OT: Turn Scope into Heart Monitor

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CJ

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if i got some of those suction cups and electrostatic gel, what what i need to do to get a heart beat on the scope?

thinking low noise inst amp shrouded in mu metal?

thanks

 

Tubetec

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Id be thinking you might need Dc coupling to get proper heart beats or at least very very good low frequency performance .Those stick on pads look to me a bit like a carbon microphone , I wonder if the input carries voltage to energise the carbon and improve sensitivity.  If you were able to bias the carbon in the pick up ,it might have enough voltage to display on the scope without amplification . I made a carbon mic from an old telephone reciever years ago ,I used a transformer to step down and balance the output from the mic but passed on 48volts phantom across the capsule , there was tons of level out of it .The other thing is that I believe you need the whole system to be floating from ground ,and then the signal is picked up between two of the pads on the chest.
 

cyrano

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Try a small speaker (.2W or so) as pickup. Attached to your chest, it should give enough signal to register on a scope, I think. I tried it as a kid, whit an audio amp as output.

Depends on what you need it for, of course. I wouldn't trust an ECG from it :D
 

benb

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CJ said:
if i got some of those suction cups and electrostatic gel, what what i need to do to get a heart beat on the scope?

thinking low noise inst amp shrouded in mu metal?

thanks
Yes, the heartbeat is a muscle and the voltage level that shows up on the skin is around one millivolt, so you need plenty of gain and good CMRR matching.  It also helps to have the circuit ground connection (and assuming you're using standard op-amps running on + and - voltages) go to a third connection on a leg, so the common-mode noise isn't so high.

Mu metal shielding won't help, as interference that gets picked up is electric, not magnetic.
 

Martin Griffith

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Just a I-Amp, more or less,isn't it
http://codinglab.blogspot.com.es/2013/07/my-heartbeat.html , simple

The normal I-Amp needs to well isolated from any AC power sources, for safety reasons.
TI has http://www.ti.com/litv/pdf/sbaa160a about front ends, and tries to sell 24 bit res ADCs
 

CJ

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holy cow , time to open up a heart clinic, thanks for the cool links!  :D

 

Martin Griffith

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This one gives more front end detail, with CMRR with a driven signal
http://www.egr.msu.edu/classes/ece480/capstone/spring13/group03/documents/ElectrocardiographyCircuitDesign.pdf
 

JohnRoberts

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Of course this should be obvious, be careful when attaching wires to your chest. This is exactly where a modest current (tens of mA) could interfere with normal heart rhythms.

JR
 

Dualflip

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Ive used piezoelectric transducers, the good old piezo buzzer works really well.

 

PRR

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You can just hold a microphone to your chest and "get heartbeat" acoustically.

I vividly recall working on this with a young musician of an  attractive shape and gender. My technical recollection is hazy, but I recall a LOT of trouble from all room sound and the LEAST motion of mike on skin. We got a few seconds which, tape-looped, was usable.

EKG is the electrical signals to the heart muscle. You read them like any other electric voltage, except they are VERY teeny. The usual techniques call one leg "common" and find the difference voltage to several points on the arms and/or chest. For medical purpose much of the noise (hiss hum buzz) must be filtered, and the wave spectra "shaped" to emphasize the medically significant details.

The electrodes are nothing special. Mostly a clothes-snap in a large round Band-Aid. Any scrap of polished metal will work. For home EKG I suspect the gel can be strong salt water (improve skin conductivity) and pressure on the electrode.
Willem_Einthoven_ECG.jpg


Mu-metal is not required. Electric shielding must be obsessive.

All input leads MUST be high-high-impedance, to control accidental current-flow into vital organs.

There may soon be an app for that.
 

JohnRoberts

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There was a popular bathroom scale that supposedly measured body fat (cough) by measuring the resistance between left foot and right foot while standing on the scale.  The scale had a micro and asked you to enter age, sex, and height.

No doubt a major part of the body fat calculation was from a lookup table.  ::)  I could perceive major swings in BF from before and after a summer run, when the significant difference was actually my hydration level.

JR

PS: The scale ran from a few C cell  batteries so not enough voltage to do harm coursing microamps of current through the scrotal region.  :eek:
 

Gene Pink

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PRR said:
For home EKG I suspect the gel can be strong salt water (improve skin conductivity) and pressure on the electrode

I suspect you would get better conductivity with ring terminals, and a handful of #10 X 3/4" brass wood screws. Don't forget the DeWalt screwgun. Try to miss bones. ;D

I sure hope this thread is only for academic interest. A sudden pulse of current can start a heart, and it can also stop it, it's all in the timing.

Gene
 

CJ

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well if i stopped my heart, how much time would i lose?  :D

but that point is mute, 79 bucks free ship gets it to the doorstep without making TI rich,

man that antique machine looks cool!  poor guy is gonna have half his skin removed with that salt water,

wonder if any of this low CMMR heart circuit stuff could be implemented in a mic pre?

what about sending a bias signal to the mic body to cancel noise like the third leg? (what?)

wonder how the first guys knew what a good scope pattern should look like?  take an average i guess, or find a young person and compare with a guy that is putting away a qt and a half a day of the juniper juice like they did in London back in 1835,


 

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PRR

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Hmmm... https://www.ebay.com/itm/New-FDA-Handheld-Portable-ECG-EKG-Machine-Heart-Beat-Monitor-LCD-USB-Bluetooth-/121692751220

Yes, an ECG, except instead of stick-on leads you press it to the different points of your body. Your heart-surgeon probably won't trust this type record. But it seems to export to PC, and if you plot a Möbius-loop heart beat, he may want to check you on his machine.

I was just reading how cheap ECG has become, how docs should be using it more. (An opposing point is that the technicians *frequently* get the leads mixed up and the reading is befuddled. My tech had to check with her boss before she let me off the table. Shouldn't an app do that?)

BUT on another price-level I found these really groovy chart printers!
https://www.ebay.com/p/FDA-CONTEC-Ecg80a-1-Channel-12-Lead-ECG-EKG-Machine-PC-Software-Printer/1240651498?iid=182473297047
https://ssli.ebayimg.com/images/g/FhQAAOSwWxNYt4kv/s-l640.jpg

Remember these grid-strips packed with fancy microphones? And how you can't get the mechanism, the paper, the pens anymore? Rectify/smooth the mike sweep output and call it "heartbeat". The $200 ECG plotter may not know the difference. (It may beep "Death Imminent!!" but we tape-up the peeper.)
 

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My3gger

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When working with sonorous theatre years ago they needed quite detailed heartbeat and sometimes a bit of blood flow from performers to PA. I bought ~5x1cm patches with 2 wires for use in hospitals and connected them to wireless system, through simple 9V circuit. Can't remember all the details except fet input, those patches made it easy because of quite high output.
 

Jarno

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Found some Elektor books that were still somewhere in a box from when we moved house.
Came across this:
hG60ZV1b.jpg

Display an ECG signal (somewhat more elaborate than just a heartbeat, but also more informative). Apparently, signal amplitude is about 200mV after this amp (and, do not connect it to a mains supply, use batteries).
 

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PRR

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Looks like a mike preamp, doesn't it?
 

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