ECG-EKG (Electrocardiograph)
For a university course project, we built an ECG circuit to amplify the heart's electrical activity and display it on an oscilloscope. This article walks through the amplification and filtering stages, with photographs and outputs from that project.
What is EKG?
An electrocardiogram (ECG, also called EKG) records the heart's electrical activity over time. Electrodes on the skin pick up small differences in electrical potential, which appear as a waveform.
How Does It Work?
ECG electrodes do not contain microphones or convert heart sounds into electrical signals. They detect electrical potential differences at the body surface. The measurement circuit amplifies and filters those signals. Recording heart sounds is a different measurement.
Acquiring the EKG Signal
Our project used three electrodes attached to the two arms and one leg. Electrode count is not the same as lead count: an ECG lead represents a particular view of electrical activity. Our aim was to amplify a small voltage difference and observe it on an oscilloscope.
We amplified the voltage difference between the arm electrodes with an instrumentation amplifier; the leg electrode formed part of the circuit's reference connection. Instead of using an AD624, we built the amplifier stage from operational amplifiers and resistors. This let us examine the amplification stage as part of the coursework.
The amplified signal can include baseline drift and noise as well as the ECG waveform. We used a band-pass filter to reduce unwanted components. ECG is not a single-frequency signal: the lower and upper cutoff frequencies need to preserve the waveform features being studied. Narrowing the passband too far can distort those features.
We also encountered 50 Hz mains interference and used a notch filter to reduce it. This suppresses a narrow frequency band around 50 Hz. Making that band too wide can also affect ECG components in the same range.
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Here is the screen capture of the ISIS diagram of the circuit we designed to acquire, filter, and amplify the EKG signal.
I wanted to share with you some images from our work environment and some outputs we obtained during our studies.
- Image 1: Materials used in the making of the EKG. As seen, it’s just a handful of components.
- Image 2: The printed circuit board of our EKG device.
- Image 3: The first test run of our circuit on the board.
- Image 4 and result: Our output signal as shown in the image.
We observed the circuit's output on an oscilloscope. Displaying it through a microcontroller would require analog-to-digital conversion, with a suitable sampling rate, resolution, and input range. Seeing a waveform on the oscilloscope did not establish clinical accuracy or validate the circuit's safety for use on people.
Note
This is a record of an undergraduate project, not a validated medical-device design or a construction guide for use on people. Measurements on people require equipment with appropriate patient protection and isolation. Updated October 9, 2026 to correct the explanation of ECG sensing and filtering. Sources: NHS: what an ECG records; Analog Devices: ECG filter requirements.
