Shift Register Controlling Frequency

Discussion in 'The Projects Forum' started by mxabeles, Nov 29, 2009.

  1. mxabeles

    Thread Starter Active Member

    Apr 25, 2009
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    Hi all,
    I want to have the 8 outputs of a shift register "choosing" different resistors for the frequency of a 555 timer over time. The register would be wired as a LFSR.

    Thanks,
    Max
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Did you mean Last In, First Out?

    That'll be kind of hard to do, as even though shift registers are available with 3-state outputs, those are an "all or nothing" proposition.

    Better to use a mux/demux IC connected to a counter.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    On a similar note, here's a 4017 5-stage Johnson counter and a 4093 NAND gate for a clock. The 4017 "chooses" a new delay value for the 4093 clock, every time the 4093 outputs a new clock. The diodes enable just 1 resistor at a time to charge the timing cap.

    [eta]
    Oops, used a 40106 hex Schmitt-trigger inverter instead of a 4093 quad Schmitt-trigger NAND gate.
    You can also use the 4093 NAND as four individual timers; with just a single resistor and cap needed per gate.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    LFSR means Linear Feedback Shift Register. They are used for generating pseudorandom sequences. Maximal length for an 8 bit LFSR is 255 states before repeating.
    In order to get 255 different frequencies from the 555, an 8 bit digital to analog current converter would be required.
     
  5. Ron H

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    Apr 14, 2005
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    Here is a place to start. There are other ways to do the D/A conversion, some of which might be simpler. This is just what I first thought of.
     
  6. mxabeles

    Thread Starter Active Member

    Apr 25, 2009
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    Thanks guys!
    i guys i was thinking u could somehow have a set resistor for each output ( 7 if using 1 shift register wired as LFSR i think). And the shift register could pick a resistor at random/////
    I mean, the real question is how do you switch frequency to the 555 without turning a knob (automate the frequency change...)
    Tnx for any insights, and I'll check out the DAC schema.
    -M
     
  7. Ron H

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    Apr 14, 2005
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    With 8 bits, you can have 256 (255 with an LFSR) different combinations of outputs, which means you can have up to 256 (255) different frequencies. to do this with 8 resistors, they have to be weighted in binary fashion, i.e, R, 2R, 4R, 8R,..., 128R. This is one type of D/A converter. You could also use an R-2R network (Google), which only requires 2 resistor values, but roughly twice as many.
     
  8. mxabeles

    Thread Starter Active Member

    Apr 25, 2009
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    I think I forgot to thank u for the schematic.
    Thanks buddy!
    -M
    Just to make sure, I don't need bi=polar power for this? Most of the DACs i've been looking at online require bipolar power supply. me no like....
     
  9. Ron H

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    The circuit I posted only requires one supply. Keep in mind that I haven't tested or simulated it.
     
  10. mxabeles

    Thread Starter Active Member

    Apr 25, 2009
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    I actually found a way to do this that works well. The LFSR data goes to output router (0,0,0...1,1,1,etc) of demultiplexer (chip CD4051). The common input is pin 7 of 555 and then I have 8 or more outputs with different resistors going to pin 8. Works!
    Best,
    M
     
  11. retched

    AAC Fanatic!

    Dec 5, 2009
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    If you find the time, I would like to see the schematic for this. Its got me a-thinkin'
     
  12. mxabeles

    Thread Starter Active Member

    Apr 25, 2009
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    sure, i can draw one up but I suck at it at in software/
     
  13. retched

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    Dec 5, 2009
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    However you do it is fine with me. (Just dont use pink paper with a brown marker) ;)
     
  14. mxabeles

    Thread Starter Active Member

    Apr 25, 2009
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    um, yeah. :)
    I didn't put any components (resistors, caps) cause it would be too messy, and its up to you what values anyway. shoot me a mess if u have any questions.
    Best,
    M
    **** eagle haha
     
  15. retched

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    Dec 5, 2009
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    I get the picture. thanks. I was basically interested in the wiring diagram, which you gave. Thanks.
     
  16. Ron H

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    Pin 8 on a 555 is Vcc. How does that work?
     
  17. mxabeles

    Thread Starter Active Member

    Apr 25, 2009
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    input of 4051. pin 7 of 555
    outputs of 4051. various resistors to pin 8 of 555
    LFSR/4051 circuit changes resistance between pin 7-8 of 555.
     
    Last edited: Apr 19, 2010
  18. Ron H

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    Apr 14, 2005
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    OK, I get it.
     
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