Simple Electrocardiogram Report
An electrocardiogram is a device that measures electric voltage generated inside the heart, a technique developed by Dr. Willem Einthoven. The heart actually has a number of specialized "pacemaker" cells that produce an electrical charge that travels through the heart's four chambers (right and left atria and ventricles). The rate of the heartbeat is set by the Sinoatrial Node (SA) which is located in the right atrium. When it pulses it causes both atria to contract, causing blood to fill both ventricles. The electrical pulse travels from the top of the heart downwards towards the Atrioventricular node (AV) where it branches out to the left and right ventricles. As the impulse continues, the ventricles contract while the atria reset themselves, getting ready for the next beat.
The path of the heart's voltage can be described in a linear pattern as shown using an electrocardiogram. These waves have particular characteristics that correspond to the contraction and expansion of the chambers of the heart. The first significant part of the wave is labeled P and corresponds to the depolarization (or contraction) of the Atria. The group of events that follow are referred to as the QRS complex which corresponds to the ventricular depolarization. This is the where the peak voltage occurs as the ventricles fire and push blood throughout the body through the pulmonary artery to the lungs and then through the aorta to the body. The last stage in the pattern is the T wave, something of a denumont as the ventricles repolarize and reset themselves for the next beat.
EKG/ECG description (Ramsey ECG1C - Electrocardiogram Heart Monitor)
There are a number of EKG kits available for the DIY enthusiast, it's a very common circuit project. I decided to work with a kit available through Ramsey Electronics as a way to familiarize myself with the circuit design. The kit costs 44.95 and comes with a packet of electrodes and all the necessary parts.
For completed EKG boards there are a number of options, though it seems as though a good choice would be these heart rate monitors from Polar
The core of an EKG circuit is what's known as a differential amplifier. A differential amplifier multiplies the difference between two inputs. This is a particularly useful circuit to use when dealing with noisy signals that need to be amplified. The signals an EKG is reading are extremely low-voltage and will be destroyed by static and surface noise on the skin. A differential amplifier will eliminate all the common noise between the two inputs and give you an amplified difference between the two common voltages.
In the Ramsey Kit, an LM324, single supply dual op amp is used. These are very useful op amps, specifically due to the fact that they are single supply eliminating the need for a voltage divider circuit. datasheet
Beyond this part of the circuit, the key to building an EKG is the filtering stage. Ramsey designed a series of low pass filters to eliminate as much high frequency noise using a few combinations of resistors and capacitors. The board is powered by a 9 volt battery. It is essential that a homemade ekg kit operates on batteries as running AC current through your body could be fatal.
Electrodes and Electrode Placement
The electrodes are the input device that read the heart's electrical signals. There are a number of options for electrode placement, but the simplest standard configuration describes a triangle with the heart at the center. In other words, electrode 1 placed at the left wrist, electrode 2 at the right wrist and electrode 3 (ground) at the left ankle.
These electrodes are all attached to a stereo 1/8" plug...two wires (electrodes 1 and 2) are connected to the inside of the jack and a single ground wire (electrode 3) connected to the outside of the jack. These wires can then be soldered to the ends of electrodes or the electrodes can be connected using aligator clips.
another option for electrode placement would involve placing electrodes 1 and 2 at the left and right parts of the upper chest.
The above illustration comes from a Scientific American article that describes in very good detail how to build another, even more simple EKG using an amplifier IC available from Analog Devices, AD624AD. See below for more detail.
Ramsey includes a set of somewhat lackluster electrodes with their kit. I found them to loose their adhesiveness rather quickly.
There are other options for purchasing electrodes that are more reliable. Discount Disposables offers a number of options, specifically the TD-429H Reusable EEG Electrode which I've also used for EEG monitoring. Another option is the electrode store. I also used some "superior silver" reusable self-adhering electrodes with permagel, purchased from medicalsupplygroup.com, which I felt gave me the best results.
The Ramsey ECG features a handy "monitor out" 1/8" jack. Using a mono 1/8" plug wired with the ground, again, on the outside and the power on the inside of the plug, the output can easily connect to an analog input of a microcontroller.
here's a video of the ekg's output on an oscilliscope:
The EKG has a very nice audio output feature that I found useful to record the sound of my heartbeat:
which I then used as source material for a short musical sketch