Heart rate sensor?

[chebeba]

For heart rate sensing, I have seen many products that you wear close to your body, usually as chest straps.
I wanted to design a similar sensor into a thingie I'm building.
The web has plenty of app notes and DIY descriptions of how to do it when you shine a light through a body part, most of the time a finger. But my Google skills fail me, I cannot seem to find any description of how to do it when you just have proximity, as with a chest strap.

Anyone done such a sensor and want to share how it works?

[NiHaoMike]

Look up EKG and pulse oximeter.

[chebeba]

OK, seems I need to clarify ;-)

Oximetry (in the implementations I have found so far) requires, as I mentioned in the OP, to shine a light through a finger. So you need a ring or clamp or something like that. Not what I am looking for.

EKG (in the implementations I have found so far) requires electrodes attached to the skin using conductive wet gel. Not what I am looking for.

There are many product that sense heart rate that does not require this (Google "heart rate chest belt"). But I have not been able to find schematics or even textual descriptions on how these sensors work.

So my question is basically: How do these chest belts work, I'd like to make a similar circuit.

[Dago]

OK, seems I need to clarify ;-)

Oximetry (in the implementations I have found so far) requires, as I mentioned in the OP, to shine a light through a finger. So you need a ring or clamp or something like that. Not what I am looking for.

EKG (in the implementations I have found so far) requires electrodes attached to the skin using conductive wet gel. Not what I am looking for.

There are many product that sense heart rate that does not require this (Google "heart rate chest belt"). But I have not been able to find schematics or even textual descriptions on how these sensors work.

So my question is basically: How do these chest belts work, I'd like to make a similar circuit.

The belt just basically has a couple of electrodes (conducting rubber) that are attached to some very high input impedance op-amp amplifier circuit which amplifies the signal (also has stuff like low-pass filters, 50/60 Hz rejection filters etc. usually) which is then sampled by a microcontroller and after some signal processing you can figure out the heart rate.

I think these might be good for what you are doing: http://www.plesseysemiconductors.com/epic-plessey-semiconductors.php I was thinking of doing something with them for fun at some point.

[chebeba]

Thanks, the EPIC looks like a good product for me. A little pricey at EUR 25 per chip for a glorified OpAmp, but that's OK 

[Dago]

Thanks, the EPIC looks like a good product for me. A little pricey at EUR 25 per chip for a glorified OpAmp, but that's OK 

The Plessey EPIC-sensors are under 7,5€/piece at Mouser: http://fi.mouser.com/Search/Refine.aspx?Ne=254016&N=1323038+7454634+4290079590