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An Arduino-based USB el Here’s an easy-to-build Arduino project which will let you take your own electrocardiogram (ECG) and display it on a laptop PC. The software lets you read, display, save and print the electrical waveform generated by your heart – or anyone else’s. It connects to your laptop via a USB cable, which also provides the low power it needs to operate. A N ELECTROCARDIOGRAM or “ECG” is a piece of medical equipment used to measure and record the voltages produced as a result of heart muscle activity. By attaching a pair of electrodes (or “leads” as they are known in the trade) to the skin of your wrists, ankle or chest, this PC-Driven ECG project can display, record or print out the same kind of ECG waveform via your personal computer. Why would you want to build one? 54  Silicon Chip Just looking at the waveforms generated by your heart can be both fun and educational. You can monitor changes to your heart under various conditions, as your heart is affected by many things including emotions and mental and physical activity – even breathing. All of these factors have a demonstrable effect on the heart’s ECG waveform. Being able to show this easily, safely and at low cost is an added bonus. Professional ECG machines can cost anything from $5000 up and while this project is not intended to be used as a diagnostic device, the displayed, recorded and printed waveforms are of a quality approaching that of professional machines. In many ways, this new ECG Sampler can be seen as a much improved Mk2 version of the project described in the February 2005 issue of SILICON CHIP. The new design is based on a low-cost Arduino Uno/Freetronics Eleven micro­computer module, which siliconchip.com.au ectrocardiogram By JIM ROWE controls the actual sampling and sends the samples back to the laptop. Note that to ensure complete safety, the unit should only be used with a laptop PC running on batteries and doisconnected from all other external devices. It should not be used with a PC (desktop or laptop) connected to the 230V mains supply – see warning panel later in this article. (1000x/2000x) differential amplifier input stage, plus a 3-pole low-pass filter to reduce the sampler’s susceptibility to 50Hz hum. DISCLAIMER This project has not been designed for medical diagnosis. Correct interpretation of ECG waveforms and tracings is a complex and skilled procedure and requires proper medical training. The USB/ECG is presented here as an instructive and educational device only. If you are concerned about the health of your heart, consult your GP or a heart specialist. The Arduino and our ECG Sampler Shield are both powered from the laptop PC via the USB cable, so there’s no need for a separate power supply. The total current drawn by the sampler is less than 65mA. It’s easy to use, with all the Sampler’s functions controlled by a Windows-based GUI program running on the laptop PC and written in Visual C++. Both the Arduino micro’s firmware program sketch and the Visual C++ PC program executable can be downloaded (free for subscribers) from the SILICON CHIP website: www. siliconchip.com.au To allow the laptop to communicate with the Arduino via a USB cable, you’ll also have to download and install a special USB virtual COM port driver. This can be downloaded from either the main Arduino website or the Freetronics website. While you’re hooked up to the Arduino website, you’ll also need to download and install the latest version of the Arduino IDE (integrated development environment) package. Arduino shield It does this under the direction of a small firmware program “sketch” stored in the micro’s flash memory. To adapt the Arduino module for sampling the low-level signals picked up by ECG electrodes, we have designed a front-end “shield” module which plugs into the top of the Arduino module in the usual way. The shield provides a high-gain siliconchip.com.au This is the Arduino shield board that you have to build. It plugs into an Arduino Uno or Freetronics Eleven module. October 2015  55 DIGITAL I/O K A 5 6 8 1 2 3 K A 5 6 7 8 LEDS 4 470Ω K SAMPLING LED2 A λ 1 µF 6.8 µF 1.2k 20k 1X/2X AMPLIFIER IC2a ARDUINO ECG SAMPLER SHIELD SC SHIELDED LEADS (EQUAL IN LENGTH) ELECTRODE 2 20 1 5 TO CON1 INSULATED RCA PLUGS ELECTRODE 1 IMPORTANT: INSULATE ELECTRODE ENDS OF LEAD SHIELD BRAIDS 10 µF 100nF CON2 TO CON2 20k 2.2M 2.2M 1nF 1% 4.7k 0.1% 1 µF 5% 47nF 1nF 1% 4.7k 0.1% 3.0k +2.5V HI LO 2 100Ω 1 4 IC1 AD623ARZ 8 3 7 5 6 3.0k 100nF BALANCED INPUT AMPLIFIER MMC GAIN S1a 2 x100 µF 2 1 µF 100 µF 82Ω LP FILTER 10k 3 8 1 100nF SIL HEADER PINS IN THIS AREA MATE WITH HEADERS ON ARDUINO UNO OR COMPATIBLE 6 11k IC2: NE5532D 2.7k +5V 5 LP FILTER 10 S1b 9 HI LO SDA 4 AREF 7 IO13 GND IC2b IO12 ELECTRODE LEAD INPUTS CON1 1 µF 5% Fig.1: the ECG Sampler Shield circuit uses just two ICs. The low-level ECG signals from the electrodes are first amplified by differential amplifier IC1, a specialised instrumentation amplifier. Its output is then low-pass filtered and amplified by op amp IC2a, while IC2b provides additional low-pass filtering to reduce 50Hz hum. 8 3 4 1 IC1, IC2 2 6 5 4 1 2 3 A 7 1N5711W7F 1 4 A5 A4 A3 A2 A1 A0 Vin GND 8 GND 7 6 5V 3.3V 4 5 RST IOREF 3 IO11 SCL 1 100 µF IO10 PWM 2 ANALOG INPUTS K K IO9 PWM A POWER D2 λ LED1 1N5711 W7F IO8 PWM A IO7 2.2k IO5 IO6 470Ω PWM PWM D1 1N5711 W7F IO3 IO4 K PWM +5V IO2 POWER TXD IO1 L1 100 µH RXD IO0 56  Silicon Chip That’s because you’ll need this to upload our sampling firmware sketch to your Arduino micro (more about all this later). How it works As mentioned above, the project is essentially in two parts: (1) a standard Arduino microcomputer module which does the ADC (analog-to-digital conversion) sampling and sends the samples back to the laptop PC; and (2) the high-gain ECG Sampler Shield which you need to build. We’ll discuss the operation of the shield first. The muscles of the human body are controlled by electrochemical impulses which are distributed by the nervous system. On reaching their destination, the nerve impulses cause the muscles to contract and produce much larger electrical voltages. A small proportion of these voltages is conducted out to the surface of the skin, where they can be detected using sensitive equipment like an ECG. Because the heart is a large, complex group of muscles which contract cyclically in a preset sequence (see panel), it’s possible to study its overall condition by measuring the amplitude, timing and waveform of the heart muscle voltage components found on the skin. This is the reason for capturing ECG waveforms, which are obtained using two or more electrodes attached to the skin via a conductive saline solution or paste. Capturing ECG waveforms is quite a challenge, because the voltage components found on the surface of the skin are quite low in amplitude – around 1mV peak-to-peak, depending on the positions of the electrodes and the resistance between them and the skin. That’s about 1/10,000th of the voltage of a standard 9V battery! So we need to feed these tiny voltages through a high-gain amplifier, to display or record them. To make the job that much harder, the tiny voltages we want to measure are usually completely swamped by 50Hz hum, picked up by our bodies from the fields surrounding the AC wiring in our homes and offices etc. Fortunately, we are only interested in the voltage differences between the two electrodes that are being used, whereas the 50Hz hum picked up by the electrodes is virtually the same regardless of their position on the body. In other words, the 50Hz hum siliconchip.com.au siliconchip.com.au 50kW A1 Fig.2: inside the AD623ARZ instrumentation amplifier. Op amps A1 & A2 are matched gain input stages and these feed a balanced subtractor output stage based on op amp A3. The resistors are lasertrimmed to achieve the required pre­ cision. 50kW 50kW 1 Rg 6 A3 50kW Vout 8 NONINVERTING INPUT 50kW A2 OUTPUT REF 50kW 5 3 AD623ARZ INSTRUMENTATION AMP leads (or the subject’s body). This is the purpose of the 1nF bypass capacitors on each input of IC1 and also the 47nF capacitor between the two inputs. All three capacitors form a balanced lowpass filter, in conjunction with the two 4.7kΩ input series resistors. The rest of the ECG Sampler Shield’s amplifier and filter circuitry is based around IC2, an NE5532D dual lownoise op amp. The output from IC1 is fed to the input of IC2a via a low-pass filter formed by a series 10kΩ resistor and a 1µF capacitor, to give a corner frequency of about 17Hz and an attenuation of about 9dB at 50Hz. IC2a provides a small amount of DC VOLTS INPUT fixed amplification for the ECG signals. The gain here is 1x or 2x, as set by switch S1. The LO position of the switch gives unity gain (1x), while the HI position provides a gain of 2x. The overall ECG signal gain for the two switch positions is thus 1000 and 2000 times, respectively. IC2b provides additional low-pass filtering, to further reduce 50Hz hum. With the R and C values shown, this filter stage has a corner frequency of about 15Hz and provides a further attenuation of about 21dB at 50Hz. At the same time, it has unity gain for the low-frequency ECG signals. So at the output of IC2b (pin 7) we RESET 1 VIN RESET/PC6 SCL POWER CONTROL AND 5V REGULATION AREF GND 3.3V VUSB +5V SDA SCLK/PB5 RESET MISO/PB4 +3.3V MOSI/PB3 +5V GND PD5 GND VIN PD4 TX LED 11 MICRO USB-B 30 29 ATMEGA PD3 8 16U2 D– D+ PD2 XTALI XTAL2 A5 A4 A3 A2 A1 A0 PB2 RX λ LED PB1 PB0 10 9 1 2 3 4 5 λ 1 2 ATMEGA 328P RXD/PD0 TXD/PD1 16MHz PD7 PD6 9 XTAL1/PB6 PD5 PD4 16MHz 10 28 27 26 25 24 23 XTAL2/PB7 PD3 ADC5/PC5/SCL PD2 ADC4/PC4/SDA TXD/PD1 ADC3/PC3 RXD/PD0 19 18 17 16 15 14 13 12 11 6 5 4 3 2 IO13 IO12 IO11/ PWM IO10/ PWM IO9/ PWM IO8 DIGITAL I/O You can see how this is all done by referring to the circuit of Fig.1. The shielded electrode leads are brought into the ECG Sampler Shield via connectors CON1 & CON2 and then fed through 1µF capacitors and series 4.7kΩ resistors to the inputs of IC1. IC1 is an Analog Devices AD623­ ARZ, a specialised instrumentation amplifier offering very highly balanced differential inputs and hence very high common-mode signal rejection, combined with high gain. A simplified version of the circuitry inside the AD623 is shown in Fig.2 and it is essentially three op amps in one: two matched-gain input stages feeding a balanced “subtractor” output stage. The overall AD623 gain for differential-mode signals is set by external resistor Rg, which gives a gain of 1000 times (60dB) when using a value of 100Ω. To ensure that IC1 can deliver maximum undistorted output level (and to ensure that the Arduino ADC used for sampling the amplified signals can handle the largest signal swing), we connect IC1’s reference signal input (pin 5) to a low-impedance source of +2.5V DC (ie, half the 5V supply). This is provided by a voltage divider comprising the two 3.0kΩ resistors and thereby sets the zero-signal output level of IC1 to the same level. The two 2.2MΩ input bias resistors for IC1 are also returned to the same +2.5V point. Since IC1 operates with such a high gain, we also need to prevent it from amplifying any stray RF signals that may be picked up by the electrode 2 POWER Circuit details INVERTING INPUT ANALOG INPUTS is a “common mode” signal, while the tiny ECG voltages are “differential mode” signals. By using a highly-balanced differential amplifier as the input stage of the ECG amplifier, we can cancel out most of the common-mode 50Hz hum before the differential ECG voltages are amplified. By the way, the connections between the electrodes and your skin play an extremely important role in this hum cancellation, because if one connection is poor, this can upset the balance of the input amplifier. Most of the remaining 50Hz signals are removed by low-pass filtering in the later stages of the amplifier. So the output of the amplifier provides relatively clean amplified ECG signals, with little residual 50Hz hum. IO7 IO6/ PWM IO5/ PWM IO4/ PWM IO3/ PWM IO2/ PWM IO1/ TXD IO0/ RXD ADC2/PC2 ADC1/PC1 ADC0/PC0 Fig.3: block diagram of the Arduino Uno/Freetronics Eleven module. It’s based on two Atmel microcontroller chips: an ATmega328P and an ATmega16U2. The 328P micro is used as the module’s main CPU, while the 16U2 handles communication with the PC via the module’s USB port. October 2015  57 ELECTROCARDIOGRAM SAMPLING SHIELD For Arduino Uno RXD TXD IO2 IO3 IO4 IO5 S1 GAIN 20k CON1 +IN 1nF A5 1 A3 1 IC1 623 100 µF 100nF 100Ω 1nF 47nF 100nF 1210 10 µF 2.2M 2.2M 3.0k 82Ω 10k 20k IC2 5532 4.7k 4.7k CON2 –IN A4 D1 A2 1N5711 1N5711 D2 A1 GND GND 1.2k 100nF +5V 1 µF 3.0k MMC A0 2.7k 11k 2.2k RST +3.3V 100 µH A IOREF 470Ω 100 µF H 1 µF 2x100 µF 1 µF 470Ω LED2 L1 R 102 C C 52015 15180170 07108151 SILICON CHIP A LED1 POWER SAMPLING REV1.2 6.8 µF C 2015 07108151 IO6 IO7 IO8 IO9 IO11 IO10 I012 GND IO13 SCL SDA AREF L 1 µF Fig.4: follow this parts layout diagram to build the shield PCB. Fit the SMD devices first before installing the larger through-hole components (see text). Compare this photo with Fig.4 when building the shield PCB. The completed PCB is shown here plugged into the Arduino module on the case lid. end up with reasonably clean ECG signals (although still with some residual 50Hz hum), amplified either 1000 or 2000 times, depending on the setting of S1. Diodes D1 & D2, together with the series 2.2kΩ resistor, ensure that the amplified ECG output signals fed out to the Arduino ADC via the A0 pin are prevented from swinging below -0.3V or above +5.3V. This is to protect the Arduino’s ADC input from overload damage. The purpose of the second pole of gain switch S1b is to allow the Arduino to sense the current switch position, so that it can inform the software running in the laptop. As shown, S1b’s rotor is connected to the Arduino’s 58  Silicon Chip IO7 pin (used as an input), so this pin is pulled low (ie, to 0V) in the LO switch position and high (+5V) in the HI gain position. The only other main circuit components are indicators LED1 & LED2. LED1 is a power indicator, to show that the ECG Sampler is connected to your laptop and “ready to roll”. LED2 is turned on by the Arduino during sampling via the IO8 pin, to indicate that sampling is taking place. Arduino in brief Now let’s take a quick look at the other half of the ECG Sampler: the Arduino Uno microcomputer module or its 100% compatible Australian incarnation, the Freetronics Eleven. Arduino Uno seems to have been the one primarily responsible for Arduinos becoming a worldwide phenomenon so quickly. The Freetronics Eleven is a direct equivalent of the latest version of the Uno, so when we talk about one we’re also talking about the other. Basically, they’re a very compact (69 x 54mm) single-PCB microcomputer based on two Atmel microcontroller chips: an ATmega328P and an ATmega16U2. The 328P device is used as the module’s main CPU, while the 16U2 is used to handle communication with the laptop via the module’s USB port. There’s not much else, apart from a few low-level chips used for power control and regulation. Inside the 328P there’s a reasonably fast 8-bit RISC processor with 32 8-bit working registers, 32K bytes of flash memory, 1K bytes of EEPROM and 2K bytes of static RAM. There are also two 8-bit timer/counters, one 16-bit timer/ counter, a real-time counter with its own oscillator, six PWM channels, six 10-bit ADC input channels, a programmable serial USART, a master/slave SPI serial interface, an I2C compatible byte-orientated 2-wire serial interface and an on-chip analog comparator. The 16U2 device is actually not far behind the 328P in capability, with 16K bytes of self-programmable flash memory, 512 bytes of EEPROM and 512 bytes of internal SRAM. It also provides 8-bit and 16-bit timer/counters, three 8-bit PWM channels, an analog comparator and so on. More importantly, it provides a full speed USB 2.0 communications module, with a 48MHz PLL (phaselock loop), 176 bytes of USB DPRAM for endpoint memory allocation, four programmable endpoints and the ability to handle bulk, interrupt and isochronous transfers with a programmable packet size of up to 64 bytes and single or double buffering. Fig.3 shows the simplified Uno/ Eleven configuration. On the right is the 328P CPU, with its 14 digital I/O pins brought out on its right and its six ADC inputs at lower left. It uses a 16MHz crystal for its main clock (on pins 9 & 10), while a tiny reset switch is connected to pin 1 (for emergency use only). At centre left is the 16U2, with its USB data pins (29 & 30) connected to the corresponding pins on the USB socket. It also uses a 16MHz clock siliconchip.com.au crystal, which forms the reference for the internal PLL (3 x 16MHz = 48MHz) driving the USB module. The Uno/Eleven provides a choice of either deriving its power from the laptop via the USB cable or from an external 7-12V DC source via a 2.1mm concentric power socket (at upper left in Fig.3). The latter is used mainly when the Arduino is being used in free-standing applications, ie, not connected to a PC. In the case of the ECG Sampler circuit, we derive power from the laptop PC via the USB connector. If you compare the pin header labels of Fig.3 with those at the right and lower right of the main circuit of Fig.1, you’ll see how the two parts of the ECG Sampler are interconnected. The shield derives its +5V power via pin 5 of the 8-pin power header and its earth/0V from pins 6 & 7 of the same header (plus pin 7 of the 10-pin digital I/O header). It provides the amplified ECG signals to pin 1 of the 6-pin Analog Inputs header (A0/ADC0), while S1b’s switch rotor connects to pin 8 of the 8-pin digital I/O header (IO7) and LED2 is driven from pin 1 of the digital I/O header (IO8). When the ECG Sampler is working, the sequence of events is quite straightforward. Before the PC software initiates sampling, it sends a request to the Arduino to report the position of gain switch S1. The Arduino sends back a 1-character response, giving that information. Then, each time the software wants an ECG sample to be taken, it sends a 1-character “take a sample” command to the Arduino, which gets its ADC to take a 10-bit sample of the amplified ECG signal at its ADC0 input. The sample value is then sent back to the laptop, the overall sampling cycle taking less than 4.13ms. Construction All the ECG Sampler circuitry, except for the Arduino Uno/Eleven microcontroller board, is mounted on the PCB shield. This is designed to plug into the top of the Arduino board in piggyback fashion. The shield PCB measures 93 x 54mm (only 24mm longer than the Arduino itself, and the same width) and is coded 07108151. The stacked board assembly fits easily inside a standard diecast aluminium box measuring 119 x 93 x 34mm. siliconchip.com.au Your Heart & Its Electrical Activity R T P Q S ONE HEART BEAT/PUMPING CYCLE Most people are aware that the heart is basically a pump which pushes blood around the body via its blood vessel “plumbing” – the arteries and veins. A typical human adult heart is about the size of a clenched fist and weighs about 300 grams. It’s located near the centre of your chest and pumps about once per second, although this can vary widely due to age, fitness, exertion, health etc. The pumping action is triggered mainly by a nerve centre inside the heart, called the sino-atrial (SA) node. Each pumping cycle is initiated by a nerve impulse which starts at the SA node and spreads downwards through the heart via preset pathways. The heart itself is made up of millions of bundles of microscopic muscle cells, which contract when triggered. The muscle cells are electrically polarised, like tiny electrolytic capacitors R (positive outside, negative inside), and as the trigger pulse from the SA node passes through them, they depolarise briefly and contract. 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, ankle and the front of your chest. It’s these voltages (about 1mV peak-to-peak) which are captured and recorded as an electrocardiogram or ECG. The actual shape and amplitude of the ECG waveform depends on the individual being monitored and the positioning of the electrodes but the general waveform is as shown above. 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 repolarisation of the ventricles, ready for another cycle. Doctors are able to evaluate a number of heart problems by measuring the timing of these wave components and their relative heights. They can also diagnose problems by comparing the way the wave components change with the various standard electrode and lead connections, as shown below. L V1 V2 V3 V4 CHEST CROSS-SECTION V5 V6 SINO-ATRIAL (SA) NODE HEART STANDARD CONNECTION POINTS V6 V5 V1 F V2 V3 V4 LEAD NAME ELECTRODE 1 ELECTRODE 2 LIMB LEAD 1 L R LIMB LEAD II F R LIMB LEAD III F L LEAD aVR R L+F R+F LEAD aVL L LEAD aVF F R+L PRECORDIAL (x6) V1 — V6 R+L+F October 2015  59 Fig.5: this diagram shows how the Arduino module and the ECG Sampler Shield board are mounted on the lid of the case. Note that the Arduino module sits on M3 Nylon nuts which are used as spacers (do not use metal nuts). BASE OF 119 x 93 x 34mm DIECAST BOX (BECOMES THE COVER) ECG SAMPLER SHIELD MODULE CON2 M3 NUTS LED1 RFC1 USB MICRO-B PLUG 15mm x M3 TAPPED SPACERS ARDUINO UNO OR FREETRONICS ELEVEN ADHESIVE FEET CROSS-SECTIONAL VIEW OF BOX LID (BECOMES BASE) M3 NYLON NUTS (AS SPACERS) 2 x 20mm M3 SCREWS The box is used upside down, with the PCB assembly mounted on the inside of the box lid and the box itself lowered down over the assembly to form a shielded enclosure. The two RCA connectors (CON1 and CON2) used for the ECG electrode leads are accessed through two 12mmdiameter holes in one end of the box, with miniature toggle switch S1 accessible via a 6.5mm-diameter hole in the same end. The two indicator LEDs protrude up through a pair of 3.5mm holes in the “top” of the box, while a small slot at the far end allows entry of the USB cable. Most of the components on the ECG Sampler’s front-end shield PCB are surface-mount devices, the exceptions being input connectors CON1 and CON2, mini toggle switch S1, the two 1µF MKT input capacitors, the two LEDs and the four SIL headers used for the interconnections to the Arduino module. Fig.4 shows the parts layout on the shield PCB. We suggest that you fit the SMD resistors first, followed by the SMD capacitors and then the diodes (D1 & D2) and ICs (IC1 & IC2). The main thing to watch with the diodes and ICs is to orientate them correctly, as shown on the overlay. When these smaller parts have all been fitted, you’ll find it quite easy to add the largest SMD component: L1. The leaded/through-hole parts can 60  Silicon Chip 2 x 10mm M3 SCREWS M3 NYLON NUTS 2 x 6mm M3 SCREWS (2 MORE ON TOP OF ECG SAMPLER PCB) then be added, taking care to fit CON1 & CON2 so that their moulded spigots pass down through their corresponding holes, thereby ensuring that each connector sits flat against the PCB. Note that you may need to enlarge the PCB holes to allow this and it may also be necessary to bend up the centre earthing pin of each socket to clear the top of the PCB. When you’re fitting the two LEDs, keep their leads quite straight and position each LED so that the underside of its body is 10mm above the top of the PCB. A 10mm-wide cardboard spacer inserted between their leads can be used to ensure that the LEDs are soldered in at the correct height. Finally, the four interconnecting SIL headers can be added. These mount on the top of the PCB with their pins passing down through it and soldered underneath. Make sure you don’t apply too much solder to the pins themselves though, because they will need to mate with the SIL sockets on each side of the Arduino board. That completes the shield PCB assembly. It can now be placed to one side while you drill the metal box. Preparing the box There aren’t very many holes to be drilled in the aluminium case but they must be accurately positioned so that the PCB assembly will fit without problems. We’ve prepared a drilling and cutting template for the case and this diagram can be downloaded in PDF format from the SILICON CHIP website and printed out. It can then be attached to the case and the holes drilled. Note that it’s best to use a small (eg, 1mm) pilot drill to start the holes to ensure accurate positioning. The 6.5mm and 12mm-diameter holes in one end of the box can be initially drilled out to 4mm and then carefully enlarged to size using a tapered reamer. The square cut-out at the other end of the box can be made by drilling a series of small holes around the inside perimeter, then knocking out the piece and filing the job to a smooth finish. Mounting the modules Once the box has been prepared, you’re ready for the final assembly. This mainly involves mounting the two PCB modules on the inside of the box lid but this needs to be done in a particular order. Fig.5 shows how it all goes together. Just follow this assembly diagram and the internal photos and you shouldn’t have any problems. Begin by attaching M3 x 15mm tapped spacers to the two holes spaced 18mm apart at one end of the lid. These should be secured using M3 x 6mm machine screws, as shown in Fig.5. That done, feed M3 x 10mm machine screws through the next pair of holes (spaced 28mm apart) and fit an siliconchip.com.au M3 Nylon hex nut on each of these screws. These Nylon nuts act as short spacers, to position the Arduino PCB just clear of the lid. Similarly, feed M3 x 20mm M3 machine screws through the final two holes in the lid (spaced 48mm apart) and fit these with M3 Nylon hex nuts as well, again to act as short spacers for the Arduino module. The next step is to turn the Arduino module upside down and check that the mounting lugs on its 2.1mm power connector don’t protrude down from the underside of the PCB by more than about 1.5mm. If they do, trim them back using a pair of sharp side cutters. This is necessary to ensure that they don’t contact with the metal lid when the Arduino module is mounted in position. Once that had been done, plug the USB cable’s micro-B plug into the matching socket on the Arduino module. The module can then be fitted to the four mounting screws on the lid, so that it rests on the four Nylon nut spacers. A pair of Nylon nuts can then be fitted to the shorter mounting screws at one end of the module to secure it in place. You won’t be able to fit nuts on the two longer screws though, because there isn’t room on the Arduino module for this to be done. Instead, this end of the assembly is secured later. The next step is to plug the ECG Sampler Shield PCB into the Arduino board, as shown in Fig.5. Make sure that all the SIL header pins go into the SIL socket holes on the Arduino. Make sure also that the mounting holes at the “LEDs end” of the shield PCB go over the two M3 x 20mm mounting screws. Push the shield PCB down until its input end rests on the two 15mm spacers. The other end (the LEDs end) should rest on top of the 2.1mm DC power socket. Once it’s in position, attach a pair of M3 hex nuts to the M3 x 20mm mounting screws, to hold both PCBs in place. The final step is to use another pair of M3 x 6mm machine screws to fasten the input end of the shield PCB to the two M3 x 15mm spacers. It’s a good idea to fit a small star lockwasher under the screw between CON1 and CON2, to make sure that the screw makes a good electrical connection with the earth copper of the PCB. This connection is used to connect the metal case to the PCB earth, for siliconchip.com.au Fig.5: the photo at top shows the Arduino module (a Freetronics Eleven has been used) mounted on the case lid, while immediately above is the completed assembly with the shield board plugged in and secured in place. proper shielding. Both screws should be firmly tightened. Final assembly Once the lid assembly has been completed, it can be fitted into the case. That’s done by first tilting it at an angle of about 20° at the RCA connector end, then lowering it into position so that these connectors and switch S1 pass through their respective holes in the case. The other end can then be lowered into position, at the same time making sure that the two LEDs on the shield PCB go through their 3.5mm holes in the base (which becomes the top). It’s then simply a matter of screwing the cover and lid together using  Datafelex/Datapol Labels (1) For Dataflex labels, go to: www.blanklabels.com.au/index. php?main_page=product_info& cPath=49_60&products_id=335 (2) For Datapol labels go to: www. blanklabels.com.au/index.php? main_page=product_info&cPath =49_55&products_id=326 the four supplied countersunk-head M4 screws. Front panel The front panel artwork is available for download as a PDF file from the SILICON CHIP website. You can then October 2015  61 15 40 15 5 40 MATERIAL: 0.15mm BRASS SHIM DIMENSIONS IN MILLIMETRES Fig.6: the electrodes are made using 40 x 40mm pieces of 0.15mm thick brass shim (see text). outer sleeve and the earth braid wires by about 15mm from the end, then fit a 25mm length of heatshrink sleeving so that the shield braid cannot make contact with anything. Only the centre conductor is soldered to the rear of the crocodile clip and you will need to remove not more than 5mm of the inner dielectric insulation before doing this. This view shows the completed unit with the front-panel label fitted. The electrode leads are terminated in RCA connectors. either print it out and hot-laminate it to protect against scratches and finger grease or you can print out a synthetic Dataflex or Datapol self-adhesive label (see above panel). Once you have the label, cut out the holes for the LEDs with a hobby knife and then attach it to the case. You can attach a laminated label using either double-sided tape or silicone adhesive. It’s also a good idea to fit four small self-adhesive rubber or plastic feet to the box lid (which becomes the base), so that the heads of the PCB mounting screws cannot scratch any surface the unit is placed on. Electrode leads Although it’s easy to obtain commercial ECG electrodes at relatively low cost, this doesn’t seem to be the case with electrode leads. So regardless of which type of electrodes you use, the simplest approach is to make up a pair of leads yourself. For this, we suggest you use a 3m 62  Silicon Chip length of reasonable-quality figure-8 stereo audio cable – the kind with a decent earth braid around each of the two centre conductors. Don’t use “el cheapo” ready-made stereo leads, because many of them don’t provide adequate shielding. The first step is to split the figure-8 cable apart over a distance of about 120mm at one end and fit each lead with an insulated RCA plug. The other end of the cable is then split over a distance of about 1.5m and the leads connected to the ECG electrodes. The simplest approach is to fit the electrode ends of the cable with small insulated crocodile clips. That’s because this type of clip is the easiest way to connect commercial ECG electrodes, which all seem to be fitted with a small metal contact stud. Presumably, commercial electrode leads have a matching clip for these studs but small crocodile clips make a good substitute. When you’re fitting these clips to the lead ends, strip back the cable’s The electrodes Although you can use the adhesive electrode pads sold in pharmacies for use with TENS machines, these are generally rather expensive. Adhesive ECG electrode pads are also available via a number of suppliers on eBay and these come at a much more reasonable cost. However, when we tried these electrodes, they didn’t seem to give a reliable low-resistance skin connection, resulting in a surprisingly high level of hum pick-up. In practice, we found that we could get much better results using a pair of simple home-made electrodes, each made from a 40 x 40mm piece of 0.15mm brass shim. Fig.6 shows the details. Use tin snips to trim the shims to size, then make two 15mm-long cuts along one side of each one, leaving a 10mm space between the two cuts in the centre. Next, bend the two ends of the 15 x 5mm strips up and towards each other, to form a pair of loops as shown in the diagram. These loops then make convenient attachment points for the alligator clip at the end of each lead. Before they’re used, be sure to snip off each corner and smooth the edges with a small file and/or fine garnet paper, so they won’t scratch the skin. And that’s it – they are simple to make siliconchip.com.au SILICON CHIP ECG SAMPLER CONTROL & DISPLAY APPLICATION ARDUINO IDE (NEEDED TO UPLOAD ECG SKETCH FIRMWARE TO THE ARDUINO) WINDOWS OPERATING SYSTEM AND GUI (GRAPHICAL USER INTERFACE) ECG ELECTRODES ECG SAMPLER SHIELD (PCB MODULE) ARDUINO USB VIRTUAL COM PORT DRIVER (USB CABLE) LAPTOP PC ARDUINO UNO OR FREETRONICS ELEVEN (WITH ECG SKETCH IN FLASH MEMORY) ECG SAMPLER Fig.7: the software block diagram. The large box on the left represents a laptop PC running Windows XP/SP3 or later, while the ECG Sampler unit is shown at right. Follow the instructions in the text to install the software. and they work extremely well. Before each electrode is applied to an ankle or wrist, or any other part of the human anatomy, both the underside of the electrode and the surface of the skin should be well moistened with saline solution, to ensure that a good low-resistance contact is made. If you don’t do this, you’ll see a lot of hum in the ECG traces. So how do you hold the electrodes firmly (but not-too-firmly) against the subject’s skin? The answer is two simple adjustable straps, each made from a 250mm length of 20mm wide Velcro felt strip, along with a 50mm length of the matching hook-strip affixed to the back of one end of each strip. In practice, each strap is run around the subject’s forearm or ankle and over its electrode, before being pulled reasonably tight to hold the electrode in place. It’s simple but it works surprisingly well. Installing the software As mentioned earlier, there are a number of software items that need to be installed on your laptop in order to use it to take ECG samples. In addition, a software “sketch” has to be uploaded to the Arduino in the ECG Sampler so that it can carry out its tasks. Fig.7 shows the software block diagram. The large box on the left represents a laptop PC, with its Windows XP/ SP3 or later operating system and GUI shown at lower left. The ECG Sampler is shown on the right, linked to the laptop via a USB cable. The ECG Sampler Application (upper left of Fig.7) needs to be installed on the laptop, together with a virtual COM port driver (lower right, in the PC siliconchip.com.au box) to allow it to communicate with the Arduino module. These are the two main items of software required in the laptop for the ECG Sampler to run. However, there’s another item of software which needs to be installed on your laptop, at least temporarily: the Arduino IDE. This is needed so that you can upload the ECG Sampler sketch to the Arduino. We suggest that you download and install this software in the following order: (1) Download the Arduino IDE from the main Arduino website at https:// www.arduino.cc/en/Main/Software We used the 1.6.5-r2-windows.exe version of the IDE but there may be a later version available by the time you read this. There’s also a zipped-up version. When you download and install the Arduino IDE, it comes with a USB virtual COM port driver to suit the Arduino Uno. This is installed in the /Drivers folder of the IDE installation. As a result, if you are using an Arduino Uno in your ECG Sampler, you’ll already have its matching USB port driver. Alternatively, if you’re using a Freetronics Eleven, you will have to download the matching USB driver from http://www.freetronics.com.au At the time of writing, this was in a zip file named FreetronicsUSBDrivers_v2.2.zip. After downloading it, unzip it into a folder so that it’s ready for installation – see below. (2) Plug the cable from your ECG Sampler into one of the USB ports on your laptop. The ECG Sampler’s power LED should immediately light but the Windows OS will probably flag a problem, indicating an error when it tried to install the driver for this “new and SAFETY WARNING To ensure complete safety, this ECG Sampler should be used only with a battery-powered laptop PC; ie, one that’s NOT connected to the mains via its charger. You should also disconnect all external cable connections to the laptop, eg, printers and network cables. Do NOT use it with a desktop or laptop PC that’s connected to the 230VAC mains, either directly or indirectly. These precautions are necessary to eliminate the remote possibility that a fault in the power supply of a mains-powered device could result in a high AC voltage being applied to the electrodes. unknown” device. Even if this doesn’t happen, you still have to install the correct driver, though. (3) Go to Control Panel on the laptop and then to Device Manager. This will show an error icon alongside an “Unknown device” listing. If you rightclick this item and open Properties, you’ll see that the problem lies with the driver for the device – it’s either not properly working or not installed at all. To install the driver, click on the “Driver” tab, select “Update Driver” and then click “Browse my computer for driver software”. You then browse to either the /Drivers folder of your Arduino IDE installation (to get Arduino’s Uno driver) or to the folder where you unzipped the Freetronics driver (to get the Freetronics Eleven driver). In either case, you should be able to October 2015  63 Fig.8: this screen grab shows the ECG Sampler program running in Windows 7 on a laptop PC and displaying a typical ECG waveform. The tiny regular oscillatory noise component in each cycle is residual 50Hz hum. see the .inf file that Windows needs to install the new USB driver. When you return to the Device Manager, Windows should be able to install the driver and you should then see the “This device is working properly” message. (4) Point your web browser to www. siliconchip.com.au and download both the Windows software for the ECG Sampler (SiliconChipECGSamplerSetup.zip) and the matching Arduino firmware sketch (sketch_for_ ECGSampler.ino). These files should be saved in your /Documents folder, in a sub-folder called /Arduino sketches. (5) Launch the Arduino IDE and direct it to that sub-folder to find the sketch. Open this and upload it to the flash memory in your ECG Sampler’s Arduino (you’ll find this process is quite straightforward). (6) Finally, unzip the SiliconChipECGSamplerSetup.zip file and double-click the .msi file to install our Windows ECG Sampler application. That’s it – you should now be ready to roll with your new ECG Sampler. Taking an ECG Apart from the Sampler’s gain switch, which is set to either LOW (1000) or HIGH (2000), all functions of the USB/ECG Sampler are controlled using the ECG Sampler program. This is easy to use because when you fire it up, it provides a GUI window (see Fig.8) which provides combo-box buttons along the top so you can set the sampling configuration: the Baud rate to be used (115,200) for communication with the Sampler, the COM port it’s connected to (usually either COM3 or COM4) and the sampling time you want (5, 10 or 20 seconds). You then start an ECG recording simply by clicking on the “Start Sampling” button. The software then shows a progress bar at the top of the application window and a sample plot display which “grows” in the accompanying graph graticule. As shown on Fig.8, there are two drop-down menus at the top, with the familiar labels “File” and “About”. As usual, the first menu gives you options for saving, reloading and printing your ECG recordings, plus an option to close the application when you’re finished. The “About” menu item simply brings up a small dialog box which shows the version number of the software. Lead configurations The electrodes can be held in place on the forearm or on an ankle using adjustable straps made from Velcro hook and loop material. 64  Silicon Chip Finally, which lead configuration should you use, just to take a basic look at your own ECG or that of someone else? Our recommendation is that you use the “Lead II” limb configuration, with lead 1 connected to the subject’s left siliconchip.com.au Parts List 1 PCB, code 07108151, 93 x 53mm 1 set of Arduino stackable shield headers (1 x 10 pin, 2 x 8 pin, 1 x 6 pin) 1 diecast aluminium box, 119 x 93 x 34mm 1 Arduino Uno or Freetronics Eleven module 1 USB cable, type A to micro-B connectors 2 RCA sockets, PCB-mount (CON1, CON2) 1 100µH 1.6A SMD inductor (L1), Murata 48101SC (element14 2112367) 1 miniature DPDT toggle switch, PCB-mount (S1) 2 M3 x 15mm tapped spacers 4 M3 x 6mm machine screws (round head) 2 M3 x 10mm machine screws (round head) 2 M3 x 20mm machine screws (round head) 6 M3 Nylon hex nuts 1 M3 metal hex nut 4 adhesive rubber/plastic mounting feet, small ECG electrode parts 2 insulated RCA plugs 3 metres of figure-8 shielded stereo cable 2 40 x 40mm squares of 0.15mm brass shim (see text) ankle and lead 2 connected to their right wrist or inside forearm. This usually gives the largest waveform amplitude, providing your electrodeskin connections are good. If you get weak waveforms with a relatively large amount of hum, this is usually a sign of poor electrode contact. So take them off, apply a bit more saline solution and try again. The exact positioning of the limb electrodes is not critical, as the limbs are really just being used as convenient conductors joined to the four “corners” of the subject’s trunk. The most important thing is to get the best possible contact to the skin. If you want to try some of the chest positions for the lead 1 electrode, the electrode positions are then fairly critical. You really need to have some medical background to know the right siliconchip.com.au 2 32mm insulated alligator clips (one red, one black) 2 50mm lengths of 20mm wide Velcro hook strip 2 250mm lengths of 20mm wide Velcro felt strip 2 25mm lengths of 4mm diameter heatshrink sleeving Semiconductors 1 AD623ARZ instrumentation op amp, SOIC-8 package (IC1) 1 NE5532D dual op-amp, SOIC-8 package (IC2) 1 3mm green LED (LED1) 1 3mm red LED (LED2) 2 1N5711W7F Schottky diodes, SOD-123 package (D1,D2) Capacitors (1206 SMD) 4 100µF 6.3V X5R ceramic 1 10µF 6.3V X5R ceramic 1 6.8µF 16V X7R ceramic 2 1.0µF 5% 100V MKT (leaded) 2 1.0µF 16V X7R ceramic 3 100nF 16V X7R ceramic 1 47nF 50V X7R ceramic 2 1nF 1% 50V C0G ceramic Resistors (0.125W, 1%, 1206 SMD) 2 2.2MΩ 1 2.7kΩ 2 20kΩ 1 2.2kΩ 1 11kΩ 1 1.2kΩ 1 10kΩ 2 470Ω 2 4.7kΩ 0.1% 1 100Ω 2 3.0kΩ 1 82Ω chest electrode positions, so it’s best to leave this to the professionals. Note that if lead 1 is used with a chest electrode, lead 2 should be connected to electrodes in all three of the limb positions so that it provides a “whole body” reference signal. In practice, this means that you’ll need to make up at least two more electrodes and connect them in parallel with the original lead 2 electrode. That’s done by connecting the additional electrodes to the ECG Sampler’s CON2 input socket via leads that are the same lengths as the original leads. If you really want to play around with all the lead configurations, you might want to make up a set of nine electrodes and leads, plus a small switch box to allow you to select any of the standard lead configurations (see SC diagram on page 59) at will. MaxiMite miniMaximite or MicroMite Which one do you want? They’re the beginner’s computers that the experts love, because they’re so versatile! And they’ve started a cult following around the world from Afghanistan to Zanzibar! Very low cost, easy to program, easy to use – the Maximite, miniMaximite and the Micromite are the perfect D-I-Y computers for every level. Read the articles – and you’ll be convinced . . . You’ll find the articles at: siliconchip.com.au/Project/Graham/Mite Maximite: Mar, Apr, May 2011 miniMaximite: Nov 2011 Colour MaxiMite: Sept, Oct 2012 MicroMite: May, June 2014 plus loads of Circuit Notebook ideas! PCBs & Micros available from On-Line Shop LOOKING FOR A PCB? PCBs for most recent (>2010) SILICON CHIP projects are available from the SILICON CHIP On-Line Shop – see the On-Line Shop pages in this issue or log onto siliconchip.com.au/PCBs You’ll also find some of the hard-to-get components to build your SILICON CHIP project, back issues, software, panels, binders, books, DVDs and much more! Please note: the SILICON CHIP On-Line Shop does not sell kits; for these, please refer to kit supplier’s adverts in this issue. October 2015  65