The Story of My ECG
Chapter 1 - Introduction
What is an ECG?
An ECG is a three letter acronym for ElectroCardioGraph. It is also known as an EKG which stands for ElektroKardioGraph (the German translation of the word). Basically, this device detects and displays small voltages generated by the human heart. If you would like to know more about the details (ie, how are the voltages generated, why they exist, etc), please refer to the fine books I have in the Notes section.
Voltages? What voltages? You said voltages are dangerous.
As many people know, voltages are not the killer in electricity. It is the current that people have to worry about. The current can cause a person's heart to stop beating or to beat irregularly. The amount of current needed to kill a person varies. Of course by age/weight/size/etc, but it also varies by where the current is applied and how much goes through the heart. Basically, if more than 1mA is applied to the skin, a person can feel it. Anything higher than about 5mA causes pain. About 20mA is what's necessary to paralyze someone because of involuntary muscle spasm. And around 80mA causes the heart to lose it's timing. If electricity is applied directly to the surface of the heart, only 80uA is needed to kill someone.
History - no one likes history
This web page has a good summary of the history behind the ECG. To summarize the summary, there once was a guy named Willem Einthoven. He found out that the heart creates voltages. However, in his time - 1887, there were no good ways to detect and display it. He found that if he stuck his hands and his foot in buckets of saline, and then using a string galvanometer to amplify and display the signal, he could view the signal generated from the heart. Since then, the ECG has gone the electronics way and is amplified with silicon amplifiers and displayed various ways.
Been there, done that, why you?
So now you may be asking yourself, "Okay, it's been invented, why would you want to build one?" Good question! Having being recently laid off from my employer. (Oops, the politically correct term should be "Workforce managed"). I found a little bit more time on my hands. So while looking for a job, I began to think, "Why not biomedical engineering?" Even though I'm an Electrical Engineer with no background in biomedical engineering, I figured, "Why can't I be in that field?" I've always wanted to save people's lives and help make society better (doesn't everyone?). So I bought the books, and studied the information. Suddenly, while reading about the history of the ECG, it hit me. If there's a guy that could make an ECG in 1887, I should be able to do it too. Not only that, but if these things have been in use for 200 years, I should be able to make one cheaply. After all, it's just an amplifier (I snicker here because after designing it, I now know it's not that easy).
Money doesn't grow on trees
So now I needed to make up some constraints. The first one - cost. After all, I'm unemployed. I can't go out spending thousands of dollars building this thing. I wanted to build this thing as cheaply as possible. In fact, one of my biggest plan was to make it cost less than $10. This is a far cry from the ECG's you can buy.
My second constraint was simplicity. I wanted to make sure anyone could go out and build this device. I didn't want to make people go out and buy thousand dollar oscilloscopes or need to have access to a special lab. Not only do I not have access to these labs, but I don't have oscilloscopes or other test equipment. All I have is my trusty power supply, some various electronic parts, and this multimeter. Of course, since I wanted to make this simple enough that anyone could build it, I wanted to design things such that you wouldn't need a multimeter or a power supply.
Trusty power supply
Various electronic parts - that's not all of it, but you get the idea
El-Cheapo multimeter. Purchased from Harbor Freight
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