How to make 555 astable osc between V and 1/3V

Discussion in 'General Electronics Chat' started by doughboy, Dec 5, 2010.

  1. doughboy

    Thread Starter New Member

    Dec 5, 2010
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    Hello,

    I have a basic 555 astable oscilator oscillating between V (or whatever the highest output pin 3 has), and 0.

    What I need is for the output to oscillate between V and say 1/3 of V instead of 0. preferably if the fraction can be adjustable from 0 to 100%.

    thanks for the help!
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    What do you want to drive with that output?
     
  3. doughboy

    Thread Starter New Member

    Dec 5, 2010
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    a tunze 6055 controllable powerhead for aquariums.
    It is controllable via a voltage between 0 and 8v to make the pump run from 0 to 100%. The manufacturer recommends the lowest speed be 30%.
    This DIY project use a basic 555 circuit but drives the output between 0 and V instead of 2.4v (30% of 8v) and V
    http://www.reefcentral.org/forums/showthread.php?t=1411393

    thanks
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    A simple diode with a hard voltage source will do this. Or you could connect the negative power lead of the 555 to a power supply voltage. Something like a LM317 will go as low as 1.30V (depending on tolerance, maybe 1.20VDC).

    [​IMG]

    Thinking about it, this won't work, because the LM317 is meant to source current, not sink it. I discuss this problem in my Creating a Virtual Power Supply Ground article. So modify the above schematic as shown.

    [​IMG]

    This design will have a minimum voltage of 1.9 to 2.0VDC out. A load (the 555 circuit) must be there for it to work properly. Drop the value of R1 to 62Ω.
     
    Last edited: Dec 5, 2010
  5. mik3

    Senior Member

    Feb 4, 2008
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    If you just need to provide an analog signal to the 6055, then a simple potentiometer with a resistor in series to limit the low speed is fine.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Agreed....
     
  7. doughboy

    Thread Starter New Member

    Dec 5, 2010
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    the output needs to be pulsed (to create the wave).

    I need the pulse voltage to be 8v and 2.4v (and not 2.4v and 0, which is what a potentiometer will do right?) I currently get 8v and 0v using the standard astable 555 circuit.

    I read the virtual ground post, but still not sure if it will be able to provide the 8v and 2.4v oscillating output.

    basically, I need help with a circuit that will output V if input is V and output 1/3V if input is 0.

    Or is there a circuit that will OR two input voltages?
    I could create two complementary astable 555 and then use the potentiometer to control each output, but then need to OR it to get the output I need.

    thanks!
     
  8. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    a zener diode clamp on the output.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Which will burn up the 555 if you don't use a current limiting resistor, and is a fixed voltage. I've already drawn a circuit that will do it, and is variable to boot. This is not the power supply for a 555, it is the ground circuit. Are you needing help on how to use it?
     
  10. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    [​IMG]

    Three diodes instead of a zener
     
  11. doughboy

    Thread Starter New Member

    Dec 5, 2010
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    thanks for the replies!
    Kermit2, do you have a larger picture? I can't read the labels in the schematic. the output waveform is what I am looking for.

    The ground of the output is still the same as the ground of the power supply right? (and not a virtual ground?), since the power supply comes from the motor controller, so it must be on the same ground.
     
  12. Kermit2

    AAC Fanatic!

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    These are just stock devices that come with Ltspice but the concept is sound.

    [​IMG]
     
  13. doughboy

    Thread Starter New Member

    Dec 5, 2010
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    thanks.

    I am trying to understand the circuit.

    The output I need signals the "electronic controller" of the motor (and not the motor itself, so there is no high current requirement). So I am not sure how I can supply that output to the controller box (so I just need a line where the output is 8/2.4v square wave) . From the diagram, I can see it puts the motor in series, but not sure how to translate that to what I need. Do I just remove motor from the diagram and just connect the mosfet drain to the motor controller line? (I think the control line is a sink so do I replace the motor with a pullup resistor?)
     
  14. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The mosfet is just a switch in this config. a circuit that present a higher resistance than the 3 ohms shown for the motor will mean that the mosfet just conduct less current. It essentially 'shorts' itself out of the circuit and becomes more like a piece of copper wire in function. So the current drawn would only depend on the resistance seen at the point where the 'motor' is. Plus the voltage drops across the diodes to get the low voltage up to 2.4 will further limit the current, but not as much as the 'motor' resistance.

    Check the input point of your motor controller with an ohm meter. This will give you an idea of what current will be.
     
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