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Mini audio osc
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400 Hz oscillator

400 Hz oscillator V 1.0
400 Hz oscillator top view

This project was the first one I built using my brand-new Dremel tool... I used an engraving bit to etch the copper, and a very tiny drill bit to make the holes for the component leads... but I went a little overboard with some of the holes! ;)

Aside from Dremel tool practice, I needed to run some tests on some audio amps. I needed a nice, clean sinewave oscillator. Since I don't own a commercial unit, and didn't really want to use the computer soundcard, I decided to build this little guy! It's a 2-transistor ckt, with one as the osc, the other a buffer/driver. This allowed me to have two signal levels/impedances: One at the Q2 collector with 1K ohm Z at a higher signal level, the other from the Q2 emitter at 100 ohm Z, with a much lower level. (It works great for testing hearphones!)

Bottom of 400 Hz osc
400 Hz oscillator bottom view

Here is the bottom view. This was a scrap PC board that I rescued from some old 1970's vintage stereo system. It was for a lamp, and it had a rather LARGE blank copper area... which is why I used it! :) Dremel tool in hand, I etched that puppy and built my oscillator on it. (This was before I splurged and BOUGHT some copper clad boards, but I digress.) That is why you see some odd-looking text on the corner of the board.

This project also served as a good place to get used to the new Dremel tool. The engraving bit worked OK, but I later discovered that it's much faster and easier to use the cutting wheel. (Just make the traces & lands all straight lines.) Save the curvy stuff for only where it's absolutely needed! ;) Besides, the cutting wheel lasts a whole lot longer than the engraving bit, which starts to get dull after only a few small PC boards about this size have been etched with it.

400Hz osc schematic
400 Hz oscillator schematic

Here's the schematic. As you can see by Q1, it's the classc phase-shift circuit, using 3 caps and 3 resistors, but with the typical N1NKM twist. ;) Rather than try to figure out the values of the bias resistors to make that last r/c network, I just add an extra cap of larger value, and use standard high value bias resistors. More parts, yes, but it's my lazy way of avoiding punching a hundred buttons on the calculator! ;)

Those of you with a sharp eye may have noticed that there are TWO diodes in the top photo, but only ONE is shown in the schematic. That's because I only have 5v and 3v zeners, and needed an 8v... so there ya go. ;) By giving it this simple regulator, the frequency and purity of the sinewave stays pretty constant as long as the supply voltage stays between 8 to 20v.

"Stay Tuned!" ;) There's ALWAYS MORE TO COME!!

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