555 timer

Discussion in 'The Projects Forum' started by GirishC, Jan 23, 2009.

  1. GirishC

    Thread Starter Active Member

    Jan 23, 2009
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    Hi

    I am currently using LCM555 timer for generating variable frequency based on relative Humidity (RH) of surrounding. The LCM555 is configured as astable multivibrator and centered at 20KHz. RA = 24K, RB = 200K and C is capacitive sensor having value of 160pF at 30C, 0%RH. The C is given by formula

    C = C0*[1 + HC0*RH]

    C0 = 160pF at 30C, 30%
    HC0 = 3420ppm/%C
    RH = % relative humidity

    temperature derating factor is
    dC= - 0.0019*(T-30) pF
    T = temperature in C

    When I sweep the RH from 0% to somewhere 45% my 555 timer stops working. I have to replace the LCM555 timer and keep RH below 45%. I have similar circuit with RA = 40.2K, RB = 360K and C is 105pF and centered at frequency 18KHz and it works very fine till 98%.

    When I talk to National about this problem, I received a feedback saying I can not use this chip beyond 5.6KHz (max), according to datasheet. I have queries based on my experiment

    1. How can upper limit being 5.6KHz is possible. We have many application where 555 used goes much above 100KHz.
    2. What causes failure of LCM555 and why it works in other configuration? I have verified the current requirement of discharge pin and trigger pin. What am I missing?
    3. What is the solution to my problem.

    Could anyone through light on this?

    Girish
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    Do you have the datasheet of the RH sensor ?
    Does it function from 0 to 100 % or is there a limitation ?

    Greetings,
    Bertus
     
  3. GirishC

    Thread Starter Active Member

    Jan 23, 2009
    58
    0
    Hello

    Yes, I do have datasheet for the sensor. You can google it for HC105 and you will get it. Yes the sensor do work from 0% to 100% RH. As such there is no limitation mentioned on the datasheet.
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    I found the data sheet. The limitation is above 60 °C.
    In the datasheet is said the optimum working frequency is 20 khz.
    Can you post a schematic of the 555 circuits you made ?

    Here is also an other idea from EDN for an oscillator with a humidity sensor :

    [​IMG]

    Greetings,
    Bertus
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Did you mean LMC555 timer?
    Please post your schematics. Use the "Go Advanced" button, then "Manage Attachments". .png format files are preferred. .jpg images are too lossy.
    I have a feeling that you're using pin 7, with a low-value resistor to Vdd. Excess current will burn up the timer.
    If it's an LMC555, this is incorrect. It can approach 3MHz.
    5.6kHz is quite do-able. 5.6MHz is out of the question.
    You need to post schematics of your circuit(s), exactly as they are built.
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I see a little detail in the datasheet :

    [​IMG]

    Maximum DC < 5 mV.

    Greetings,
    Bertus
     
  7. GirishC

    Thread Starter Active Member

    Jan 23, 2009
    58
    0
    You are right, above 60C I can not take RH to 100%. But you know what, it fails at 45%, 30C. Thanks for the circuit idea. Here is the circuit
    [​IMG]
    It seems I am unable to upload the schematic...but for your refernce its a normal astable with 24K and 200K resistors.

    Girish

     
  8. GirishC

    Thread Starter Active Member

    Jan 23, 2009
    58
    0
    Here is the schematic...
     
  9. GirishC

    Thread Starter Active Member

    Jan 23, 2009
    58
    0
     
  10. GirishC

    Thread Starter Active Member

    Jan 23, 2009
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    So do you mean, I have to add a series resistor with capacitor to limit DC voltage to 5mV?
     
  11. GirishC

    Thread Starter Active Member

    Jan 23, 2009
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    The sensor range is can go to 98% but for me it fails at 45%. And national says I can run LMC555 till max 5.6KHz
     
  12. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I think that the sensor has to be used in a resonant circuit without DC on it.
    This could be an oscillator with a LC part with the sensor as the C.

    Greetings,
    Bertus
     
  13. GirishC

    Thread Starter Active Member

    Jan 23, 2009
    58
    0
    You are right, its LMC555
     
  14. GirishC

    Thread Starter Active Member

    Jan 23, 2009
    58
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    This sounds interesting...something different...I will have to give try for it...Its a huge change for me!
     
  15. bertus

    Administrator

    Apr 5, 2008
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  16. GirishC

    Thread Starter Active Member

    Jan 23, 2009
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    Thanks a lot! I appreciate your input.
     
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