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Heart rate monitor transmitter question

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Can anyone provide detail or insight into the Heart Rate Monitors that are sold for personal use - such as those sold by Polar (and others)? Typically a chest strap is worn - the chest strap contains some electronics that ultimately transmits a pulse for each heartbeat detected. The receiver, often worn on the wrist (or bike handlebar) receives the pulses then counts, displays, etc.

I was able to "listen" to the transmitted pulses with my AM radio held within a few feet of the strap (while it was on me). Then I took a coil of wire and connected the coil to a scope and I could see the pulse waveform if I held the coil reasonably close to the chest strap. It appeared that the transmit frequency was fairly low- maybe 100 kHz but with no storage or synch capability I had to guess.

At the same time I was having trouble getting decent readings from my monitor while on some equipment in the gym - apparently because of the rather large amount of electronic noise. I had contact a couple of manufacturer's reps for HRMs to see if anyone made one that was more immune to the noise however the reps barely understood my question. One offered that the IC used in their receiver was the same as everyone else used (only one made) so that it was unlikely any one brand or model would be superior - not sure if true.

Ultimately I'd like to solve this problem. One thought was to contruct a repeater that would be attached to the chest strap or my belt. I thought I'd post the situation here to see what others might know.
We have several of these polar units here at the uni. It has been my experience that bad or intermittend readings are caused by poor contact of the chest band electrodes. Sweat or a hairy chest do affect the electrodes for a good heart pulse pick up.
Try the equipment at the same location when you are sitting still and the sensor has a good contact on dry skin. If it works then you have found your problem. If it still plays up, you do not have a hairy chest and you are sure there is a good contact to the electrodes, check the batteries. They use largeish button cells which do not last forever. Take care when opening the units.
Klaus - I've pretty much ruled out the contact question with water, sweat and the special paste used by medical folks. As I walk toward "noisey" equipment the problem gets bad - as I walk away it goes away. Others in the gym experience similar problems. The problem is that the electrically noisey area are the cardio area where I need the monitor the most.

Thanks! I do agree with the "make sure of the basics" thinking because we often look for more complex solutions to simple problems.
Steve, your only choice then is to limit the noise source(s) :(
Since these Polar units operate at radio frequencies they also will pick up noise. I was surprised to read you could pick up the signal on the AM range. I would have thought they work in the FM mode, which is much less prone to noise interference.
I never saw the technical specs for them, maybe they are on the 'net somewhere. If you know the exact operating frequency you can easier guess how to tackle the noise source. If that option is practical is an entire other matter.

Good luck,
Klaus- several years ago I had searched the web for info and found little so I didn't bother checking again. Yesterday I had a minute and searched on "heart rate monitor schematic" and came up with a couple of interesting websites. Let me know and I'll send you the URLs. One author describes the transmitted signal as at 5 kHz and shows the waveforms on a scope.

Like you, I presumed the transmitter had to be FM or something like that. I had considered building a repeater and if I did that I'd probably use a low power FM transmitter to deal with the noise problem. Getting rid of the noise in the gym would be tough. There's at least 50 machines that generate the RF.

Anyway, I may use the receiver design that I found as a start. Thanks for the help.
Steve, that is interesting. I would have never thought these units work that way. 5Khz suggests some kind of inductive wire loop transmitter, its really too low to push a low power signal off a reasonable sized aerial.
Luckily our units are used for teaching and research and the noise interference in the labs is less of a problem. We have more trouble with EMG machines and the computers that record their signals.
Klaus - I would never have guessed that such a low frequency was used either but my ealier measurements showed something like 40 kHz - though I had to guess as a waveform was traced - the one thing that was certain is that the frequency was very low. I would love to hear from someone with some expertise on the electronics of these things. It's possible that the receiver is very broadbanded and responsive to powerful pulses in some range like 5 kHz to 50kHz - but that's a guess.

I know that in general the chest straps and receiver/computers are interchangable in that one mfr's strap works with another receiver. The signal from my chest strap is also easily rec'd by the treadmill and other high end equipment at the gym. Now and then when people are side by side with a HRMs on there is some question as to which person's heart rate is displayed on the computer. When these were first developed runners used them and I would imagine that some rather short range was a requirement given that runners often run in packs - at least for a while.
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