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Using Electronic Modules with Jim Rowe Heart Rate Sensor Module This Jaycar XC3784 kit features an Analog Devices AD8232 heart rate monitor front-end IC which forms the ‘heart’ of this module. It provides a low-cost way to monitor the operation of the heart via an Arduino MCU or similar. It comes complete with a matching three-electrode lead; a pack of additional electrode pads is also available. E lectrocardiograms (ECG) are medical tools for measuring and recording the tiny voltages produced on the skin due to heart muscle activity. By attaching two, three or more electrodes or ‘leads’ to the skin of your wrists, ankle or chest, a professional ECG costing upwards of $5000 can record ECG waveforms to allow a GP or cardiac specialist to check your heart’s health. In the October 2015 issue of Silicon Chip, we described an Arduino-based project which allowed you to do all of this using a Windows-based laptop PC (siliconchip.au/Article/9135). The project was not intended for use in medical diagnosis, but simply for use in exploring the way your body works. It can be fun, as well as educational. You can monitor changes to your heart under various conditions, as it is affected by many things, including emotions, mental and physical activity – even breathing. All of these things can have a demonstrable effect on the heart’s ECG waveform. Being able to show this easily, safely and at a low cost is a bonus. To adapt an Arduino Uno module for sampling the low-level signals picked up by ECG electrodes, in 2015, I designed a small ‘front-end shield’ that plugged into the Arduino. It provided a high-gain (1000/2000 times) differential amplifier plus a three-pole low pass filter to reduce the sampler’s susceptibility to 50Hz hum. The Duinotech XC3784 kit comes with everything shown. While it’s called a kit, the module is already assembled. 72 Silicon Chip Australia's electronics magazine The heart rate sensor module we’re discussing in this article is basically a much-improved version of the frontend shield in our project, compressed into a single 4mm-square 20-lead SMD chip: the Analog Devices AD8232. This is a very impressive device, as you’ll soon see. This comes on the Duinotech module from Jaycar (Cat XC3784), which combines the AD8232-based module with a colour-coded three-electrode cable and a set of matching adhesive sensor electrode pads. Jaycar currently has this kit for $27.95, with 12 additional electrode pads (Cat XC3785) sold separately for $8.95. Inside the AD8232 Analog Devices describe the AD8232 as a “Heart Rate Monitor Front End”, or an “integrated signal conditioning block for ECG and other biopotential measurement applications”. A simplified version of the circuitry inside the AD8232 is shown in Fig.1. As you can see, it includes an instrumentation amplifier (InstA) to process the incoming low-level ECG signals plus three further op amps: A1, A2 and A3. A1 provides low-pass and high-pass filtering plus additional gain. A3 is used to buffer the half-supply reference voltage, ensuring that the main amplifier InstA can handle the full signal swing. A2 is used to drive the right-leg electrode lead (RLD) with an inverted siliconchip.com.au Fig.1: a simplified block diagram of the AD8232 IC. It’s described as a singlelead ECG front-end and implements various low- and high-pass filters using internal op amps. version of any common-mode signal present in the inputs to the instrumentation amplifier, InstA. This improves the common-mode rejection of the system, giving a significantly cleaner reproduction of the ECG signal. There are also two comparators, C1 and C2, used to provide ‘lead-off’ signals if either of the main electrodes is not in good contact with the skin of the wrists or arms. The result of this complexity inside the AD8232 chip is that when its inputs are connected to electrodes attached to the skin of a human body, and it’s provided with suitable support circuitry, it gives a clean analog ECG output signal. The module circuit Fig.2 shows the full circuit of the AD8232-based module. There’s very little in it apart from the AD8232 chip and a handful of passive components. It all fits on a small PCB measuring 30 × 35mm, including the mini 3.5mm TRS jack socket used to connect the three-electrode lead. Connectors CON1 and CON2 provide alternative connections for the input electrodes, with CON2 being the 3.5mm input jack and CON1 being just a set of three holes in the PCB to receive a 3-pin SIL header. CON3 is a 6-pin SIL header that provides all the power and output connections. As the labels suggest, pins 1 and 2 of CON3 are used for ground and +3.3V power, respectively; pin 3 is the ECG signal output, while pins 4 and 5 provide the ‘lead-off’ error signals. Pin 6 of CON3 is a logic input that allows the AD8232 to be placed in shutdown (standby) mode to save power when ECG readings are not needed. It is normally pulled high by a 10kΩ resistor, so all that is required to place it in standby mode is to pull it low. The rated current drain of the AD8232 chip is less than 250μA in operating mode, dropping to less than 500nA (0.5μA) in shutdown/standby mode. So it is suitable for battery-­ powered portable use. As well as being taken to pin 3 of CON3, the ECG output from pin 10 of IC1 also connects to LED1 via a 1kΩ series resistor. This allows the LED to be used to monitor the heartbeat visually. But if this is not required, the LED can be disabled simply by cutting the PCB track between the two pads of LK1. LED1 is on the module PCB at upper left, in the centre of the printed ‘heart’ symbol. LK1 is visible just to the left of Fig.2: the full circuit of the heart rate monitor module. Apart from IC1 and LED1 the circuit consists of a small number of passive components. The module also features alternative input connectors (CON1 & CON2) for the electrodes. siliconchip.com.au Australia's electronics magazine February 2023  73 the ‘heart’, above the connections for CON3. The latter is fitted underneath the PCB, ready to connect to a breadboard or another PCB. Electrode placement Fig.3 shows two of the suggested placements of the three electrodes with this kind of ECG sensor. On the left, the RA (right arm) electrode is positioned near the right wrist, the LA (left arm) electrode near the left wrist and the RL (right leg) driving electrode is above the right knee. However, another suitable position is just above the right ankle. On the right is another way of achieving much the same result. Here the RA and LA electrodes are placed just above the armpit on each side, while the RL electrode is placed on the abdomen just below the rib cage. Although it’s shown to the right, it can be placed in the centre, just above the navel. Connecting it to an Arduino Fig.3: the typical electrode placements on the human body. Note the orientation of the person is such that their face is facing upward. It’s pretty easy to connect the AD8232 Heart Monitor module to an Arduino like the standard Uno or one of the many compatibles, as shown in Fig.4. The GND and +3.3V pins on CON3 connect to the corresponding pins on the Uno, as shown by the grey and red wires, while the OUTPUT pin connects to the A0 pin of the Uno (blue wire). If you want to try using the LO- and LO+ pins, these can be connected to the Uno’s IO11 and IO10 pins (green and purple wires). And if you envisage wanting to make use of the SDN pin (pin 6) to save power, this can be connected to the Uno’s D8 pin (not shown in Fig.4). It’s also relatively easy to connect the module to an Arduino Nano, as shown in Fig.5. Note that the connections shown in both Fig.4 and Fig.5 are those expected by the sketches I found to put the module to use. Other configurations are possible as long as the software is adapted to match. Firmware and software Fig.4 (above): the connection diagram for the heart rate monitor module to an Arduino Uno or similar. Fig.5: the connection diagram to an Arduino Nano. 74 Silicon Chip Australia's electronics magazine I couldn’t find sketches or PC software on the Jaycar website for use with this module, but after searching the internet, I found references on Sparkfun’s website to a simple sketch called “Heart_Rate_Display.ino”, available to download from: https://github.com/sparkfun/ AD8232_Heart_Rate_Monitor This sketch was written by Casey Kuhns at SparkFun Electronics and seems to have been written originally for the Mini Arduino Pro. It simply sends numeric samples of the ECG signal back to the PC, where they can be displayed as a listing in the Arduino IDE’s Serial Monitor. If you have a recent IDE version (v1.6.6 or later), you can display them as a waveform using the Serial Plotter tool instead. To try out the module and kit with an Arduino Uno, I adapted the Kuhns/ SparkFun sketch to make it work with siliconchip.com.au the Uno. The adapted sketch is called “AD8232_heart_monitor_basic.ino” and is available for download from the Silicon Chip website. Trying it out I connected the Jaycar XC3784 module up to an Arduino Uno, as shown in Fig.4, then connected the Uno to a PC via a USB cable. After that, I started the Arduino IDE (v1.8.19), opened the “AD8232_heart_monitor_basic.ino” sketch, verified and compiled it. After that, I connected the plug on the end of the electrode cable into the 3.5mm jack on the module and fitted the red electrode to my right wrist, the green electrode to my left wrist, and the yellow electrode to my right leg just behind the knee. The next step was to upload the compiled sketch to the Arduino, after which it began running, with the little ‘heartbeat’ LED on the module blinking away cheerfully. When I opened the IDE’s Serial Monitor tool, I was greeted by a scrolling list of numeric samples of my ECG waveform. Of course, it is not easy to deduce much from a scrolling list of numbers, so I closed the Serial Monitor tool and opened up the Serial Plotter tool instead. This gave a waveform that was a lot easier to interpret, although there was a fair bit of noise present. So I tried moving the electrode positions a few times and kept checking the result. The plot shown in Fig.6 is about the best I could get, and as you can see, there’s still a fair bit of noise between the main QRS spikes, almost obscuring the smaller P and T bumps. Your heart & its electrical activity Most people know that your heart is basically a pump that pushes your blood around your body via its blood vessel ‘plumbing’ – the arteries and veins. The typical human adult heart is about the size of a clenched fist and weighs about 300g. It’s located near the centre of your chest and pumps about once per second. The pumping action is triggered mainly by a nerve centre inside the heart, called the sino-atrial or SA node. Each pumping cycle is initiated by a nerve impulse that starts at the SA node and spreads downwards through the heart via preset pathways. The heart comprises millions of bundles of microscopic muscle cells, which contract when triggered. The muscle cells are electrically polarised, like tiny electrolytic capacitors (positive outside, negative inside). As the trigger pulse from the SA node passes through them, they depolarise briefly and contract. So with each beat of the heart, a ‘wave’ of depolarisation sweeps from the top of the heart to the bottom. Weak voltages produced by this wave appear on the outside surface of your skin, and can be picked up using electrodes strapped to your wrists, ankles and the front of your chest. It’s these voltages (about 1mV peak-to-peak) that are captured and recorded as an electrocardiogram or ‘ECG’. The actual shape and amplitude of the ECG waveform depend upon the individual being examined and the positioning of the electrodes, but the general shape is shown in the adjacent graph. The initial ‘P’ wave is due to the heart’s atria (upper input chambers) depolarising, while the relatively larger and narrower ‘QRS complex’ section is due to the much stronger ventricles (lower output chambers) depolarising. Finally, the ‘T’ wave is due to the repolarisation of the ventricles, ready for another cycle. Doctors can evaluate several heart problems by measuring the timing of these wave components and their relative heights. They can also diagnose problems by seeing how wave components change with the various standard electrode and lead connections. Conclusion Although I think some of this noise could be removed by further experimenting with electrode placement, I also gained the impression that some of it was being picked up by the AD8232 module itself and the wiring between it and the Arduino. I suspect that, for the best results, it would be a good idea to place the module and the Arduino inside an Earthed metal box. So the AD8232 module and accompanying electrode kit provide an easy way to check your heart rate. If you get one, I suggest you also get one of the packs of extra electrode pads (Jaycar Cat XC3785), since the pads are only suitable for a single use. SC siliconchip.com.au Fig.6: a heart rate plot taken using the sample software and the Arduino IDE’s built-in Serial Plotter. Australia's electronics magazine February 2023  75