request,if you can plz send me a circuit with CD 4538 that will not have this short coming.
    Thank you.
     
  6. AnalogKid

    AnalogKid

    2,505
    719
    Jun 10, 2015
    Please post your schematic.
    It might be possible to solve your problem with a circuit modification.
    Please post your schematic.
    Think about the question you are asking.
    Please post your schematic.

    ak
     
  7. WHONOES

    WHONOES

    1,154
    313
    May 20, 2017
    Have a look at the data sheet (see link below). There are quite a few variations on the nomenclature. the 14538 is the same as the 4538.
    http://www.ti.com/lit/ds/symlink/cd14538b.pdf
     
  8. Karthik rajagopal

    Karthik rajagopal

    238
    9
    May 9, 2016
    Thank you so much!!
     
  9. Bluejets

    Bluejets

    4,868
    1,017
    Oct 5, 2014
    I remember using an addition to the 555 circuit to give the same time at startup but will have to find it.
     
  10. AnalogKid

    AnalogKid

    2,505
    719
    Jun 10, 2015
    Yes, there are two or three modifications to the standard timer circuits that will reduce this problem, but without knowing the circuit he is using it is just random speculation.

    ak
     
  11. Karthik rajagopal

    Karthik rajagopal

    238
    9
    May 9, 2016
    Sorry for the late response.
    I have attached the circuit in my reply.
    This is the circuit to monitor the heat rate ( it will display he number of beats per minute). So I used this 555 circuit to turn the output on after a delay of 1 minute.
    In this circuit you can notice that I have connected the capacitor to the DPDT relay. This will disconnect the capacitors connection from the positive terminal and discharge the capacitor via a resistor (resistor connected to ground)when the reset button is pressed.
     

    Attached Files:

  12. AnalogKid

    AnalogKid

    2,505
    719
    Jun 10, 2015
    There is nothing wrong with your reset approach. Just because it is not all solid state, that doesn't make it "crude". If it already is built and works, there is no automatic reason (inability to get parts, long term reliability, etc.) to change it.

    BUT - that relatively gigantic timing capacitor's value will drift with both ambient temperature and aging. A more stable monostable approach is a high frequency oscillator and a divider. With a much smaller, 1%, metalized film capacitor, the circuit would be much more stable. With a crystal it would not need calibration. Look into the CD4060 and CD4521.

    And, only because someone will say it, a PIC, arduino, etc. can do both the timing and the counting, reducing the other digital electronics to nothing more than a display driver.

    ak
     
  13. Karthik rajagopal

    Karthik rajagopal

    238
    9
    May 9, 2016
    Thank you so much for helping me .
     
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