Help with understanding how to connect headphones audio stereo cable with electrodes

Thread Starter

Argow0921

Joined Oct 15, 2019
3
Hi,

I'm lacking some basic knowledge about electronics to finish a project and need help from you guys. I am working on project based on this: https://www.hackster.io/protocentral/single-lead-ecg-heartrate-variability-using-max30003-d20057.
I connected arduino with Protocentral MAX3003 board but I need help with understanding how should i connect electrodes with board used in that project. I don't have these special clips with audio jack and electrodes. I want to use my headphones cable and coins as electrodes. I don't understand however how should I connect coins with my headphones wires so it would send signal properly. I plug in audio jack and on the other side I have four wires of headphones cable: left audio, right audio and 2 ground. Problem is that I don't know what should I do with those ground wires. Should I connect only audio wires with coins and leave ground wires? Or I need to connect ground wires too. Based on what I thought I need to connect both audio and ground wire to close a curcuit, but connecting both signal and ground wire to one coin would result in grounding that signal wire and no signal would be sent to board at all. So I don't know what I should do.

I would appreciate any help.
 

Thread Starter

Argow0921

Joined Oct 15, 2019
3
Maybe I'll describe my problem differently. There is mini jack 3.5mm on Protocentral board to connect electrodes.
First, what kind of jack should I use? Like on the picture:
https://ibb.co/74zdmQd

Do I need microphone or stereo type of jack?
Second, I want to solder wire to jack and coin (my electrode). Should I solder wire only to audio and coin? Or microphone and coin? And what about ground, should I solder wire also to ground and coin or just one signal wire?
 

djsfantasi

Joined Apr 11, 2010
5,704
I’m not familiar with a heart rate sensor, but that won’t stop me from commenting.

First, I see no use for the audio outputs at all. Neither L Audio nor R Audio...

In your application, I don’t see the ground connections being useful. This is where my unfamiliarity with heart sensors come into play. Somewhere you need a ground reference to the body. I just don’t know where this is.

Otherwise, ignore the ground connections.

That leaves the microphone. It’s the only input you have, so connect it to the coins.

I hope this is helpful, especially if someone can tell you how to handle the ground issue.
 

Thread Starter

Argow0921

Joined Oct 15, 2019
3
Thank you very much for your reply.
First, yes, there has to be ground reference to the body, maybe someone will know how to do this.
Second, you are of course right with the fact that I need to connect microphone not audio, thank you for that.
But how to implement it with two electrodes then? Should I just solder two wires to the same microphone place on jack? Wouldn't it end up with some kind of sum of signals? Shouldn't signals of electrodes be somehow seperated?
 

Audioguru

Joined Dec 20, 2007
11,251
A heartbeat sensor uses a shielded audio cable where the signal is on the shielded inner conductor and the shield is the sensor and the preamplifier grounds.
A headphones cable usually does not use a shielded cable so it should not be used.
The sensor should be completely isolated from the preamplifier so that electrocution of the patient is impossible.
 

Dodgydave

Joined Jun 22, 2012
8,472
Heart monitors usually use 3 wire pads, left and right chest, and left leg as ground. Your system uses 2 wires, so just use left and right chest pads no headphones needed.
 
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