- Home made ECG number 2

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My new ECG is a work in progress (See my first working ECG here). You've got to admit - the orange decal is both stylish and back in fashion. The idea is to use an antiquated and cheap ECG from the 80s (or is it the 70s?) connected to a computer which is running CRO software.

A trace can then be recorded. Ideally I would use a cheap laptop and attach it to the ECG where the old paper recorder used to be (blending old with new). These units could be created for literally a few dollars (all old parts) - and sent to countries that really need diagnostic equipment.

And even better than the 'heaven' ecg mk 1 - there is electrical isolation built in to just about every old ECG you can buy. Safety, style & the seventies. What more could you want?

Here's the first post:

Some of you may remember my first attempt to build an ecg
Which was terrific fun, but never gave me a nice signal.

So i've changed tack. The idea was to buy a used ECG for cheap, then convert it so that it plugged into the computer and could be used to take a good quality trace.
My purchase was this:
small ecg
Hugely expensive at $10, it comes complete with a few problems:

1) Broken nylon cog means that it will never trace onto paper again (likely).
broken cog

2) No patient cable - the one that you hook up to your body
And i can't find any reference for this model (MES-2000) on the web, probably because it is seriously old.

3) No tested interface with computer. I suspect I can use the CRO output and connect it to my sound card, then use CRO software to display the ecg trace, just like my old ecg.
interface panel

So, what I would like to do is the following:

  • Get a new patient cable, or make one. This means that I need to work out what the individual parts of the plug correspond to. If you know what each of the numbers actually refers to (I assume it refers to the actual sensor number, i.e. a 12 lead EKG has 12 sensors and hence 1 is sensor 1 etc), and if you know where to get either a replacement plug/cable, please let me know! Only problem is that the leads are not usually numbered 1 to 11, so i don't know what corresponds to what.
  • Make sure the ECG actually outputs a good signal via the CRO port. I think it is just a standard earphone jack, and if so, is it possible that it will output to a sound card just like my last ECG?
  • Work out the polarity and voltage of the power DC port. I suspect it is 12v (it takes 8 D cells), but how does one know if the tip or sleeve is positive?
  • Get it all working. Any hints or tips would be appreciated. If, for some ludicrous reason, a replacement cog exists, please let me know! It would be great to have the option of paper trace.

So, Please give me some input on how to solve the above issues!

As you can see, this ECG is still a total work in progress! If you are reading this and know the answer to any of questions, please click on More Info at the top of the page and mail me. I'd love to know your answers.

Update 23/5/08 - Computer interface completed

I have soldered an audio jack to the CRO port, and plugged it into my computer. The result using the '1mV' test tone button the CRO is shown in the image below


Please note, these are not QRS complexes, rather, they are JUST the test tone on the ECG. However, a new problem has emerged.

When anything is plugged into the patient port (I used three resistors, hypothesising that port 1 is R arm, port 2 is L arm, port 4 is patient ground) a huge noise feedback event takes place. This results in a totally unreadable signal. It does not occur when nothing is plugged into the patient port - so this begs the question - what kind of circuitry occurs after the plug?

Since this is likely to be a constant thing in all ECGs, what occurs between the plug and the patient? Will not having all patient cables connected cause the feedback described? Could it be as a result of not grounding either the ECG or the patient correctly?

I need your help!

So, as you can see - there is a small problem working out which cable corresponds to the correct plug. Needless to say - it WILL be solved. If you have a suggestion, please email it through.

Update 27/5/08

I've received some mail about the lead pattern.

From Jamie comes this:


Jamie here - I have a few paramedics standing next to me saying:

Pin 1 is likely ground, the next 4 are likely limbs, and the next 6 are

One says - since the electronic isolation is INSIDE the unit, there is
almost certainly nothing but copper to the patient - no external wiring.

This isn't to be taken as true true true fact - but it's the best info
we have here!

Hope that helps


The only point for clarification is what 'ground' acutally means. For example, I though that electrode 4 (RL) is the patient ground, if so, what is pin one on the ecg? Jamie suggests the following:

Hi Marcus

I believe they were referring to patient ground. (bed)

And you need minimum 3 leads to get a signal - positive, negative, and
ground (in my limited comprehension).

Good luck!



Bed ground - just a term - grounded to the patient, not the earth.

If they were on (and touching) a metal table, it would work...


So perhaps pin 1 is the patient, pin 5 (i.e. electrode 4) is RL (patient ground). But why then have another pin for the patient? This seems like duplication. If you know - let me know!

Further update 27/5/08

A helpful visitor has sent me the following link to a great (free) piece of ECG monitoring software

Unfortunately it appears to be uncompiled. I have neither the software nor the expertise to compile the program into an executable. It would be great if a more techno-literate reader could email me a compiled version.

Update 1/6/08

A very helpful suggestion has led to the further dissection of the machine. By a process of deduction (and plainly simple PCB markings) the ECG leads can be identified.

First the shell was removed


What's that I can see? Coloured wires?

Further dissection


And, hiding at the bttom of the image below those resistors, you might notice some markings on the pcb.

They correspond as follows (plug port #, colour, PCB marking, theorised role)

1 brown R - Presumably the right arm

2 red L - Presumably the left arm

3 orange F - Apparently stands for left leg

4 yellow C1 - Chest electrode 1

5 blue C2, 6 purple C3, 7 gray C4, 8 white C5, 9 red C6

10 black RF - I presume that given F stands for left leg, RF means right leg (patient ground)

11 brown S - (not in line with the others, i assume this is the shielding plug for the cable)

This hypothesis would be supported by the slide plugs on the bottom of the switching unit


Which do look curously like described ones. The second set of two reads 'pre input' - slightly cut off by the shadow in the photographs.

So where does this project now stand?

The computer interface is complete. All that remains is:

1) Hope that the plugs align with the colours & thus electrodes as described.

2) Plug it in (i.e. right arm, left leg, right leg) and try to take a rhythm strip (Lead II)

Thankyou everyone who has contributed so far, it's much appreciated. Further comments certainly welcome.

Update 2/6/08

Jerry rigged the cables to provide an interface to the ecg leads. Plugged the machine in to display lead I (as in, RA LA & RF (patient ground)) and here's the results


As you can see, the P wave, QRS and T wave are all clearly visible. There is still some noise, probably caused by:

1) The fact that I am using unshielded cables
2) The fact that the machine itself is not grounded
3) The fact that my electrodes have been used about 20 times each, and that I didn't prep the skin, and that I placed them over biceps - so possibly some EMG noise

I'll work to eliminate all these factors - but as you can see for now the result is pleasingly good.


I've finished the hardware installation. From the outside, you can't see any changes


But as soon as you lift the service hatch (orange) you might notice a few changes


There are a set of audio plugs mounted. Top is RA, middle is LA or LL (whatever you want really) and the bottom one is Patient ground. Each of the plugs has now been shielded, so the ecg cables have also been shielded. I have installed a small plastic box below this unit to store the wires when it is not being used. You can now pull the audio interface cable from this center console. And yes, I will use a real label rather than tipex.s

At the same time, I tracked down the DC supply (hopefully 12v 1A) and ran a cable to supply it - perhaps in the future. Although the unit is still running on batteries, 8D cells are likely to last a long time - especially now i've removed the hot stylus and motors etc.

And here's one of the traces.


That's pretty nice compared to the noise I had before. Very happy with this.

So the only thing from here is SOFTWARE!!!

Please email me any suggestions you have about software that might suit. As you can see, I am currently using a VB program which is very basic. It does not record the trace, and there are no processing options (eg software 50hz removal).

I'm still looking for a kind programmer who knows how to compile this. If this is you, please email me :)

Update 25/6/08

I have now finished the software (at least for now). Based on Jason's original source code, I have added some functionality myself, and freelanced out the other work here to an excellence coder at GetAFreelancer. The product spec was to add saving & playback ability, bulletproof the program a little and make it more usable for the average joe.

I have every intention of releasing this software as freeware. The only condition is that you don't sell it to anyone, as that's not reasonable to any of the developers involved in the production.

You can see screenshots & download FreeECG here

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