555 timing pattern changes with input voltage

Discussion in 'The Projects Forum' started by zjose86, Aug 6, 2010.

  1. zjose86

    Thread Starter New Member

    Aug 6, 2010
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    I have a square wave generator using a 555 timer and a duty cycle less than 50%. I want the output to be high for 1.5 seconds and low for 3 seconds. When I built the circuit I was using a +5V power supply, but I noticed that my output was only about 3.65V. This circuit was working perfectly fine, but I need output voltage to be at least 4.2V. I decided to use a +12V power supply with a trim pot at the output to get the 4.2V I wanted. The problem is that when I'm using the +12V power supply, my timing changes. I have an LED light and a stopwatch. When the +12V power supply is used, I see the light on for about 1.5 seconds and I see the light off for about 1.5 seconds as well. I then change the power supply to +5V without changing anything else, and the timing goes back to what I want, 1.5 seconds on, 3 seconds off.

    Does anyone have any idea what could be the problem? I thought time on and off was only dependent on the resistors and capacitors being used.

    Thanks in advanced for you time and help.
     
  2. tom66

    Senior Member

    May 9, 2009
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    It would help if you posted a circuit schematic.

    Also, if you use a CMOS 555, such as the TLC555, you can get it working up to +5V. The problem is, with the bipolar 555, the output is 1.4V (or so) less than the supply voltage, because the transistors drop 0.7V each.
     
  3. zjose86

    Thread Starter New Member

    Aug 6, 2010
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  4. tom66

    Senior Member

    May 9, 2009
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    I think the diode is causing the problem. How does the circuit work without it? Also, try putting a 47 microfarad capacitor across your supply terminals, as the 555 can cause significant glitches in the power supply, which will mess up other devices connected.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    A standard 555 will do it and be immune from power supply voltage. Tom is right, the diode is the problem.

    [​IMG]

    Of course, you want precision this is the way to go...

    [​IMG][​IMG]

    In this circuit the freq doesn't change, it is stable, while the duty cycle is a variable.

    LEDs, 555s, Flashers, and Light Chasers
     
  6. zjose86

    Thread Starter New Member

    Aug 6, 2010
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    So I took off the diode from the circuit and used both the +5V and +12V power supplies and the circuit worked in the same way. The LED light went on and off at the same time with the different power supplies. I put another diode in parallel with R2 and again I get the same problem. The timing of On and off is different with the different power supplies when the diode is used.
     
    Last edited: Aug 6, 2010
  7. tom66

    Senior Member

    May 9, 2009
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    The problem is, the diode is a nonlinear device, so it will change its response depending on the voltage; this changes the frequency. The circuit Bill posted will work over a variety of supplies.
     
  8. zjose86

    Thread Starter New Member

    Aug 6, 2010
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    I understand now.

    Thanks Tom and Bill
     
  9. marshallf3

    Well-Known Member

    Jul 26, 2010
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    That circuit, with the diode, is commonly used when you need to run a 555 out of its preferred duty cycle range by altering the charge vs discharge times of the timing capacitor. In effect it bypasses the bottom resistor during charge but unlike the RC part of the circuit the diode is going to have a set voltage drop so the 1/3 - 2/3 formula is altered.
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    A schottky diode, being a bit closer to ideal, will work better for this application. You will still have the drift, but it will be less.
     
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