Sine wave code practice oscillator

Discussion in 'Homebrew and Kit Projects' started by WR5AW, Aug 13, 2008.

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  1. WR5AW

    WR5AW Ham Member QRZ Page

    I'm looking for a schematic for a code practice oscillator that can be hooked up to a keyer. I have a Picokeyer so I'd like to be able to just plug the oscillator into the key jack of the Picokeyer. Output would be to an 8 ohm speaker. Tone should be adjustable. Power could be a 9v battery.

    I've found lots of references to square wave code oscillator circuits. I found only one for a sine wave oscillator. Unfortunately, I can't remember where I saw it and haven't been able to find it again.

    I'd settle for a square wave oscillator but I find the tone a bit annoying so I'd prefer a sine wave output.

    Any input would be appreciated.
  2. W4HAY

    W4HAY XML Subscriber QRZ Page

    How about a Twin-Tee oscillator like this one? Add a transistor emitter follower to drive the 'phones. Wein Bridge and phase-shift circuits also give good sine wave output. Use your search engine.

    The easiest way to get good tone is from a simple Pierce oscillator using any crystal you have on hand, and monitoring it with your receiver. I used a color-burst rock from an old TV set. It's in the 80 Meter band to boot. I substituted a 1K resistor for the inductor in the drain circuit and tied the crystal directly between the drain and gate, eliminating the resistor "R" and .001 capacitor. The gate resistor is 100K rather than 10M. It's powered from a 9-Volt battery.
    Last edited: Aug 13, 2008
  3. KL7AJ

    KL7AJ Premium Subscriber QRZ Page
  4. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

  5. AB8RO

    AB8RO Ham Member QRZ Page

  6. K7JEM

    K7JEM Ham Member QRZ Page

    I suppose if a tunable frequency is required, other circuits might be better. One that works well is the XR2206, but it's pretty pricey, almost $4 for the chip. But, a single pot could make the tone frequency adjustable from 300-3000 Hz or more.

    A good design project might be to design a complete twin T code osc using just an LM324 and maybe a couple of transistors. It would be good and cheap, but possibly not easily variable in pitch. It could be able to drive a headphone without any additional amplifier.

    Set a goal of sine wave and decent sound, and set an upper limit of $2 for all of the parts (not including board and PS).

  7. AB8RO

    AB8RO Ham Member QRZ Page

    I like that challenge Joe. That pretty much rules out any special purpose chips. One cheaper way to get a decent variable pitch sine wave oscillator is to use the LM13700 which is a dual transconductance amp. The data sheet, or an ap-note I can't remember which, includes a sin wave VCO. Unfortunately, the LM13700 isn't THAT much cheaper than the XR2206.
  8. K8ERV

    K8ERV QRZ Member QRZ Page

    Not the greatest, but cheap. Feed the speaker line back into the volume control thru a cap or cap+resistor (whatever works) in an old transistor radio.

    TOM K8ERV Montrose Colo
  9. K7JEM

    K7JEM Ham Member QRZ Page

    Something like this might work:

    Last edited: Aug 13, 2008
  10. K7ELP

    K7ELP Ham Member QRZ Page

    Here is on I built several years ago. It uses a phase shift oscillator(741 op amp). The jack is for a key so in operation the key just turns on the speaker.
    actual frequency is 660 Hz, calculated is 690 Hz.
    I also built one that you can vary the frequency. It uses a 555 timer followed by a couple of intergraters, the first integrator converts the square wave to a triangle wave and the second one converts the triangle wave to a sine wave. If you want the schematic let me know and I will draw it with my cad package.
    Last edited: Sep 11, 2009
  11. WA9SVD

    WA9SVD Ham Member QRZ Page

    Some good suggestions, but I still prefer the LM380 over the LM-386. It's MUCH easier to tame, (at least in my experience) and can run off 12/13.8 Volts directly, with 2 Watts out, enough for code practice in even a crowded room, with a decent sized speaker...
  12. AB8RO

    AB8RO Ham Member QRZ Page

    Boy I agree. The LM386 is a harsh sounding beast.
  13. K7ELP

    K7ELP Ham Member QRZ Page

    It can be if it is overdriven and if you want hi-fi sound, but for voice frequencies and lower power, I think it is fine.
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