Believe it or not, but the human body is full of electrical signals which can be picked up and analyzed.  These signals come from nerve impulses, which can cause muscle contractions.  Voltages range from microvolts (brain waves and some muscle contractions) up to around 2 millivolts p-p (peak-to-peak).  Certain conditions or exercise can increase voltages further.  Currents tend to be no more than probably a couple microamps or so.  Thanks to God's wonderful design of the body, these signals can be picked up and analyzed non-invasively through the use of skin electrodes.  Because skin resistance can be high (usually in the mega-ohm range), conductive pastes or gels are often used to make it easier to pick up these signals.  Luckily, newer equipment eliminates a lot of mess, due to higher input impedances (input resistance).  It is the varying VOLTAGES that are picked up and analyzed.  These voltages are amplified greatly (typically by a factor of 1000 X on EKG and around 10,000 X for EEG).  The output then can go to a video monitor, paper plotter, radio transmitter, and/or to an analog or digital recording system.
 
 

    Please note that limb leads ( I, II, III, aVL, aVR, and aVF) do NOT require exact placement.  However, better R-waves can be seen when using the limbs as opposed to the chest.  However, the chest versions of these leads are more convenient when monitoring for several hours or days at a time.  Unless you are doing an EKG for diagnostic reasons, many people would probably oppose the idea of shaving hair off their chests.  Also when using limb connections, be sure to place the electrodes over fleshy areas (NOT over bony areas).

    Please note that the green marks on the drawings represents the ground electrode.  The black marks represent the positive electrode.  The white marks represents the negative electrode.  If you want to do a quickie and use just two electrodes, just short the negative and ground electrodes together.  However, this technique does make the output MUCH MORE susceptible to outside interference, such as people walking around (not necessarily the patient).
 
 

    First of all, be sure to check your work when building this circuit.  I will NOT BE LIABLE for injury or death resulting from the use of this circuit.  Built it at your own risk.  When I built this circuit using 9-volt batteries for its power, I had a maximum of 7 microamps leaking from any two electrodes.  Leakage currents greater than 20 microamps renders such hardware NOT ACCEPTABLE for medical use.  Moreover, the medical profession prefers leakage currents to be no more than 10 microamps.  Please be aware that some people can begin to feel a shock when even 200-300 microamps are passed through the body.  However, be sure to test all three electrode connections (positive to negative, negative to ground, and positive to ground) after building this circuit.  Please be aware that maximum leakage currents will vary due to supply voltages, evenness of individual supply voltages, and tolerances of the components themselves.  If you have any doubt of any voltage spikes coming out of a computer, oscilloscope, or A/D (analog-to-digital) converter, please add opto-coupling.  In order to make this possible, the output of this circuit will need to have 5-9 volts (depending on individual supply voltage) added,  and then have a voltage-to-current circuit added.  The output of this extra circuit is to then go to an opto-coupler chip.  Please be aware that if opto-coupling is used, a small amount of extra programming is required due to the loss of linearity.  Probably an exponential function is in order here.