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When i started to make this very low frequency oscillator, i
reckoned that an L.C. arrangement would need an un-reasonably
large inductor and capacitor, and to get active feedback at
frequencies that are barely more than a slowly changing D.C.
level would make normal inductive coupling quite impractical.
So i opted for a phase shift oscillator, i know them to be
fairly simple, but until then i had never built one. I looked
at all the designs i could find on the internet, and they all
were arranged like this:
After i had tried making the resistors very high values, many
megs actually, and making the capacitors as high as i had, that
could be used in that circuit, i soon decided that to get higher
values of capacitance i would have to use condensers, or
electrolytic capacitors as they are now known.
The problem here is that electrolytics are uni-directional,
unlike the ones in all the circuits i could find, which are A.C.
So i tried staggering them, with one up, and one down, so as to
keep the uni directional voltage, and also pass the signal,
When that didn't work either, i was most perplexed. It took me until
the next day to realise why it wouldn't work. Quite simply, the
attenuation is so severe that it needs more gain. I stuck another
transistor in as a darlington pair and bingo, works a treat.
Unfortunately i cant find the notes i made at the time of
construction, except a few scrawly bits without any values
But i think the caps were 100 mfd, just ordinary ones,
and i think the resistors were about 30 k-ohms.
you will have to mess around with the values to get the sort
of frequency you're looking for.
I've been interested in this as well...
I"ve designed a square-wave EM wave generator controlled by software (communicates with hardware via parallel port)...
It's intended use is with brainwave entrainment... if the frequency is high enough, the sudden polarity changes may have the same effect on the human brain (essentially) as sine waves, however at lower frequencies (beta/alpha/theta/delta) aren't efficient with square waves...
The device must be accurate to .01 CPS, and preferrably have a wide range... say up to 1KHz...