gain up high and put a finger on the probe.
You’ll likely immediately see a very distorted
sine wave with a base frequency of 50 or 60
Hz, depending on your AC mains frequency.
These fields will easily induce a voltage on an
amplifier’s high impedance input.
One way to reduce the effect (it cannot
be eliminated), is to both lower the gain of
the amplifier and lower the input impedance.
We can do this with a resistor network
attenuator at the input to the amplifier
(Figure 3). More current will be required
to get an equivalent voltage, thus lowering
the impedance. We can then increase the
gain of the amplifier (turn up the volume) to get the
Vout = Vin R2/(R1+R2)
Another possible way that the hum in getting in is
what you mentioned: a “ground loop.” I find ground to be
an ill-defined term, so let’s just think of this case as stray
voltage differences and currents flowing through the AC
mains, including the so-called safety ground. These voltage
differences can become currents that flow in the wire
coupling the devices together; in this case, the Amazon
Dot and the amplifier. There are numerous causes for a
condition like this, including faulty house wiring.
To break the circuit that may be carrying such currents,
we can couple with magnetic fields instead. We do this
with an audio isolation transformer. There are many that
are targeted at audio frequencies and typical audio system
impedances. RadioShack used to carry these in their stores,
but there aren’t many RadioShacks left.
I did find them, however, at Jameco on the web for
only a few dollars. Keep in mind that the response of a
transformer will be frequency dependent, so it will tend to
roll off the low frequencies and emphasize the high. You
might have to compensate for this using some simple filters
to get music quality audio back again.
It would probably be worth trying the attenuator first,
since it’s so easy to implement. If that doesn’t work, try the
Computer Mind Tricks
QI want to get into brain control — especially for flying drones. I’ve read the sub-$100 EEG monitors are a joke, and that the best system out there is an open source monitor for over
$1,000. Do you know of an affordable brain interface that
ALet’s take a look. It depends on what you mean by “works.” They all output some data, but whether you can make sense of those data is another matter.
I’ve been interested in this area for a while. A couple
of years ago, I bought a NeuroSky device and got the SDK
to play with sensing brain states. My interest is in sleep. I
have always had trouble sleeping and was hoping to use
some brain state data to drive how to induce sleep using
binaural beats. I’ve had some success using binaural beats
in iOS apps I wrote for myself to help my sleep, and I
wanted to enhance that. Interestingly, a device called the
Sleep Shepard Blue independently implemented some of
my ideas, but not all.
In principle, it was an interesting idea, but what I
found was that the data coming from the EEG device were
unreliable indicators of my brain state. It was also difficult
to sleep with that thing strapped to my forehead and
earlobe, or at least to keep it on while I was lying in bed
trying to sleep.
Looking around the web for the current state-of-the-art,
I found that there are some very cheap hacks that can be
done with EEG based toys. The Star Wars™ Force Trainer
seems to be a popular device to modify.
It has a NeuroSky EEG board inside that can be used,
along with the toy headgear. Serial data are output from
a pin on that board, and an Arduino or Raspberry Pi can
be used to read the output. I found an Instructables article
that shows how to do it at
Control-A-Computer-With-Your-Mind/. That’s a $40
solution, but it’s probably not very good. There may only
be three channels of data, which doesn’t give you much to
QUESTIONS and ANSWERS
Post comments on this article and find any associated files and/or downloads at
n FIGURE 3. Voltage divider based amplifier attenuator.
September/October 2018 9