I hate to ask, but i need help with 555 PWM

Discussion in 'General Electronics Chat' started by xbox hacker, Feb 6, 2010.

  1. xbox hacker

    Thread Starter Member

    Sep 9, 2008
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    Ok, i have been looking for a soultion for a PWM i need, but it eludes me :(

    I want a 555/556 PWM with 50% duty cycle. With 2 outputs:
    First output will have frequency controlled via pot.
    Second output will have the EXACT same output as first, BUT with a second pot create a delay in the wave form of the second output.

    And if the first output frequency is increased, the secondary output will increase as well, but maintain the same delay ratio set by the second pot. Kind of like a offset...

    Yes i know..."why not just use 2 PWMs". The project calls for strict offset prams, so i think 556 would be the best choice.

    Thank you for your help in advance!
     
  2. Wendy

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    Mar 24, 2008
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    Does any of this need to be shifted in phase or duty cycle?

    What is the loading requirements of this circuit? If it is light a CMOS 556 might work better.

    [​IMG]

    [​IMG]

    You can get the same effect from two flip flops and a couple of AND gates.
     
    Last edited: Feb 6, 2010
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Should the 1st output always have a 50% duty cycle? If so, then it is a variable frequency clock, not a PWM circuit. If the duty cycle changes, then it is a PWM circuit.

    You need to clarify this statement.
    Do both the rising and falling edges need to be delayed, or just the leading edge, or just the trailing edge?

    A diagram would be most helpful.
     
  4. Wendy

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    Mar 24, 2008
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    We've had similar requests for this before. The questions are valid, but usually a 90° phase shifted square wave is what is wanted.

    555 circuits have their own sets of problems. The circuit I drew would work, but it may have some stability issues, unlike the digital equivalents.

    There are huge numbers of ways to do this.
     
  5. xbox hacker

    Thread Starter Member

    Sep 9, 2008
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    OK, here is a image that i hope will help...

    [​IMG]
     
  6. Wendy

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    See post #2, that is what I drew. I included the data sheet to help design the 555 oscillator. If you need information on how to use it just ask. Without specific parameters such as frequency, power supply, and load we're flying blind.
     
  7. xbox hacker

    Thread Starter Member

    Sep 9, 2008
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    frequency, power supply, and load are not important. I have made many 555 projects before, i can figure out all that. I dont know how to make it have 2 outputs with the same frequency, and have output B with a offset or shift or delay...but still have the same frequency as output A

    but if your really need those prams... 5v 1-50hz 50%duty
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    The second 555 segment in the 556 in my diagram is what you asked for, a variable phase shifter. By definition the frequency will be the same. Just use the same general resistor values and capacitance you have for the 1st one.

    Just to be sure you know, a 555 drops 1.2V on the plus side due to it's output drivers. The output of a 555 with a 5V power supply (which is on the edge of their specs, which is no less than 4.5VDC) will be 3.8V max.

    Load matters, because if it low you can use a CMOS 555/556, which doesn't have these problems. It is also a lot easier to design a 50% duty cycle circuit using one, as explained here...

    The 555 Projects

    555 Hysteretic Oscillator

    I'll be back with something for a conventional 555.
     
  9. Wendy

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    OK, I spent zero time calculating the shifter, you may need to drop C4 down to 0.01µF.

    [​IMG]
     
  10. xbox hacker

    Thread Starter Member

    Sep 9, 2008
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    Thank Your so much, i will give it a try!! :D
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Bill,
    Double-check the left side of that circuit; it's not going to work properly like that.
     
  12. SgtWookie

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    xbox_hacker - hang on a few, the circuit needs to be looked at a bit more.
     
  13. SgtWookie

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    Ahhh, I'm afraid you're off a few decimal points on your C1 calculation, too ... should be a total of around 27uF instead of 0.27uF.
     
  14. xbox hacker

    Thread Starter Member

    Sep 9, 2008
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    Yeah, i see that now...

    .27uf would be way too high in the frequency.

    27uf should be lower freq as i need.

    Thanx!
     
  15. Wendy

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    Drats, nailed again. I'm also concerned about the P-P amplitude of the triangle wave. 5V is not much drive.
     
  16. SgtWookie

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    Bill, I think you're going to need to use 10k for the fixed, 500k for the pots, and 2.7uF, .47uF and 1uF for the caps. Otherwise, it's going to load a CMOS timer too badly, and a BJT timer won't work well at 5v.

    Use the output of the 1st half to charge/discharge C1; something like this:

    [​IMG]

    Oh, and 10nF (0.01uF) is all you need to keep the control nice and quiet.

    Upping the resistance will help a lot on the drive. It'll still be tight at the high frequency end, but not as bad as using the bigger caps and lower resistors.
     
  17. SgtWookie

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    xbox_hacker,
    OK, I think the schematic I posted will do it for you - know that you will have to adjust R5 after adjusting R2. If you have R2 adjusted for a higher frequency, you may wind up with a constant high output on Out2 until you adjust R5 downwards.

    Make sure you use 10nF caps on the Ctrl inputs (pins 11 and 3) to ground.

    You should also use a 0.1uF and a 100uF cap across the Vcc/ground terminals.

    You must use a CMOS 555 timer, as a standard transistorized 555 won't work properly at such a low voltage.
     
  18. Wendy

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    Mar 24, 2008
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    The problem I see is the hysteretic oscillator won't work at 5V, as you pointed out to me in another post a way back. This is why I went with the schematic I did. I could have gone with a CMOS, but I don't think it has the drive the OP needs.

    Revamping my drawing again...

    [​IMG]

    My big concern is drive, the 555 doesn't go peak to peak, which doesn't give us much room to work with. The transistor I added gives the drive the 555 version would need.

    I think I'll add the TTL version using a 7474 dual flip flop later.
     
  19. SgtWookie

    Expert

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    OK, well I'm looking at the specs for a Maxim ICM7556 (dual CMOS 555 timer).

    With Vcc=5v, output high Vout is 4v min when sourcing 2mA.
    The worst case source requirement is if both of the 500k pots are set to minimum resistance, so two 10K resistors in parallel charging caps works out to be around 5k, for 1mA if charging from 0v - but that will only happen on startup; the rest of the time it'll be charging from 1/3 Vcc - so, 3.33v/5k = 0.666mA of the beast. :eek:

    That leaves around 1 mA drive for the xbox input circuitry; no clue how much that needs.

    The CMOS 555/556 will work at low voltages with the output charging/discharging the timing cap, as it doesn't have the Darlington follower like the bjt version does - that is, if the output is not otherwise heavily loaded.

    But, with the Darlington output on the bjt version, you're barely getting up to 3.7v even if the load is light. That may or may not be a problem with the xbox input; I don't know what the trigger levels might be.
     
  20. xbox hacker

    Thread Starter Member

    Sep 9, 2008
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    So to clear up any more confusion about what i am driving...

    Its a GM HEI Module. Connecting the HEI to a coil and pulsing the trigger in will give you a HV spark out of the secondary on the coil. But i came to find out that the module only needs about 3VDC sq. wave to work. My original tests i was using 12VDC Vcc into the 555. So i wanted to use 5VDC for the 555 Vcc.

    And no, i have NO specs on the module... i just know it works :rolleyes:
     
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