Wireless Heart Rate Monitor

Polar is (Swiss) a manufacturer of wristwatch-Heart Rate Monitors (HRM) that are used by people working out. They have a separate transmitter that straps around your chest, and a receiver in the wristwatch. They're a consumer product, albeit a niche one. Their manuals are a bit.. opaque.

Best regards, Spehro Pefhany

Are you talking MICS (Medical Implant Communications Service)?

Mark Witczak wrote in news:nCwQd.36312$Vg3.2296 @lakeread05:

First, Polar makes a small stand-alone receiver unit for their transmitters. The transmitters output a "burst" at 5KHz for each qrs detected. At this frequency, you don't use a conventional antenna, but rather a resonance-tuned coupling coil. The range is *very* limited (less than 2 meters) and is prone to interference. I gave up on the Polar stuff and made my own system using off-the-shelf RF modules at 413MHz.

The battery in the wrist watch is a consumer changeout, but the transmitter must be returned to the service center when the battery dies- every two-three years or so- it is a sealed unit.

The range is only slightly more than arms length because it is designed to be used in a group setting where several people can use them without interference- and this works *well*. The receiver is locking to a transmitter rate in addition to just simple detection so there is selectivity not only on signal frequency but also by steady heart rate. The transmitter requires a moisture layer between the skin and strap for best sensitivity- so if you're just sitting there in your Lazy-Boy versus getting off your ass and sweating, then the Polar is not for you.

Un bel giorno Dan Major digitò:

But it's a bit expensive (150 euro IIRC).

Actually it seems to be at 5.5 kHz.

With this method I've got some good results. More are the coil turns, better is the result (you need however a good preamplifier). Then you need something to discriminate the 5.5 kHz carrier from the noise, and something to make the moving average of the hertbeats (a small microcontroller could do both things).

I wonder why Polar used this carrier frequency; the typical use of those chest belts are for road bicycles, and the cars/motorcycles generate most of their noise in that frequency band.

And the "real" heartbeat sensor? What did you use?

Hello, I am very interest myslef in being able to log and analyze in a computer overnight heart rates with the Poalr chest band or other systems.

can you post more details?

Thanks

Luca

Dan Major wrote in news:Xns95FEC23895A26soonerboomergbronlin@68.12.19.6:

Could you post some details?

Thanks, Dave

Fred Bloggs wrote in news:421339E8.3050102 @nospam.com:

I'd like to see a consumer of a S410 change the battery. It is a bitch and is a testament to cheap shit/unobtainable customer service.

Polar is cheap crap, with .009mm AL metal type battery cage and soft plastic, coupled with nessicity for complete dissasembly - separation of LCD panel from PCB! Short battery life.

I would have paid hundreds of dollars for the 6 month battery life of this POS, if I didn't have the nuts to change it myself 4 times. In two years. The plastic tabs that hold the battery are wearing out. Short battery life + expensive HRM + needing to line up the 20+ graphite pads on the display to the PCB = f*ck you polar.

A $12 Chinese Timex is a bargain (battery changing wise) compared to the this abomination.

Shop around.

Venting, thanks!

Dave

Most of them appear to be in that area anyway. I had a cheapy wireless HRM that measured around 4.7KHz.

The polar devices are coded as well though, to identify between one and another unit. They don't deal well with beat inteference, so if you're riding in a pack, it will likely misbehave.

Sounds like a good application for a Zigbee link.

Best regards, Spehro Pefhany

I've been thinking of something similar and bought a cheap (Aldi) HRM to experiment with. This uses a similar method to the (uncoded) Polar units, in this case a 5.3kHz burst. Seems to be close enough to work with standard gym machines.

If you google back through the sci.electronics newsgroups you'll find a few comments about the Polar units - the reciever apparently uses a Neosid encapsulated inductor as the receive coil.

Given the audio frequency, it seems to me that a very cheap logger could be built by constructing a circuit built around an open-cored inductor and tuned to about 5kHz. Such a circuit will show 5kHz pulses even on an insensitive scope if held close to the HRM band. Add, perhaps, a little more filtering and a microphone preamp and then send the resulting signal to an audio recorder - one of the tiny flash-based music players with a microphone socket would be convenient, but experiments could be done with a PC sound card.

Finally, extract the 5kHz pulses from the recorded data file using software on the PC. I don't know what effect compression will have on the recording, but it would be easy to generate a test file without building any hardware at all.

-adrian

dalai lamah wrote in news: snipped-for-privacy@40tude.net:

I used the Polar plastic chest strap, just because I was lazy, and it worked. For the EKG amp I used a simple programmable-gain instrumentation amp (free sample from Analog) which gated a 555. The 555 output was hooked to the input to an RF module that was sold by Radio Shack at the time. Several other mfgrs make similae (Lynx, Abacom). Unit was slightly larger than Polar xmitr as it used two 9v batteries, but still very small.

"Thot" wrote in news:VDUQd.80789$bu.11978@fed1read06:

I used the Polar chest band, and connected to the two snaps. The EKG amp was a simple insrumentation amp, actually a free sample from Analog Devices. There are a number of such devices on the market today. I *did* have to add a ground reference electrode, just a disposable EKG electrode. The output of the amp gated a 555 on the qrs. The 555 was hooked up to an RF module from Radio Shack. The RF tx/rx set is no longer sold, however there are plenty of similar products from Linx, Abacom, etc. For the receiver I used a simple dipole or yagi antenna, depending on range needed, with the output going to a 567 tone decoder. Battery power was two 9v. If you bought everything new, it should cost less than $50.

If you want to home brew the entire thing (and not use the polar chest strap etc) you could look at feb's silicon chip

Trev

Trevor,

thank you for the link. Meanwhile, from another group, those somewhat related links have been suggested:

Luca

You might find this helpful:

I'm sure the design could be improved, but it's worked for me; and at least a few others have built working copies.

Rick Moll