It's probably cheaper to use just a single IR led and phototransister,
but I thought the original desings included both red and IR, and you can
effectively subtract the red (which doesn't change with blood flow, but
does change with movement and other factors you want to exclude) from
the IR and you get a much better waveform with pulse information.

It's going to be much less trouble, however, if you simply go with a
heart rate transmitter such as a polar.  Of course you're trading off
cost for complexity/rapid development, but you didn't tell us what your
goal actually are.

The average, cheap chest strap exercise HR monitor simply sends a pulse
of 5kHz through a coil each heartbeat.  It's fairly simple to use
another coil nearby with some filtering and amplification to detect this
pulse.  Overkill, but working, is the following example:
http://www.ricksunfinishedstuff.com/hrm/hrm_circuit.html

I'm actually more interested in the oximetry - could you send the URL of
the oximeter you found?

-Adam


Russell McMahon wrote:

>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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