I wish to measure human pulse rate in an exercise environment.

Known options include (but are not limited to) ecg direct contact
measurements using various sensor locations (chest (eg polar), hand to hand
etc) and blood flow variation (typically thumb or ear clip.)

I want to try to produce good results using an ear clip sensor but may in
due course end up using some other method.

As ear clip sensors (IR LED to photo-detector via ear lobe & blood) are
common enough one might expect that effective circuitry would be easy enough
to develop. Getting reliable signals is more annoying than I expected :-).
Interestingly (or annoyingly) the sorts of problems that I experience match
those reported by users when I search the web. So maybe nobody ever really
sorted this out well. A major factor is the wide variation in optical
transmission through the ear lobe,  time to time, and user to user, and in
session. Sensor movements or variations in clip tension on earlobe lead to
shifts in the DC operating point as effective ear transmissivity varies
which is seen as a substantial signal level change. As the AC levels are low
(millivolts) such changes swamp the detector circuit until the DC level
again stabilises. Driving the detector via a feedback loop to keep its
operating point stable helps but is not a perfect solution. The received
signal must be reasonably aggressively low pass filtered to remove 50 (or
60) Hz hum artefacts. The low frequency of the desired signal leads to large
capacitor and resistor values (around the 1 uF and 1 to 10 Mohm range) which
makes DC operating point susceptible to capacitor leakage (so electrolytics
no good) and offset bias currents.

I have a few more ideas to try (eg feedback modulate the IR LED rather than
changing the detector operating point) but I think it's liable to prove an
annoying application still.

Interestingly, I was unable to find a single circuit diagram on web although
there are many mentions of equipment using ear clips.
Did turn up the complete circuitry for a pulse oximeter along the way :-)

SO - anybody tried this or can suggest any useful circuit ideas?



        Russell McMahon

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