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Pulse Width Modulation

Discussion in 'General Electronics Discussion' started by colum, Aug 12, 2019.

  1. colum

    colum

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    Jul 25, 2013
    I have a small DC motor that is driven by 2 555 timer circuits and 2 Pots and a Transistors in PWM mode...I know the first 555 is the timer and the second 555 does what...does it amplify each pulse at its peak or does it make the pulse longer duration.
    I would greatly appreciate it if someone would show me this circuit layout
    Thanks...Jeff
     
  2. Minder

    Minder

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    Apr 24, 2015
    Can you post the circuit?
    Normally you only need one 555 for PWM for a motor.
    M.
     
  3. Alec_t

    Alec_t

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    Jul 7, 2015
    Hopefully you can show us the circuit, or post a link to it. Otherwise we just have to guess what the second circuit does. Normally you would get PWM using just one 555.

    Edit: Minder beat me to it!
     
  4. Minder

    Minder

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    585
    Apr 24, 2015
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Without seeing the circuit diagram all we can deliver is pure guesswork.
    Does the 2nd 555 amplify? Very most likely not. Why should it be able to deliver more drive than the first?
    Why 2 pots? For pwm you need only one.

    One possibility is that the first 555 defined the pwm frequency and the 2nd 555 defined the pulswidth (used as monostable multivibrator). But that would be an awkward circuit and really hard to adjust.
    Upload a schematic. Note that there is a size limit for uploading images (but I don't know what this limit is :( ). Should your attempt to upload fail, resize the image (but keep it clearly readable, please).
     
  6. colum

    colum

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    Jul 25, 2013
    I built this circuit many years ago using wire wrap and is very difficult to untangle all I know is that the second 555 has a big effect on the motor and I want to build another one...I did see a wiring diagram of a double 555 in one IC (forgot number) for PWM
    This circuit is going to aim a Laser and I want to use up my stash of parts.....Thanks....Jeff
     
  7. colum

    colum

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    Jul 25, 2013
    Thanks Harald you brought back my memory. The second 555 was set up as a monostable so now I must find the second 555 hook up ...I don't have an Oscilloscope so I would imagine that a Monostable would give a higher Volt at the peak as opposed to a longer pulse altho it is diagrammed as a longer pulse. Thanks again ...Jeff
     
  8. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Crystal Ball [1024x1024].jpg
    Okay. No more guessing. No more trying to pull information we need from this unresponsive poster. I have uploaded an image of my Crystal Ball, the one that I use to divine the intent and purpose of posters who cannot or will not provide the information we responders need... if we are to provide a reasoned, somewhat rational, response that may offer the original poster some, or maybe even all, of the answers they seek.

    As you can see, it is somewhat cloudy today, but feel free to use it if you can.

    In the meantime, I have some comments for @colum...
    I have often built, as well as supervised the building of, circuits from wire wrapped circuit boards. We could do double-sided boards with plated-through holes, but the layout became unwieldy for complicated circuits. Wire wrapping seemed to be a good alternative to designing multi-layer circuit boards involving dozens of LSI and MSI as well as ordinary integrated circuits.

    The way we did this was to label each IC socket and its pins with little paper puppets attached between the pins on the wire-wrap side of the board. These puppets were used to identify where the wires were connected. Then, referring to the schematic for guidance, we made a from-to wire list for each wire-wrap wire. There was no such thing as a "daisy chain" in our wire lists. Every pin initially had one, and only one, wire connected. The other end of that wire went to one and only one pin on the same or another socket. We tried to avoid wrapping more than two layers deep, but sometimes that was un-avoidable when the engineer (me) made changes in the circuit.

    After the board was completely wrapped we performed continuity checks using the from-to wire list for guidance. This was made easier because in generating the from-to list from the schematic, it was naturally easier to include common connections first. So usually one continuity probe was fixed on a node while the other was rapidly moved to the other nodes, shifting the second probe as soon as a beep was heard. You can use this same procedure to reverse engineer your PWM motor control board.

    Remove all ICs from their sockets. Start at pin 1 of socket 1 and test for continuity between that pin and every other pin on every socket on the board. Write down the results as a wire list for wire number 1. Move the test probe to the next socket pin and repeat for wire number 2. Continue in this manner until every pin that has a wire attached has been mapped. Don't worry if you wind up writing down the same connection twice. This will be corrected later when you edit the wire list.

    Perform these continuity measurements for every pin on every socket of the circuit board. When you are finished you should have a complete wire list where every connection is identified twice. Using a magic marker, identify and cross out the duplicates on your wire list. If you find a connection that doesn't have a duplicate, you have either measured your continuity wrong or one of your wires is wrapped on the wrong post. Find and fix that before continuing.

    With a good wire list in hand, it should be a trivial task to draw up a schematic based on the parts you do know. If some of the sockets contain unknown parts, you will have to make educated guesses using parts you do know. For example, if you are certain that a dual version of the 555 timer was used, look at a 556 datasheet. If you think maybe a quad version of the 555 timer was used, look at a 558 datasheet. Compare their application note wiring diagrams with what you have discovered about what you made.


    See above paragraph on how to reverse engineer what you already have.

    Yessir! I always try use up my stash of parts when designing and building something new! Why go to all the trouble of doing it right when with a lot more effort you can do it over... and over... and over again. Good luck with your Laser aiming project, Jeff! Come back and visit here more often and let us know how it turned out!
     
  9. AnalogKid

    AnalogKid

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    Jun 10, 2015
    That is not how a monostable works. A monostable makes a constant-width (and constant-amplitude) output pulse whenever it is triggered.

    Based on your description, it sounds like PPM - pulse position modulation. The first 555 is variable frequency and the second one is constant-width. As the frequency increases, the output pulses get closer and closer together. This changes the overall duty cycle sorta-kinda like PWM. The constant pulse width guarantees that the energy to the motor never drops below some minimum value, probably to prevent stalling.

    ak
     
    hevans1944 and Harald Kapp like this.
  10. colum

    colum

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    Jul 25, 2013
    Thanks Analog Kid that is the information I was looking for...colum
    General statement
    Is Hevens 1944 a Troll or what
     
  11. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    No, definitely not. Maybe at some times a bit irritable. If you stayed on this forum long enough you too would get fed up with lots of posts lacking teh required information. Be honest to yourself: you haven't exacly provided good details and we had to do a lot of guesswork.

    It always pays to do at least some documentation...
     
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