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Largest range of frequency&duty cycle from 555 circuit

Discussion in 'General Electronics Chat' started by gentlywiringit*, Aug 6, 2017.

  1. gentlywiringit*

    gentlywiringit* Member

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    Looking for an easy and reliable way to get the largest range of frequency and duty cycle from a 555 timer circuit using ~8v source.

    Both of these circuits look like they fit the bill but there was no actual frequency or duty given for them.

    Which one will most reliably will give me the fullest range?
    Is there a more suitable 555 circuit I haven't found?


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  2. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    Frequency will be given in the appropriate datasheet. The CMOS version is higher than the NE555/LM555.

    As for duty cycle, it goes pretty reliably from 5% to 95%. Some people get a wider range. That may depend on the specific circuit you use. There is a small variety of circuits. The classic like you show. There are others that do not use Pin 3 as the output. Paisley has a pretty good list here: http://home.cogeco.ca/~rpaisley4/LM555.html

    John
     
  3. crutschow

    crutschow Well-Known Member Most Helpful Member

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    The second circuit will allow the PWM duty-cycle to go from 0% (fully off) to 100% (completely on) by the adjustment of VR2 and appropriate values for R2 and R4 (possibly slightly smaller than the given values).
     
    Last edited: Aug 6, 2017
  4. dave

    Dave New Member

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  5. hackware

    hackware New Member

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    Just found this site the other day via google... Very good site to learn the 555!

    william...
     
  6. gentlywiringit*

    gentlywiringit* Member

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    Can't say I'm looking to lean how to do it, looking more to just copy a schematic if I'm honest. Feels like I'm getting to old to be taught new tricks. :grumpy:

    I'll grab some cmos 555's.

    If anyone has a schematic for achieving something like 1hz - 1Mhz with 0-100% duty using any type 555 in any configuration I'd be greatful for seeing it, it's a pretty popular Google search term but there are no straight 'here's the circuit you want' answers.
     
    Last edited: Aug 6, 2017
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Why do you want a wide frequency range? At low frequencies the motor will be noisy and jerky and you cannot hear above 20kHz so a little above 20kHz is fine.
     
  8. gary350

    gary350 Well-Known Member

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    This is from a 40 year old TTL book. Get a newer TTL book or CMOS book it has extra information the old book does not have. There are about 10 pages that tells you exactly what you need to know with circuit drawings too. Download a 555 simulator build a circuit on your computer see if does what you want. Page 183 tells how to do duty cycle.

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  9. AnalogKid

    AnalogKid Well-Known Member

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    I agree with Wally that the second schematic is a better way to go. The pulse width part does not require timing components, so it tracks whatever it is fed with a nearly constant duty cycle output. Note that it will not operate up to 1 MHz because an LM393 is not fast enough.

    An improvement would be to use a different 555 configuration that produces a nearly 50% duty cycle output at all frequencies.

    upload_2017-8-7_0-46-13.gif

    This in turn makes a nearly symmetrical sawtooth wave at the top of C1 to feed the comparator. R1 would be a combination of a small fixed resistor and a large-value pot.

    ak
     
  10. gentlywiringit*

    gentlywiringit* Member

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    Hey audioguru.
    Just for testing purposes really, I figure if I'm going in it might as well be with both feet :D
    I find myself having 'issues' relating math and schematics to real world causality. I need a few hours of twiddling knobs to physically observe the affect it has on various loads. There'll be a usable range I'm sure, I just want to understand the why's and wherefores from a hands on viewpoint. Plus it'll be handy to have for future projects :)

    Hey AnalogKid, thanks for your suggestion.

    You lost me a bit though, are you saying I can graft your 555 circuit and (if I had one) a faster differential comparator together in the topology of the second schematic? Or just scrap that one entirely in favour of yours?
    Ive had a quick look before writing this, the LM2903 is a faster ddc but it's got twice the number of pins, I guess it's not suitable? I'll keep looking for a direct replacement.


    Would the small resistor be above or below the pot on your schematic? 1ohm resistor 1Mohm pot about do it?
     
    Last edited: Aug 7, 2017
  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    But less than about 10Hz the motor will be jerking on and off.
    10Hz to 20kHz the motor will be buzzing or whining.
    Above 30kHz the PWM is not efficient since the 555 and driver transistor cannot turn on and turn off fast enough.
     
  12. AnalogKid

    AnalogKid Well-Known Member

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    In the second schematic, a 555 circuit drives a comparator circuit. Within some limits, the two halves are pretty much independent of each other, so you can change one side and keep the other. The 555 datasheet has minimum and maximum recommended values for the timing resistors. The minimum value would be the fixed resistor, and the maximum value would be the variable resistor. In the post #8 schematic, the two resistors are in series, so it doesn't matter which one comes first.

    Not that a single range will not be very usable. At the higher frequencies, a small adjustment of the pot will cause a large change in output frequency. That is why the second schematic has C1 through C4. For the same R1 and VR1, each capacitor creates a different range of output frequencies. The equation to calculate the output frequency is in the datasheet.

    The LM2903 has the same number of pins as the LM393. Both are dual comparators - two independent comparator circuits in one package. For both parts, if you are using only one comparator then the inputs to the unused circuit must be connected to something, not left floating. Connecting one to V+ and one to GND is safe. It doesn't matter which goes where.

    How are you going to assemble the circuit (perf board, solderless proto-board, etc.)? You should consider getting a small function generator or PWM circuit kit as a starting point.

    ak
     
  13. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Here's an LM339/393 PWM circuit that goes from 0% to 100% duty-cycle.
    It's upper frequency limit is likely in the neighborhood of a half MHz.
     
  14. AnalogKid

    AnalogKid Well-Known Member

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    Yeah, I thought about mentioning that the 555 could be replaced by the unused half of the 393. I figured someone would do it.

    ak
     

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