> >  But the majority of current commercial exercise heart
> >rate monitors use a strap that goes across the users chest.  You
> >dampen the two pads on the strap and it picks up your heart rate,
> >transmitting it to a display. Anyone have some thoughts on how to
> >build one of these?
>
> I have one of these. What I find amazing is that the electronics has been
> running on one 3V watch battery for about 5 years or so, which also includes
> RF transmission from the monitoring strap to a watch. Amazing. I assume it
> must be simple single pulse RF with each heart beat.

I was recently involved with a Major consumer electronics company and we
designed a commercial heart rate monitor. While ours does have unique aspects
which I cannot discuss, I can mention that which is common to all the other
chest-based heart rate monitors on the market (we have studied almost 10 of
them).

The circuit is actually quite straightforward, but it is virtually impossible to
simulate due to the low currents. The chest signal is only about 1mVpp so you
need about 100dB gain or so. Build a differential amplifier with op-amps or
discretes. You cannot trigger on amplitude alone, since the ECG signal varies a
LOT from person to person. Every time the person moves, various signal artifacts
are also generated. Add a BP-filter at around 16-20Hz and check for phase shift
instead. As for the RF portion; most products on the market transmit a quick
burst of a 5kHz or so signal. They also power down in between pulses to further
conserve battery life - typically you sense the pad-to-pad impedance.

____________________________    ______________________________________
Michael R. Hansson (M.Sc.)  \\// Synapse, Inc.
President & Systems Designer \/  Door 2-B, M&C Bldg., A.S. Fortuna St.
Tel/Fax: (+63) 32 346-2836/7 ||  Banilad, Mandaue City, Philippines
E-mail:  synapse@gsilink.com ||  -- "The Automation Design Specialists"