making a 555 'organ' monophonic

Discussion in 'The Projects Forum' started by ivan., Nov 13, 2008.

  1. ivan.

    Thread Starter New Member

    Nov 13, 2008
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    basically, im trying to create a circuit that uses a 555 oscillating circuit that will be able to give a range of frequencies - by using different resistor values in the potential divider to change the input at pin 7, each triggered by a switch of some sort.. but im stuck at how to only allow one resistors value at a time. btw, this is for an audio type project.
    i presume their will be a way of using logic for this, but we havent really been taught enough for me to work it out :p
    any advice would be appreciated, even if its 'your doing everything wrong'!.

    ivan.
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    The "real" way to do this is to put all the resistors in a string, and short out more or less of them. You can do this with a mosfet between each resistor junction and ground.


    eric
     
  3. bertus

    Administrator

    Apr 5, 2008
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    Last edited: Apr 11, 2009
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Ooops...I meant short them to Vcc...not ground. !

    eric
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Well, for a basic "toy" monophonic organ, you could use something like this:
    http://www.design-technology.info/KS3/Y8/page9.htm

    If you used pushbuttons, limiting it to a single resistor at a time is going to be a trick.

    You might have to resort to using something like a series of CMOS 4066 analog switches, coupled with a decoder/multiplexer.

    [eta]
    I like the schematic Bertus posted better than the one I found; the 1st schematic avoids the problem of more than one button being pressed at a time. However, you'd get better results if you used potentiometers wired as rheostats for the multiple 1k resistors between the switches.
    It will be quite a chore tuning the thing.
     
    Last edited: Nov 13, 2008
  6. ivan.

    Thread Starter New Member

    Nov 13, 2008
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    cheers people, the Experimentalists post was alot of help.
    il try out some stuff tmorra, and i shall look up the CMOS stuff, see what is achievable with high school resources!
    ivan.
     
  7. mik3

    Senior Member

    Feb 4, 2008
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    You can take resistor with values of about 1K, 2K, 4K, 8K, 16K and so on (or whatever values you want). Connect the one end of each one to pin 7 of the 555 and leave the other end of each one unconnected. Then use a switch for each one of the resistors to connect/disconnect them from pin 6 of the 555. By closing and opening the switches you can make different combinations and change the total resistance. With only 6 resistor you can make 63 tones.
     
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