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Back in 1980 I realized that I could buffer a 555's timing cap,
or one of a SN76477's timing caps, and get a decent low-impedance
waveform from it, with a cheap three-resistor op-amp circuit.
The ideal-value calculations were a snap; the tricky part was
selecting the right standard resistor values to get the gain and
offset right.
ampsr
does all the calculating and selecting. It's useful for
designing three-resistor attenuator networks using the 5% or 1%
standard-value arrays. Over the years I've written HP-11C,
HP-41CV and Turbo C versions to do some or all of that; here's
the Perl version.
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synctone
There's a great little program available for Linux, written by Steven
James, called
AutoZen
. It uses your soundcard to put a slightly different pitch into
each earpiece of your headphones. Result: the two halves of your
brain will try to synchronize at the frequency which is the difference
of the two pitches, which can be any brainwave frequency. It's a
little like alpha-state biofeedback -- biofeedback can
tell you when you get there, and AutoZen can lead you there.
That's great for when you're sitting at the computer. What about
when you'd rather not lug a computer around?
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One quad op-amp and a bunch of passive components equals the
cheapest hemisphere entrainment hardware I could build. It
doesn't do programs or sequences, and I have to dial in the
tones by feel, but I can clip it to my belt and walk around with
headphones on. I stay at the brainwave that works for what I
want to do, dodging distractions and staying on-track, and the
controls are right there if I want to change things. Will it
work for you? If you've got decent hearing in both ears, it
should. It works for me, and has since I designed and built the
first one around 1988.
With an alkaline 2U6-type 9v battery as a power-supply, it's
loud. If you don't like that, either feed it 6 volts (4 AA
cells) or reduce the R3 and R8 values to your liking. If you're
inclined to improve designs, the first step towards putting this
circuit under microprocessor control is replacing R2 and R7 with
either OTAs such as CA3080, or DACs. Remember, this is
designed to affect the brain: if you're prone to seizures, have
someone around to pull the headphones off you the first few
times in case you start going under, or give this one a pass.
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