audio oscillator

Discussion in 'General Electronics Chat' started by leila, Jan 4, 2008.

  1. leila

    Thread Starter New Member

    Jan 4, 2008
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    I need to design a voltage controlled audio oscillator. Its said in the voltage oscillator page that the output timing is depended on the source voltage... does anyone know more details? can I vary the input voltage use it to aobtain a range of frequency?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    Does the output need to be in the form of a sinewave or is it sufficient that the output is simply a signal in the audio frequency range?

    hgmjr
     
  3. hgmjr

    Moderator

    Jan 28, 2005
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    If you can find a source for the IC part number MAX038 made by Maxim ( www.maxim-ic.com ) that would give you good starting point to your project.

    I checked Digi-key, Mouser, and Jameco without any success. There are some less well known sources but I am not able to recommend them since I have never done business with them.

    hgmjr
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Or you can get an NTE864 from Mouser. It's the replacement for an ICL8038. Sine, triangle and square outputs.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Well, he could also use a 555 or 1/2 of a 556 IC. They're cheap and available just about anywhere.
    Here's a software program that's a big help for designing 555/556 timer circuits:
    http://www.schematica.com/555_Timer_design/555_Timer_PRO.htm
    It has a "lite" mode that can be run as freeware indefinitely. There's a "sound effects" helper that shows how to connect two halves of a 556 together to generate various sound effects - and the signals shown could be quite useful.
    In astable mode, pin 5 of the 555 can be used to vary the frequency of the output by varying the voltage level.

    The MAX038 has sadly been obsolete for awhile; if you can even find them in an online auction, they're generally in excess of $25 each.

    The NTE864 is even more expensive at over $30/each.

    What kind of signal are you wanting to obtain as output? What frequency range? Like 30Hz-20KHz?
     
  6. leila

    Thread Starter New Member

    Jan 4, 2008
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    The thing is that I am doing an obstacle detector and so far I have a DC voltage that is proportional to the distance the obstacle is at. So now I want to convert this into an audible frequency so that it could guide movement.. so I need a range of audible frequencies
     
  7. hgmjr

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    Jan 28, 2005
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    Then I think you would benefit from using a simple voltage-to-frequency converter IC.

    hgmjr
     
  8. leila

    Thread Starter New Member

    Jan 4, 2008
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    I can't use an IC
     
  9. hgmjr

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    Jan 28, 2005
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    Then I would recommend you take the path that has already been suggested to an earlier poster whose topic is pretty much along the same lines as yours.

    You can use a twin-t or wien-bridge oscillator that is modified so that its frequency of oscillation is variable with voltage. You can use a JFET as the variable resistance element.

    hgmjr
     
  10. hgmjr

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    Jan 28, 2005
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    Another scheme for varying the resistance in the Twin-T or wien-bridge oscillator would be to take a Light-Dependent resistor (LDR) and encapsulate it together with an LED. The resulting assembly becomes a straightforward variable resistor that is a function of the brightness of the LED. The brightness of the LED is in turn a function of the current with which it is driven. You could build two of these and use them in the wien-bridge oscillator as a means of tuning the frequency.

    hgmjr
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    This sounds like homework. Is it?
     
  12. hgmjr

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    Here is a link to a wien-bridge oscillator build entirely of transistors.

    It should be possible to take this circuit and subsitute two of the white-led plus photocell(LDR) assemblies for the two 47K ohm resistors. You would need to size the capacitors accordingly to obtain the frequency range you desired.

    hgmjr
     
  13. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You can make one with a two-transistor astable. Use a current mirror to convert the control voltage to a proportional current which controls the frequency. I simulated one, and it had a fairly linear transfer function.
    It uses 5 transistors.
     
  14. leila

    Thread Starter New Member

    Jan 4, 2008
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    Hi thanx all for the help, I found a very simple oscillator which uses 1 LM741 opamp.
    I need a dc chopper now that can provide me with a sufficient current to drive a light bulb (2 ohms). The current that I get now is around 40mA and I need it to be 10X larger.
    If anyone has another idea than a dc chopper it would also help
     
  15. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Add a compound (npn/pnp) emitter follower to the op amp output and take the load from the centre. This will supply the current directly.
     
  16. studiot

    AAC Fanatic!

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    Attached some follower configurations that would boost the current capability of your 741. These should be included within both the negative stabilistaion feed back loop and the positive (wein) loop.
     
  17. Ron H

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    The improved follower won't work unless you add something, like maybe a wire from the output to the junction of the two emitters.
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    Santa didn't give me a scanner.
    Just a box of rulers.
     
  19. Ron H

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    Yep. That's what I had in mind. Not sure what studiot had in mind.
    It may cause the closed loop amplifier to oscillate, though, as it adds another pole in the loop.
     
  20. studiot

    AAC Fanatic!

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    You are right, Ron I missed a connection out in the first complementary emitter follower sketch.

    I think the principle comes through though that there are lots of ways to boost the current drive capacity of an op amp. Even a simple power transistor/FET with the bulb as an emitter or collector/source/drain load, will be capable of supplying the 400 mA required.

    The wein oscillator has two feedback loops as stated. The negative feedback is shown in all the sketches and is used to set and maintain the gain at exactly 3, to promote oscillation. This is done with two resistors across the ouput set as a potential divider with the pick off point set appropriately. Gain stabilisation is achieved by making one of the resistors output (voltage) dependant.

    The frequency determining components of the wein bridge are inserted in the positive feedback loop. At the desired frequency they provide zero phase shift to set the oscillation frequency.
     
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