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CIRCUIT NOTEBOOK
Interesting circuit ideas which we have checked but not built and tested. Contributions will be
paid for at standard rates. All submissions should include full name, address & phone number.
Biofeedback for stress management
This circuit connects to a pair of skin
electrodes on your scalp and gives you
a ‘stress level’ reading. If you can see
how your stress level varies immediately, it will help you to determine
when you are particularly stressed
and also what you need to do to relax.
It measures the magnitude of the
alpha waves produced by your brain,
which have signal frequencies in the
4-11Hz range. A small amount of these
waves (in the microvolts range) reaches the surface of your skin and can
be picked up using electrodes. These
signals are fed to a TL071 JFET-input
op amp to be amplified to a level that
can be read out on a digital voltmeter.
A small bias current (less than
750µA, varying depending on skin resistance) is applied through the skin
electrodes.
siliconchip.com.au
The AC alpha waves are coupled
to the op amp input via a 1µF MKT
capacitor, and they are DC-biased to
around -26mV by the resistor network
following that capacitor.
Higher-frequency signals are filtered
out using a 1kW/1nF RC low-pass filter plus a series ferrite bead. This filter also helps to eliminate any RF that
is picked up by the electrode leads.
The 1µF coupling capacitor and
39kW resistor form a high-pass filter
with a -3dB point of 4Hz to eliminate
very low frequency signals, below the
range of alpha waves.
The gain of this op amp stage is adjusted using trimpot VR1. It can be up
to about 1000 times (1MW ÷ 1kW). The
signal is then fed through a 15kW/1µF
RC low-pass filter which has a -3dB
point of 10.6Hz. This filters out most
Australia’s electronics magazine
signals above the 11Hz maximum frequency of alpha waves.
The signal is then rectified by diode D3, which also reduces the reading by about 0.2V, eliminating noise
from the results.
A DMM set to read millivolts connected across the output terminal
therefore gives a reading proportional
to your alpha wave activity, with VR1
correctly adjusted.
The circuit runs from a pair of 9V
batteries which produce an approximately ±8.4V split supply after reverse
polarity protection diodes D1 & D2.
100µH chokes reduce RF pickup from
the battery leads while 47W resistors
and 100µF bypass capacitors provide
further supply filtering.
David Strong,
Penshurst, NSW. ($105)
April 2021 61
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