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Signal Peak Detector Circuit for Heart Beat (Help!)

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I am looking for help designing a circuit that detects the peak of a low frequency (1-2Hz) periodic signal (the amplified pulse of the heart acquired via plethysmography -- basically just an amplified/filtered led/photodiode couple on the fingertip).

The circuit must output a TTL signal for x amount of time (10-50ms) when the peak of the pulse occurs. It must also be dynamic in detecting the peak as it will be used on many different people, so simple static thresholding to a comparator will not work. I have decided I want to stay away from a pot to adjust the threshold for each individual. The constraints are that this needs to stay analog -- nothing beyond say an A/D or the likes if necessary.

The output from my plethysmography signal is very clean, but varies between individuals from as low as 1Vp-p all the way up to 5Vp-p. Scope shot and circuit is attached (this shot was when my gain was set a little higher, but I had problems with saturation in my first amp stage in some individuals so I have sinced reduced gain). I would very much like to leave my first amplifier stages intact, tack on a follower, and go from there.

The problem with sample-and-hold is that even from heartbeat to heartbeat the magnitude of the peak can change and I need to still be able to trigger on the peak of a following, lower amplitude peak. I've tried many different things, but I'd LOVE some input.

Fixed-decay sample-hold doesnt work so well because everyone's heart rate changes . . . you get the idea. One other implementation that would work that I would still need help with would be turning my original amp stages into Automatic Gain Control amplifiers so that static thresholding afterward would in fact equate to variable thresholding in real-life. Except I am under the impression that would not work so well for such low frequency signals? (1-2Hz?)

Minimizing/eliminating false positives is a priority.

Also, I know I may get flamed for this, but I am also willing to make it worth anyone's time monetarily who is kind enough to help me towards a working end product. My next step is posting on freelance sites.

Sorry for the long post . . .


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