This application report describes a low-cost heart-rate monitor solution based on the MSP430G2 LaunchPad™ Development Kit, which supports the MSP430G2xx microcontrollers (MCUs). A daughterboard amplifies and filters the electrocardiogram (EKG) signal before it is sent to the MCU for sampling and processing. The heartbeat-per-minute data is sent to the PC by means of the back-channel UART-over-USB available on the LaunchPad kit. Additionally, an eZ430 radio frequency (RF) target can be connected to the six-pin header on the daughterboard to transmit data wirelessly using the SimpliciTI™ network protocol. The system can be powered by universal serial bus (USB) power, a CR2032 3-V coin cell, or two AA or AAA batteries.
WARNING
The application presented here is for reference design purposes only and is not intended for any life-saving or medical-monitoring use.
Project collateral and source code discussed in this application report can be downloaded from http://software-dl.ti.com/msp430/msp430_public_sw/mcu/msp430/EKG-Based-Heart-Rate-Monitor/1_00_00_00/index_FDS.html.
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The source of the human heart beat is an electrical pulse generated by a cluster of cells within the heart called the sinoatrial (SA) node [1]. This pulse travels from the SA node through the surrounding cells of the heart and then to the atrioventricular (AV) node. The AV node acts as a gate that allows the atria to finish contraction before allowing the pulse to move on to the ventricles. Each atrium pumps blood to a corresponding ventricle. The right atrium pumps blood to the right ventricle to provide blood to the lungs. The left ventricle, sourced by the left atrium, is the chamber that pumps blood throughout the body.
The electrocardiogram (ECG) or elektrokardiogramm (EKG) is a medical standard for testing the human heart for defects and diseases [2]. Figure 1 shows the anatomy of the human heart and the waveform of the EKG signal. The EKG waveform can be used for extrapolation of data such as the number of heartbeats per minute (BPM) and the values can range from 30 to 200 BPM or 0.5 to 4 Hz.
The typical amplitude of the R wave component of the EKG signal is approximately 1 mV [3]. This peak is located within a group of peaks known as the QRS complex and represents the electrical pulse flowing through the ventricles. As this pulse travels through the blood stream, it can be detected at various points on the body. The extremities and the chest have become the standard locations for placing electrodes for acquiring the EKG signal. In this application, the subject’s finger tips act as the differential point of contact with conductive pads to detect the EKG signal.
The hardware is a daughterboard design attachable to the 10-pin headers on the LaunchPad development kit. The daughterboard contains the analog front-end components, battery connectors, and headers, while the MSP430 MCU, the back-channel UART, and the eZ430 emulator circuit with the USB connector reside on the LaunchPad kit itself [4]. The schematic diagram of the hardware is shown in Section D.