555 astable problem

Discussion in 'The Projects Forum' started by imperatormk, Mar 7, 2011.

  1. imperatormk

    Thread Starter Member

    Mar 1, 2011
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    Here is the deal. Recently I built a *simple* circuit with a 555 timer using this scheme:

    http://www.google.com/imgres?imgurl...=7PV0TcK-IIr3sgao9JyEDg&sqi=2&ved=0CBYQ9QEwAA

    However, when I connect the red probe of my voltmeter on pin 3 and the black probe on GND, I get nothing but constant 0.8V. My power supply is ~12V. I have examined the connections and I can`t find anything suspicious.

    R1 = 10K
    R2 = 390K
    C1 = 4.7μF

    Any advice appreciated.
     
    Last edited: Mar 7, 2011
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    A volt meter will not show you the output pulse. At best it will vary the display so it does not settle on a single number. In your case it is showing some average voltage value due to its sampling rate being slower than the output square wave pulse.

    You really need an o'scope to see the output waveform.
     
    imperatormk likes this.
  3. imperatormk

    Thread Starter Member

    Mar 1, 2011
    96
    2
    OK, that is correct. But, I tried with a LED diode and the result was not good (the LED diode wouldn`t flash). Note: The diode has been connected in the right direction (polarity matters).

    I am really not sure what can be wrong.
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Without seeing the schematic or seeing the circuit you breadboarded we are reduced to guessing. Not a very helpful thing - guessing

    What did your simulation indicate as the period(inverse of freq) for the output waveform?

    A short period would be invisible to the human eye.
     
  5. imperatormk

    Thread Starter Member

    Mar 1, 2011
    96
    2
    Here is the schematic:

    http://www.google.com/imgres?imgurl=http://www.m0ukd.com/Calculators/555_timer_calculator/555astable.gif&imgrefurl=http://www.m0ukd.com/Calculators/555_timer_calculator/&h=297&w=370&sz=3&tbnid=rv_ikz9DQ3UxWM:&tbnh=98&tbnw=122&prev=/images%3Fq%3D555%2Bastable&zoom=1&q=555+astable&hl=en&usg=__Dm7-Kwv0Lbkhe3WqqF0d9Ov-UO8=&sa=X&ei=7PV0TcK-IIr3sgao9JyEDg&sqi=2&ved=0CBYQ9QEwAA

    As by the online calculator, the HIGH period should be 1.5s (the LOW period as well).

    Unfortunately, I can`t post a pic of the circuit, but I think it wouldn't reveal much since everything looks fine and a 800x600 picture taken with cheap camera would not be the key here.

    So, if there isn`t anything certain that I should check out (i.e. connection between a IC pin and some element), I guess starting all over would be the best shot.

    P.S. Can measuring the voltage between each IC pin and ground (or something else) help here?
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    What are the component values? That design can approach 50% duty cycle quite close, but only if R1 is quite low. There is a danger here if R1 is too low in value. What is the current limiting resistor for the LED?
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The component values and schematic are in the first post. The frequency should be low enough to see on an LED, and maybe even on a voltmeter. I'm guessing that our OP has either a wiring error or a wrong component value(s).
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    OK, missed that, that leaves the LED resistor. A resistor is required for the LED.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    http://forum.allaboutcircuits.com/blog.php?b=381

    [​IMG]

    R1 = 10K
    R2 = 390K
    C1 = 4.7μF

    F = 0.388 Hz, or 2.6 seconds
    Duty Cycle = 50.6%

    At such a slow rate you should see the 555 switch on the DVM (though there isn't time for it to settle). Ron is probably right about a simple wiring error. Is the 555 getting warm or hot?

    A similar experiment, note how the LEDs are connected.

    555 Hysteretic Oscillator
     
  10. imperatormk

    Thread Starter Member

    Mar 1, 2011
    96
    2
    Although this post is rather old, I feel that I need to share the solution.

    Everything was actually connected well and all, but somehow instead of NE555
    I used 741 op amp. Pretty simple, huh?
     
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Funny, almost that same exact thing happened to someone else on the forums, too. ;)
     
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