Need fast help! For homemade ECG

Thread Starter

y3804

Joined Apr 6, 2014
11
Hi everyone,

I'm trying to build a homemade ECG according to this schematic:

http://makezineblog.files.wordpress.com/2013/03/ecg_draft4.jpg

Question #1: I don't understand how I'm supposed to use the leg as a ground! How does it apply in real life? Where do the cables go? One end is on the leg, where do I put the other end? On the breadboard?

Question #2: I don't have a Go! Link ADC. What microcontroller/microchip could I use as an alternative?

I'm a beginner, but I need some help!

Thanks
 

Thread Starter

y3804

Joined Apr 6, 2014
11
Maybe I worded this badly

I'm just confused about the use of 'ground' in this circuit. I see that we're using 2 batteries to supply -9/+9V to the circuit, but we're also using another node as 'Ground'. What is 'ground' in this case? Is it literally the earth ground? Do I need to connect an electrode from my leg and wire it to a metal box? If so, how do I measure the voltage difference between the 'ground' and the RC Filter output?

Please answer. I really need help. Deadline for the project is in 2 days. I'd even pay Bitcoins (lol) if anyone can offer some help.
 

Thread Starter

y3804

Joined Apr 6, 2014
11
Here's another picture:



I don't understand what the 'ground' is! The leg electrode connects to the ground. The AD624AD connects to both +9/-9V and to the ground. wtf? How am I supposed to achieve that on the breadboard/IRL?
 

#12

Joined Nov 30, 2010
18,190
Ground is between the two batteries. It is also connected to a couple of resistors and capacitors in the amplifier section. The leg ground is usually done with a large piece of conductive plastic, but I wouldn't expect you to have that so a sponge or rag dampened with a weak salt solution would work. Cinch it on with a strap of some sort and connect a wire from that to the 10M resistor that is connected to ground in the amplifier section.

I have no idea what to do about a Go Link.
 

Key

Joined Jun 21, 2013
14
that go link thing seems to be there only for the software i assume, cant think of anything on the top of my head as an alternative
 

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AnalogKid

Joined Aug 1, 2013
8,307
The Go Link looks like an A/D converter with a USB interface, probably with some application software. But -

<stern voice>

PATIENT-CONTACT ELECTRONICS ARE FLAT FRICKING DANGEROUS.

ak
 

Thread Starter

y3804

Joined Apr 6, 2014
11
I don't think you're going to fry anybody with (2) 9 volt batteries and a 5 volt USB port.
Sorry about my (hilariously bad) mspaint skills, but here's what I have in mind. Is this correct/incorrect/dangerous?



Again, thank you so much for helping.
 

t06afre

Joined May 11, 2009
5,934
How about the electrodes? Using any other than proper medical grade electrodes. Will only give you a lot of noise and other artefacts. In commercial equipment as system named driven right leg, will always be used https://www.google.com/search?q=ecg+driven+right+leg
If you use two 9 volt batteries and a laptop computer running on battery. This will not in any way kill you. You can also reduce the right leg resistor to something in the range of 50 to 100 Kohm
 

Thread Starter

y3804

Joined Apr 6, 2014
11
Thanks for all the inputs.

Some more questions.

#1: I don't have a AD624 in-amp available locally. However, I can buy an INA141, LM741, etc. (Full list at local store: http://www.abra-electronics.com/search.php?search_query=op+amp&x=0&y=0)

What I plan to do is to buy a INA141 (G = 100). Is this enough? I heard I need at least G = 1000, so how to do this using what is available?

#2: Again, I don't have a Go! Link. Can't I just wire the output to an oscillator? Is this too dangerous? How else could I read the output without too much overhead?
 

#12

Joined Nov 30, 2010
18,190
Don't touch a 741 chip!
Use 2 amplifiers, each with gain = sqrt 1000 (31.6)
Attach the output to an oscilloscope.
Put all this in a metal box. A gain of 1000 picks up incredible amounts of noise.
 

Thread Starter

y3804

Joined Apr 6, 2014
11
What amplifier model should I buy? Can't I buy 2x INA141, make the first one G = 100 and the second one G = 10? (as bad as it sounds, could it work?)
 

#12

Joined Nov 30, 2010
18,190
That chip should work. And maybe some input protection to keep static electricity from frying the chip before you get it connected. (Always start by connecting the ground strap.) TO6 spoke of reducing the grounding resistor. That resistor needs to limit current to .005 amps during voltage equalization between the person and the amplifier. For the 400 volt static electricity model, that needs 80k ohms, but the grounding resistor also diminishes the voltage of the human, so I say keep it high, at least 1 meg.

Theoretically, the diodes won't have any effect, but I put a switch in, just in case. They do have capacitance, even if they aren't conducting.
 

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Thread Starter

y3804

Joined Apr 6, 2014
11
Thanks for the input about the diodes.

I'm now in the process of assembling the circuit. However, (yes, I'm a newbie) how am I supposed to connect the output of INA141 #1 to input of INA141 #2? Do I connect a ground wire to V- and connect the output of #1 to V+?
 

MaxHeadRoom

Joined Jul 18, 2013
20,022
As mentioned, earth ground is not required for low voltage, the application shown appears to use a Laptop PC, normally isolated common, if using a desk top computer, the the power supply common via the ports, USB, Parallel, Serial, is referenced to earth ground via the power supply.
Max.
 

Thread Starter

y3804

Joined Apr 6, 2014
11
As mentioned, earth ground is not required for low voltage, the application shown appears to use a Laptop PC, normally isolated common, if using a desk top computer, the the power supply common via the ports, USB, Parallel, Serial, is referenced to earth ground via the power supply.
Max.
Huhh.... then I'm doing it wrong? How am I supposed to connect the wires then? I won't be using a laptop (don't have ADC). I'll be directly connecting the output of the circuit to an oscilloscope

For now I've connected everything to the ground (between the two batteries, if that makes sense), like I imagined in post #9

Deadline is tomorrow.. I guess I won't have much luck then ;(
 

burger2227

Joined Feb 3, 2014
194
Well the circuit should work no matter what you connect it to once you connect the batteries. Ground is actually the common connection to both batteries when you use + and minus voltages.

The scope can read something no matter what two wires of the 3 you choose. Normally you would connect the common probe to 0 volts and read each side of the circuit anywhere you wish.
 

Thread Starter

y3804

Joined Apr 6, 2014
11
I tested my new circuit today. I double checked everything with my teacher. For some reason, I'm not picking any heart signal. There's nearly no noise when I leave the wires on the table. When I pick them up, a sinusoidal function (60Hz) appears on the oscilloscope. When I put the electrodes on my chest, nothing happens. The electrode on my leg doesn't change anything to the signal. I'm not using shielded cable and I'm using a penny as my conductor. However, the in-amp works fine. I tested with a microphone and I get a gain of ~100. Any suggestions?
 
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