[time-nuts] Off topic project sort of heart rate monitor NEED BEATS PRE MINUTE TO ANALOG VOLTAGE

Hendrik Dietrich don_hendi at gmx.de
Wed Feb 20 01:58:43 EST 2013

Good morning Paul,

First: I almost spilled my coffe as you bear a similar last name as a
guy at a national research lab who does exactly such things :)

Even when it makes your ECG preamplifier free for other things, I advise
you to use another sensor and put a lot of decoding and interpretation
to your brain.

ECG is not easy from a safety standpoint, as it connects to the body
immediately. The Signal is quite complex as it is composed of not a
single but a "burst" of waves during excitation and contains a lot of
moving and electrode artifacts, I wouldn't try to count BPM except AD
conversion and doing a lot of filtering and preprocessing in software.

I would use photoplethysmography instead of ECG, means you shine a
little light from a LED thru your finger or ear to a photodiode.
Actually a optocoupler with a bit of you inbetween !
The light at the photodiode will fade up and down with the local
relative blood pressure in the tissue between the emitter and receiver,
and so does the photodiode current. The resulting signal contains way
less movement artifacts and less high-frequency parts.

to record it on a PC i would say generate a audible AM-modulated tone
with the photodiode current (amplified by an TIA) as modulation, when
you feed that to the soundcard you can record with audacity and you will
see the envelope on screen where you can count the (single) peaks per
minute. For a more immediate result without PC, hook the TIA output to a
V/F converter in the audible range and hook it up to a speaker, its
easier to listen to the pitch changing.

At work I am hooked up to heart-signal-analysing thingies quite often
and i can tell you that it is easy to change heart-rate and relative
blood pressure by will in a certain range. I made it a obligatory task
to students, visitors and new co-workers to prove that they have a
heartbeat ! :)

Best regards


P.S.1.: My first post at the time-nuts
P.S.2.: Don't let the FDA see anything :)

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