### Debut of the AJ4VD QRSS Gator

Warning: This post is several years old and the author has marked it as poor quality (compared to more recent posts). It has been left intact for historical reasons, but but its content (and code) may be inaccurate or poorly written.

I re-wrote the code from the previous entry to do several things. Once of which was to make a gator rather than a fish. It’s more appropriate since I’m planning on housing the transmitter at the University of Florida. To do it, I drew a gator in paint and wrote a python script to convert the image into a series of points. I’ll post it later. One thing to note was that size was a SERIOUS issue. I only have two thousand bytes of code, and every point of that gator was a byte, so it was a memory hog. I helped it dramatically by using repeating segments wherever possible, and some creative math to help out the best I could (i.e., the spines on the back) Here’s what it looks like, and the code below it…

```#include <avr/io.h>
#include <util/delay.h>

// front top LED - PA0
// inside top LED - PA1
// inside bot LED - PA2
// front bot LED - PA3

unsigned long int t_unit; // units of time
const int tDit = 100; //units for a dit
const int tDah = 255; //units for a dah
char fsk; // degree of frequency shift to use for CW
char fsk2; // degree of frequency shift to use for HELL

char light = 0; // which lights are on/off

void delay(){
_delay_loop_2(t_unit);
}

return;
if (light==0){
PORTA|=(1<<PA0); //on
PORTA|=(1<<PA1); //on
PORTA&=~(1<<PA2); //off
PORTA&=~(1<<PA3); //off
light=1;
} else {
PORTA|=(1<<PA2); //on
PORTA|=(1<<PA3); //on
PORTA&=~(1<<PA0); //off
PORTA&=~(1<<PA1); //off
light=0;

}
}

void tick(unsigned long ticks){
while (ticks>0){
delay();
delay();
ticks--;
}
}

void pwm_init() {
//Output on PA6, OC1A pin (ATTiny44a)
OCR1A = 0x00; //enter the pulse width. We will use 0x00 for now, which is 0 power.
TCCR1A = 0x81; //8-bit, non inverted PWM
TCCR1B = 1; //start PWM
}

void set(int freq, int dly){
OCR1A = freq;
tick(dly);
}

void fish(){
char mult = 3;

char f2[]={2, 3, 4, 5, 6, 7, 4, 3, 7, 4, 7, 7, 6, 5, 4, 3, 2, 2, 2, 3, 3, 3, 2, 2, 2, 3, 3, 3, 2, 2, 2, 3, 4, 5, 6, 7, 8, 4, 9, 5, 9, 6, 9, 6, 9, 6, 9, 8, 8, 7, 7, 6, 5, 4, 3, 3, 3, 4, 5, 5};

for (int i=0;i<sizeof(f2);i++) {
OCR1A = f2[i]*mult;
tick(20);
OCR1A = 1*mult;
tick(20);
}

char f3[]={1,2,3,4,3,2};

char offset=0;
while (offset<9){
for (char j=0;j<3;j++){
for (char i=0;i<sizeof(f3);i++){
char val = (f3[i]+5-offset)*mult;
if (val<mult || val > 10*mult){val=mult;}
OCR1A = val;
tick(20);
OCR1A = 1*mult;
tick(20);
}
}
offset++;
}

}

void id(){
char f[]={0,0,1,2,0,1,2,2,2,0,1,1,1,1,2,0,1,1,1,2,0,2,1,1,0,0};
char i=0;
while (i<sizeof(f)) {
if (f[i]==0){OCR1A = 0;tick(tDah);}
if (f[i]==1){OCR1A = fsk;tick(tDit);}
if (f[i]==2){OCR1A = fsk;tick(tDah);}
OCR1A=0;
tick(tDit);
i++;
}
}

void slope(){
char i=0;
while (i<25){
OCR1A = 255-i;
i++;
}
while (i>0){
i--;
OCR1A = 255-i;
}
}

int main(void)
{
DDRA = 255;
pwm_init();
t_unit=1000;fsk=10;id(); // set to fast and ID once
//fsk=50;//t_unit = 65536; // set to slow for QRSS
t_unit=60000;

while(1){;
fish();
id();
}

return 1;
}
```

### Debut of the AJ4VD QRSS Fish

Warning: This post is several years old and the author has marked it as poor quality (compared to more recent posts). It has been left intact for historical reasons, but but its content (and code) may be inaccurate or poorly written.

Finally! After a few years tumbling around in my head, a few months of reading-up on the subject, a few weeks of coding, a few days of bread-boarding, a few hours of building, a few minutes of soldering, and a few seconds of testing I’ve finally done it – I’ve created my first QRSS transmitter! I’ll describe it in more detail once I finalize the design, but for now an awesome working model. It’s all digital, consisting of 2 ICs (an ATTiny44a for the PWM-controlled frequency modulation, and an octal buffer for the pre-amplifier) followed by a simple pi low-pass filter.

My desk is a little messy. I’m hard at work! Actually, I’m thinking of building another desk. I love the glass because I don’t have to worry (as much) about fires. That sounds scary, I know.

This is the transmitter. The box is mostly empty space, but it consists of the circuit, an antenna connection, a variable capacitor for center frequency tuning, and a potentiometer for setting the degree of frequency shift modulation.

Yeah, that’s a fishy. Specifically a goldfish (the cracker). It’s made with a single tone, shifting rapidly (0.5 sec) between tones. So cool. Anyway, I’m outta here for now – getting back to the code! I think I’ll try to make a gator next…

Here’s the code that makes the fish. It sends my ID quickly, some fish, then my ID in QRSS speed using PWM.

```#include <avr/io.h>
#include <util/delay.h>

const int tDit = 270/3;
const int tDah = 270;

char fsk;
unsigned long int t_unit;

void delay(){
_delay_loop_2(t_unit);
}

PORTA^=(1<<0);
PORTA^=(1<<1);
PORTA^=(1<<2);
PORTA^=(1<<3);
}

void tick(unsigned long ticks){
while (ticks>0){
delay();
delay();
ticks--;
}
}

void pwm_init() {
//Output on PA6, OC1A pin (ATTiny44a)
OCR1A = 0x00; //enter the pulse width. We will use 0x00 for now, which is 0 power.
TCCR1A = 0x81; //8-bit, non inverted PWM
TCCR1B = 1; //start PWM
}

void set(int freq, int dly){
OCR1A = freq;
tick(dly);
}

void fish(){
char f[]={0,0,0,4,5,3,6,2,7,1,5,6,8,1,8,1,8,1,8,1,8,2,7,3,6,2,7,1,8,1,8,4,5,2,3,6,7,0,0,0};
char i=0;
while (i<sizeof(f)) {
i++;
OCR1A = 255-f[i]*15;
tick(20);
}
}

void id(){
char f[]={0,0,1,2,0,1,2,2,2,0,1,1,1,1,2,0,1,1,1,2,0,2,1,1,0,0};
char i=0;
while (i<sizeof(f)) {
if (f[i]==0){OCR1A = 255;tick(tDah);}
if (f[i]==1){OCR1A = 255-fsk;tick(tDit);}
if (f[i]==2){OCR1A = 255-fsk;tick(tDah);}
OCR1A = 255;tick(tDit);
i++;
}
}

void slope(){
char i=0;
while (i<25){
OCR1A = 255-i;
i++;
}
while (i>0){
i--;
OCR1A = 255-i;
}
}

int main(void)
{
DDRA = 255;
PORTA^=(1<<0);
PORTA^=(1<<1);
pwm_init();

t_unit=2300;fsk=50;id(); // set to fast and ID once

fsk=50;t_unit = 65536; // set to slow for QRSS

while(1){
id();
for (char i=0;i<3;i++){
fish();
}
}

return 1;
}
```

### Microcontroller Clocking a Transmitter?

I shouldn’t claim this idea as novel. After googling around, I found another person who’s doing the same thing, and now that I read his page I remember reading his page before, which is likely where I got the idea in the first place. I want to give him credit and hope that my project can turn out successfully like his! http://clayton.isnotcrazy.com/mept_v1.

Here’s my idea: The core of any transmitter is an oscillator, and in simple transmitters (like QRSS devices), it’s often a crystal. Frequency adjustment is accomplished by adjusting capacitance to ground on one of the crystal legs. Simple oscillators such as the Colpitts design (based on an NPN transistor) are often used (pictured).

See how pins 4 and 5 allow for a crystal, and pin 6 has a “CKOUT” feature?” I’m still not sure exactly what the waveform of its output looks like. The datasheet is almost intentionally cryptic. About the only thing I’ve been able to discover from the Internet is that it’s sufficient to clock another microcontroller. However, if it’s an amplified sine wave output, how cool is it that it might be able to produce RF at the same frequency at which it’s clocked?

However in my quest to design a minimal-case long-distance transmitter, I’m trying to think outside the box. Although it’s relatively simple, that’s still several parts just to make an oscillating sine wave from a crystal. The result still has to be preamplified before sending the signal to an antenna. I’m starting to wonder about the oscillator circuitry inside a microcontroller which has the ability to be clocked by an external crystal. For example, take the pinout diagram from an Atmel ATTiny2313 AVR:

Taking it a step further, I wonder if I could write code for the microcontroller to allow it to adjust its own clock speed / frequency output by adjusting capacitance on one of the legs of the crystal. A reverse-biased LED with variable voltage pressed against it from an output pin of the microcontroller might accomplish this. How cool would this be – a single chip transmitter and frequency-shifting keyer all in one? Just drop in a crystal of your choice and BAM, ready to go. Believe it or not I’ve tested this mildly and it’s producing enough RF to be able to be picked up easily by a receiver in the same room, but I’m still unsure of the power output or the waveform. If the waveform is an amplified sine wave I’m going to pass out. More likely it’s a weak sine wave needing a preamplifier still, or perhaps even amplified square waves in need of lowpass filtering…

My wife snapped a photo of me working! It’s a funny pic – I’m in my own little world sometimes…

### DIY ECG Machine On The Cheap

Update: The DIY ECG project has had several iterations. The latest one can be viewed here: https://www.swharden.com/wp/2019-03-15-sound-card-ecg-with-ad8232/

Warning: This post is several years old and the author has marked it as poor quality (compared to more recent posts). It has been left intact for historical reasons, but but its content (and code) may be inaccurate or poorly written.

## Background

You’ve probably seen somebody in a hospital hooked up to a bunch of wires used to analyze their heartbeat. The machine that visualizes heartbeat is called an electrocardiograph, or ECG. It amplifies, measures, and records the natural electrical potential created by the heart. Note that cardiac electrical signals are different than heart sounds, which are listened to with a stethoscope. The intrinsic cardiac pacemaker system is responsible for generating these electrical signals which serve to command and coordinate contraction of the four chambers at the heart at the appropriate intervals: atria (upper chambers) first, then the ventricles (lower chambers) a fraction of a second later. Analysis of these signals reveals a wealth of information about cardiac regulation, as well insights into pathological conditions.

Each heartbeat produces a little squiggle with unique properties. The squiggle is called the PQRST wave. The smooth curve in the ECG (P) is caused by the stimulation of the atria via the Sinoatrial (SA) node in the right atrium. There is a brief pause, as the electrical impulse is slowed by the Atrioventricular (AV) node and Purkinje fibers in the bundle of His. The prominent spike in the ECG (the QRS complex) is caused by this step, where the electrical impulse travels through the inter-ventricular septum and up through the outer walls of the ventricles. The sharp peak is the R component, and exact heart rate can be calculated as the inverse of the R-to-R interval (RRi).

WARNING: This page documents how I made a simple ECG machine with a minimum of parts to view the electrical activity of my own heart. Feel free to repeat my experiment, but do so at your own risk.

## Project Goal

The goal of this project is to generate an extremely cheap, functional ECG machine made from common parts, most of which can be found around your house. This do-it-yourself (DIY) ECG project is different than many others on the internet in that it greatly simplifies the circuitry by eliminating noise reduction components, accomplishing this via software-based data post-processing. Additionally, this writeup is intended for those without any computer, electrical, or biomedical experience, and should be far less convoluted than the suspiciously-cryptic write-ups currently available online. In short, I want to give everybody the power to visualize and analyze their own heartbeat!

## Video Overview

I know a lot of Internet readers aren’t big fans of reading. Therefore, I provided an outline of the process in video form. Check out the videos, and if you like what you see read more!

## Electrical Theory

Measurement: The electrical signals which command cardiac musculature can be detected on the surface of the skin. In theory one could grab the two leads of a standard volt meter, one with each hand, and see the voltage change as their heart beats, but the fluctuations are rapid and by the time these signals reach the skin they are extremely weak (a few millionths of a volt) and difficult to detect with simple devices. Therefore, amplification is needed.

Amplification: A simple way to amplify the electrical difference between two points is to use a operational amplifier, otherwise known as an op-amp. The gain (multiplication factor) of an op-amp is controlled by varying the resistors attached to it, and an op-amp with a gain of 1000 will take a 1 millivolt signal and amplify it to 1 volt. There are many different types of microchip op-amps, and they’re often packaged with multiple op-amps in one chip (such as the quad-op-amp lm324, or the dual-op-amp lm358n). Any op-amp designed for low voltage will do for our purposes, and we only need one.

Noise: Unfortunately, the heart is not the only source of voltage on the skin. Radiation from a variety of things (computers, cell phones, lights, and especially the wiring in your walls) is absorbed by your skin and is measured with your ECG, in many cases masking your ECG in a sea of electrical noise. The traditional method of eliminating this noise is to use complicated analog circuitry, but since this noise has a characteristic, repeating, high-frequency wave pattern, it can be separated from the ECG (which is much slower in comparison) using digital signal processing computer software!

Digitization: Once amplified, the ECG signal along with a bunch of noise is in analog form. You could display the output with an oscilloscope, but to load it into your PC you need an analog-to-digital converter. Don’t worry! If you’ve got a sound card with a microphone input, you’ve already got one! It’s just that easy. We’ll simply wire the output of our ECG circuit to the input of our sound card, record the output of the op-amp using standard sound recording software, remove the noise from the ECG digitally, and output gorgeous ECG traces ready for visualization and analysis!

## Parts/Cost

I’ll be upfront and say that I spent \$0.00 making my ECG machine, because I was able to salvage all the parts I needed from a pile of old circuit boards. If you need specific components, check your local RadioShack. If that’s a no-go, hit-up Digikey (it’s probably cheaper too). Also, resistor values are flexible. Use mine as a good starter set, and vary them to suit your needs. If you buy everything from Digikey, the total cost of this project would be about \$1. For now, here’s a list of all the parts you need:

• 1x low voltage op-amp LM358N \$0.40
• 1x 100kOhm resistor (brn,blk,yel) virtually free
• 1x 1kOhm resistor (brn,blk,red) virtually free
• 1x 0.1uF capacitor (104Z) virtually free
• Microphone cable to get from the op-amp to your PC
• Electrodes 3 pennies should do. (\$0.03)

## Making the Device

Keep in mind that I’m not an electrical engineer (I have a masters in molecular biology but I’m currently a dental student if you must know) and I’m only reporting what worked well for me. I don’t claim this is perfect, and I’m certainly open for (and welcome) suggestions for improvement. With that in mind, here’s what I did!

This is pretty much it. First off is a power source. If you want to be safe, use three AAA batteries in series. If you’re a daredevil and enjoy showing off your ghettorigging skills, do what I did and grab 5v from a free USB plug! Mua ha ha ha. The power goes into the circuit and so do the leads/electrodes connected to the body. You can get pretty good results with only two leads, but if you want to experiment try hooking up an extra ground lead and slap it on your foot. More on the electrodes later. The signal from the leads is amplified by the circuit and put out the headphone cable, ready to enter your PC’s sound card through the microphone jack!

Note how I left room in the center of the circuit board. That was intentional! I wanted to expand this project by adding a microcontroller to do some on-board, real-time analysis. Specifically, an ATMega8! I never got around to it though. Its purpose would be to analyze the output of the op-amp and graph the ECG on a LCD screen, or at least measure the time between beats and display HR on a screen. (More ideas are at the bottom of this document.) Anyway, too much work for now, maybe I’ll do it one day in the future.

## ECG circuit diagram:

This is the circuit diagram. This is a classical high-gain analog differential amplifier. It just outputs the multiplied difference of the inputs. The 0.1uF capacitor helps stabilize the signal and reduce high frequency noise (such as the audio produced by a nearby AM radio station). Use Google if you’re interested in learning exactly how it works.

## ECG schematic:

This is how I used my LM358N to create the circuit above. Note that there is a small difference in my board from the photos and this diagram. This diagram is correct, but the circuit in some of the pictures is not. Briefly, when I built it I accidentally connected the (-) lead directly to ground, rather than to the appropriate pin on the microchip. This required me to place a 220kOhm between the leads to stabilize the signal. I imagine if you wire it CORRECTLY (as shown in these circuit diagrams) it will work fine, but if you find it too finicky (jumping quickly from too loud to too quiet), try tossing in a high-impedance resistor between the leads like I did. Overall, this circuit is extremely flexible and I encourage you to build it on a breadboard and try different things. Use this diagram as a starting point and experiment yourself!

## The Electrodes:

You can make electrodes out of anything conductive. The most recent graphs were created from wires with gator clips on them clamping onto pennies (pictured). Yeah, I know I could solder directly to the pennies (they’re copper) but gator clips are fast, easy, and can be clipped to different materials (such as aluminum foil) for testing. A dot of moisturizing lotion applied to the pennies can be used to improve conduction between the pennies and the skin, but I didn’t find this to be very helpful. If pressed firmly on the body, conduction seems to be fine. Oh! I just remembered. USE ELECTRICAL TAPE TO ATTACH LEADS TO YOUR BODY! I tried a million different things, from rubber bands to packaging tape. The bottom line is that electrical tape is stretchy enough to be flexible, sticky enough not to fall off (even when moistened by the natural oils/sweat on your skin), and doesn’t hurt that bad to peel off.

Some of the best electrodes I used were made from aluminum cans! Rinse-out a soda can, cut it into “pads”, and use the sharp edge of a razor blade or pair of scissors to scrape off the wax coating on all contact surfaces. Although a little unconformable and prone to cut skin due to their sharp edges, these little guys work great!

## Hooking it Up

This part is the most difficult part of the project! This circuit is extremely finicky. The best way to get it right is to open your sound editor (In Windows I use GoldWave because it’s simple, powerful, and free, but similar tools exist for Linux and other Unix-based OSes) and view the low-frequency bars in live mode while you set up. When neither electrode is touched, it should be relatively quiet. When only the + electrode is touched, it should go crazy with noise. When you touch both (one with each hand) the noise should start to go away, possibly varying by how much you squeeze (how good of a connection you have). The whole setup process is a game between too much and too little conduction. You’ll find that somewhere in the middle, you’ll see (and maybe hear) a low-frequency burst of noise once a second corresponding to your heartbeat. [note: Did you know that’s how the second was invented? I believe it was ] Once you get that good heartbeat, tape up your electrodes and start recording. If you can’t get it no matter what you do, start by putting the ground electrode in your mouth (yeah, I said it) and pressing the + electrode firmly and steadily on your chest. If that works (it almost always does), you know what to look for, so keep trying on your skin. For short recordings (maybe just a few beats) the mouth/chest method works beautifully, and requires far less noise reduction (if any), but is simply impractical for long-term recordings. I inside vs. outside potential is less susceptible to noise-causing electrical radiation. Perhaps other orifices would function similarly? I’ll leave it at that. I’ve also found that adding a third electrode (another ground) somewhere else on my body helps a little, but not significantly. Don’t give up at this step if you don’t get it right away! If you hear noise when + is touched, your circuit is working. Keep trying and you’ll get it eventually.

## Recording the ECG

This is the easy part. Keep an eye on your “bars” display in the audio program to make sure something you’re doing (typing, clicking, etc) isn’t messing up the recording. If you want, try surfing the net or playing computer games to see how your heart varies. Make sure that as you tap the keyboard and click the mouse, you’re not getting noise back into your system. If this is a problem, try powering your device by batteries (a good idea for safety’s sake anyway) rather than another power source (such as USB power). Record as long as you want! Save the file as a standard, mono, wave file.

## Digitally Eliminating Noise

Now it’s time to clean-up the trace. Using GoldWave, first apply a lowpass filter at 30 Hz. This kills most of your electrical noise (> 30hz), while leaving the ECG intact (< 15Hz). However, it dramatically decreases the volume (potential) of the audio file. Increase the volume as necessary to maximize the window with the ECG signal. You should see clear heartbeats at this point. You may want to apply an auto-gain filter to normalize the heartbeats potentials. Save the file as a raw sound file (.snd) at 1000 Hz (1 kHz) resolution.

## Presentation and Analysis

Now you’re ready to analyze! Plop your .snd file in the same folder as my [ecg.py script], edit the end of the script to reflect your .snd filename, and run the script by double-clicking it. (Keep in mind that my script was written for python 2.5.4 and requires numpy 1.3.0rc2 for python 2.5, and matplotlib 0.99 for python 2.5 – make sure you get the versions right!) Here’s what you’ll see!

This is a small region of the ECG trace. The “R” peak is most obvious, but the details of the other peaks are not as visible. If you want more definition in the trace (such as the blue one at the top of the page), consider applying a small collection of customized band-stop filters to the audio file rather than a single, sweeping lowpass filter. Refer to earlier posts in the DIY ECG category for details. Specifically, code on Circuits vs. Software for noise reduction entry can help. For our purposes, calculating heart rate from R-to-R intervals (RRIs) can be done accurately with traces such as this.

Your heart rate fluctuates a lot over time! By plotting the inverse of your RRIs, you can see your heart rate as a function of time. Investigate what makes it go up, go down, and how much. You’d be surprised by what you find. I found that checking my email raises my heart rate more than first-person-shooter video games. I get incredibly anxious when I check my mail these days, because I fear bad news from my new university (who knows why, I just get nervous about it). I wonder if accurate RRIs could be used to assess nervousness for the purposes of lie detection?

This is the RRI plot where the value of each RRI (in milliseconds) is represented for each beat. It’s basically the inverse of heart rate. Miscalculated heartbeats would show up as extremely high or extremely low dots on this graph. However, excluding points above or below certain bounds means that if your heart did double-beat, or skip a beat, you wouldn’t see it. Note that I just realized my axis label is wrong (it should be sec, not ms).

A Poincare Plot is a commonly-used method to visually assess heart rate variability as a function of RRIs. In this plot, each RRI is plotted against the RRI of the next subsequent beat. In a heart which beats at the same speed continuously, only a single dot would be visible in the center. In a heart which beats mostly-continuously, and only changes its rate very slowly, a linear line of dots would be visible in a 1:1 ratio. However, in real life the heart varies RRIs greatly from beat to beat, producing a small cloud of dots. The size of the cloud corresponds to the speed at which the autonomic nervous system can modulate heart rate in the time frame of a single beat.

The frequency of occurrence of various RRIs can be expressed by a histogram. The center peak corresponds to the standard heart rate. Peaks to the right and left of the center peak correspond to increased and decreased RRIs, respectively. A gross oversimplification of the interpretation of such data would be to state that the upper peak represents the cardio-inhibitory parasympathetic autonomic nervous system component, and the lower peak represents the cardio-stimulatory sympathetic autonomic nervous system component.

Taking the Fast Fourier Transformation of the data produces a unique trace whose significance is extremely difficult to interpret. Near 0Hz (infinite time) the trace heads toward ∞ (infinite power). To simplify the graph and eliminate the near-infinite, low-frequency peak we will normalize the trace by multiplying each data point by its frequency, and dividing the vertical axis units by Hz to compensate. This will produce the following graph…

This is the power spectrum density (PSD) plot of the ECG data we recorded. Its physiological interpretation is extraordinarily difficult to understand and confirm, and is the subject of great debate in the field of autonomic neurological cardiac regulation. An oversimplified explanation of the significance of this graph is that the parasympathetic (cardio-inhibitory) branch of the autonomic nervous system works faster than the sympathetic (cardio-stimulatory) branch. Therefore, the lower peak corresponds to the sympathetic component (combined with persistent parasympathetic input, it’s complicated), while the higher-frequency peak corresponds to the parasympathetic component, and the sympathetic/parasympathetic relationship can be assessed by the ratio of the integrated areas of these peaks after a complicated curve fitting processes which completely separates overlapping peaks. To learn more about power spectral analysis of heart rate over time in the frequency domain, I recommend skimming this introduction to heart rate variability website and the article on Heart Rate Variability following Myocardial Infarction (heart attack). Also, National Institute of Health (NIH) funded studies on HRV should be available from pubmed.org. If you want your head to explode, read Frequency-Domain Characteristics and Filtering of Blood Flow Following the Onset of Exercise: Implications for Kinetics Analysis for a lot of good frequency-domain-analysis-related discussion and rationalization.

## Encouraging Words:

Please, if you try this don’t die. The last thing I want is to have some kid calling me up and yelling at me that he nearly electrocuted himself when he tried to plug my device directly into a wall socket and now has to spend the rest of his life with two Abraham Lincolns tattooed onto his chest resembling a second set of nipples. Please, if you try this use common sense, and of course you’re responsible for your own actions. I provide this information as a description of what I did and what worked for me. If you make something similar that works, I’ve love to see it! Send in your pictures of your circuit, charts of your traces, improved code, or whatever you want and I’ll feature it on the site. GOOD LUCK!

## Fancier Circuit:

If you want to try this, go for it! Briefly, this circuit uses 6 op-amps to help eliminate effects of noise. It’s also safer, because of the diodes interconnecting the electrodes. It’s the same circuit as on [this page].

## Last minute thoughts:

• More homemade ECG information can be found on my earlier posts in the DIY ECG category, however this page is the primary location of my most recent thoughts and ideas.
• You can use moisturizing lotion between the electrodes and your skin to increase conduction. However, keep in mind that better conduction is not always what you want. You’ll have to experiment for yourself.
• Variation in location of electrodes will vary the shape of the ECG. I usually place electrodes on each side of my chest near my arms. If your ECG appears upside-down, reverse the leads!
• Adding extra leads can improve grounding. Try grounding one of your feet with a third lead to improve your signal. Also, if you’re powering your device via USB power consider trying battery power – it should be less noisy.
• While recording, be aware of what you do! I found that if I’m not well-grounded, my ECG is fine as long as I don’t touch my keyboard. If I start typing, every keypress shows up as a giant spike, bigger than my heartbeat!
• If you get reliable results, I wonder if you could make the device portable? Try using a portable tape recorder, voice recorder, or maybe even minidisc recorder to record the output of the ECG machine for an entire day. I haven’t tried it, but why wouldn’t it work? If you want to get fancy, have a microcontroller handle the signal processing and determine RRIs (should be easy) and save this data to a SD card or fancy flash logger.
• The microcontroller could output heart rate via the serial port.
• If you have a microcontroller on board, why not display heart rate on a character LCD?
• While you have a LCD on there, display the ECG graphically!
• Perhaps a wireless implementation would be useful.
• Like, I said, there are other, more complicated analog circuits which reduce noise of the outputted signal. I actually built Jason Nguyen’s fancy circuit which used 6 op-amps but the result wasn’t much better than the simple, 1 op-amp circuit I describe here once digital filtering was applied.
• Arrhythmic heartbeats (where your heart screws-up and misfires, skips a beat, double-beats, or beats awkwardly) are physiological (normal) and surprisingly common. Although shocking to hear about, sparse, single arrhythmic heartbeats are normal and are a completely different ball game than chronic, potentially deadly heart arrhythmias in which every beat is messed-up. If you’re in tune with your body, you might actually feel these occurrences happening. About three times a week I feel my heart screw up a beat (often when it’s quiet), and it feels like a sinking feeling in my chest. I was told by a doctor that it’s totally normal and happens many times every day without me noticing, and that most people never notice these single arrhythmic beats. I thought it was my heart skipping a beat, but I wasn’t sure. That was my motivation behind building this device – I wanted to see what my arrhythmic beats looked like. It turns out that it’s more of a double-beat than a skipped beat, as observed when I captured a single arrhythmic heartbeat with my ECG machine, as described in this entry.
• You can improve the safety of this device by attaching diodes between leads, similar to the more complicated circuit. Theory is that if a huge surge of energy does for whatever reason get into the ECG circuit, it’ll short itself out at the circuit level (conducting through the diodes) rather than at your body (across your chest / through your heart).
• Alternatively, use an AC opto-isolator between the PC sound card and the ECG circuit to eliminate the possibility of significant current coming back from the PC.
• On the Hackaday post, Flemming Frandsen noted that an improperly grounded PC could be dangerous because the stored charge would be manifest in the ground of the microphone jack. If you were to ground yourself to true ground (using a bench power supply or sticking your finger in the ground socket of an AC wall plug) this energy could travel through you! So be careful to only ground yourself with respect to the circuit using only battery power to minimize this risk.

## SAMPLE FILTERED RECORDING:

I think this is the same one I used in the 3rd video from my single op-amp circuit. [scottecg.snd] It’s about an hour long, and in raw sound format (1000 Hz). It’s already been filtered (low-pass filtered at 30Hz). You can use it with my code below!

## CODE

```print "importing libraries..."
import numpy, pylab
print "DONE"

class ECG:

def trim(self, data,degree=100):
print 'trimming'
i,data2=0,[]
while i<len(data):
data2.append(sum(data[i:i+degree])/degree)
i+=degree
return data2

def smooth(self,list,degree=15):
mults=[1]
s=[]
for i in range(degree): mults.append(mults[-1]+1)
for i in range(degree): mults.append(mults[-1]-1)
for i in range(len(list)-len(mults)):
small=list[i:i+len(mults)]
for j in range(len(small)):
small[j]=small[j]*mults[j]
val=sum(small)/sum(mults)
s.append(val)
return s

def smoothWindow(self,list,degree=10):
list2=[]
for i in range(len(list)):
list2.append(sum(list[i:i+degree])/float(degree))
return list2

def invertYs(self):
print 'inverting'
self.ys=self.ys*-1

def takeDeriv(self,dist=5):
print 'taking derivative'
self.dys=[]
for i in range(dist,len(self.ys)):
self.dys.append(self.ys[i]-self.ys[i-dist])
self.dxs=self.xs[0:len(self.dys)]

def genXs(self, length, hz):
print 'generating Xs'
step = 1.0/(hz)
xs=[]
for i in range(length): xs.append(step*i)
return xs

def loadFile(self, fname, startAt=None, length=None, hz=1000):
self.ys = numpy.memmap(fname, dtype='h', mode='r')*-1
self.xs = self.genXs(len(self.ys),hz)
if startAt and length:
self.ys=self.ys[startAt:startAt+length]
self.xs=self.xs[startAt:startAt+length]

def findBeats(self):
print 'finding beats'
self.bx,self.by=[],[]
for i in range(100,len(self.ys)-100):
if self.ys[i]<15000: continue # SET THIS VISUALLY
if self.ys[i]<self.ys[i+1] or self.ys[i]<self.ys[i-1]: continue
if self.ys[i]-self.ys[i-100]>5000 and self.ys[i]-self.ys[i+100]>5000:
self.bx.append(self.xs[i])
self.by.append(self.ys[i])
print "found %d beats"%(len(self.bx))

def genRRIs(self,fromText=False):
print 'generating RRIs'
self.rris=[]
if fromText: mult=1
else: 1000.0
for i in range(1,len(self.bx)):
rri=(self.bx[i]-self.bx[i-1])*mult
#if fromText==False and len(self.rris)>1:
#if abs(rri-self.rris[-1])>rri/2.0: continue
#print i, "%.03ft%.03ft%.2f"%(bx[i],rri,60.0/rri)
self.rris.append(rri)

def removeOutliers(self):
beatT=[]
beatRRI=[]
beatBPM=[]
for i in range(1,len(self.rris)):
#CHANGE THIS AS NEEDED
if self.rris[i]<0.5 or self.rris[i]>1.1: continue
if abs(self.rris[i]-self.rris[i-1])>self.rris[i-1]/5: continue
beatT.append(self.bx[i])
beatRRI.append(self.rris[i])
self.bx=beatT
self.rris=beatRRI

def graphTrace(self):
pylab.plot(self.xs,self.ys)
#pylab.plot(self.xs[100000:100000+4000],self.ys[100000:100000+4000])
pylab.title("Electrocardiograph")
pylab.xlabel("Time (seconds)")
pylab.ylabel("Potential (au)")

def graphDeriv(self):
pylab.plot(self.dxs,self.dys)
pylab.xlabel("Time (seconds)")
pylab.ylabel("d/dt Potential (au)")

def graphBeats(self):
pylab.plot(self.bx,self.by,'.')

def graphRRIs(self):
pylab.plot(self.bx,self.rris,'.')
pylab.title("Beat Intervals")
pylab.xlabel("Beat Number")
pylab.ylabel("RRI (ms)")

def graphHRs(self):
#HR TREND
hrs=(60.0/numpy.array(self.rris)).tolist()
bxs=(numpy.array(self.bx[0:len(hrs)])/60.0).tolist()
pylab.plot(bxs,hrs,'g',alpha=.2)
hrs=self.smooth(hrs,10)
bxs=bxs[10:len(hrs)+10]
pylab.plot(bxs,hrs,'b')
pylab.title("Heart Rate")
pylab.xlabel("Time (minutes)")
pylab.ylabel("HR (bpm)")

def graphPoincare(self):
#POINCARE PLOT
pylab.plot(self.rris[1:],self.rris[:-1],"b.",alpha=.5)
pylab.title("Poincare Plot")
pylab.ylabel("RRI[i] (sec)")
pylab.xlabel("RRI[i+1] (sec)")

def graphFFT(self):
#PSD ANALYSIS
fft=numpy.fft.fft(numpy.array(self.rris)*1000.0)
fftx=numpy.fft.fftfreq(len(self.rris),d=1)
fftx,fft=fftx[1:len(fftx)/2],abs(fft[1:len(fft)/2])
fft=self.smoothWindow(fft,15)
pylab.plot(fftx[2:],fft[2:])
pylab.title("Raw Power Sprectrum")
pylab.ylabel("Power (ms^2)")
pylab.xlabel("Frequency (Hz)")

def graphFFT2(self):
#PSD ANALYSIS
fft=numpy.fft.fft(numpy.array(self.rris)*1000.0)
fftx=numpy.fft.fftfreq(len(self.rris),d=1)
fftx,fft=fftx[1:len(fftx)/2],abs(fft[1:len(fft)/2])
fft=self.smoothWindow(fft,15)
for i in range(len(fft)):
fft[i]=fft[i]*fftx[i]
pylab.plot(fftx[2:],fft[2:])
pylab.title("Power Sprectrum Density")
pylab.ylabel("Power (ms^2)/Hz")
pylab.xlabel("Frequency (Hz)")

def graphHisto(self):
pylab.hist(self.rris,bins=20,ec='none')
pylab.title("RRI Deviation Histogram")
pylab.ylabel("Frequency (count)")
pylab.xlabel("RRI (ms)")
#pdf, bins, patches = pylab.hist(self.rris,bins=100,alpha=0)
#pylab.plot(bins[1:],pdf,'g.')
#y=self.smooth(list(pdf[1:]),10)
#x=bins[10:len(y)+10]
#pylab.plot(x,y)

def saveBeats(self,fname):
print "writing to",fname
numpy.save(fname,[numpy.array(self.bx)])
print "COMPLETE"

self.genRRIs(True)

def snd2txt(fname):
## SND TO TXT ##
a=ECG()
a.invertYs()
pylab.figure(figsize=(7,4),dpi=100);pylab.grid(alpha=.2)
a.graphTrace()
a.findBeats()
a.graphBeats()
a.saveBeats(fname)
pylab.show()

def txt2graphs(fname):
## GRAPH TXT ##
a=ECG()
a.removeOutliers()
pylab.figure(figsize=(7,4),dpi=100);pylab.grid(alpha=.2)
pylab.savefig("DIY_ECG_Heart_Rate_Over_Time.png");
pylab.figure(figsize=(7,4),dpi=100);pylab.grid(alpha=.2)
pylab.savefig("DIY_ECG_Power_Spectrum_Raw.png");
pylab.figure(figsize=(7,4),dpi=100);pylab.grid(alpha=.2)
pylab.savefig("DIY_ECG_Power_Spectrum_Weighted.png");
pylab.figure(figsize=(7,4),dpi=100);pylab.grid(alpha=.2)
pylab.savefig("DIY_ECG_Poincare_Plot.png");
pylab.figure(figsize=(7,4),dpi=100);pylab.grid(alpha=.2)
pylab.savefig("DIY_ECG_RR_Beat_Interval.png");
pylab.figure(figsize=(7,4),dpi=100);pylab.grid(alpha=.2)
pylab.savefig("DIY_ECG_RR_Deviation_Histogram.png");
pylab.show();

fname='publish_05_10min.snd' #CHANGE THIS AS NEEDED
#raw_input("npress ENTER to analyze %s..."%(fname))
snd2txt(fname)
#raw_input("npress ENTER to graph %s.npy..."%(fname))
txt2graphs(fname)```

### Microcontroller-Powered Prime Number Generator

Warning: This post is several years old and the author has marked it as poor quality (compared to more recent posts). It has been left intact for historical reasons, but but its content (and code) may be inaccurate or poorly written.

My microcontroller-powered prime number calculator is complete! Although I’m planning on improving the software (better menus, the addition of sound, and implementation of a more efficient algorithm) and hardware (a better enclosure would be nice, battery/DC wall power, and a few LEDs on the bottom row are incorrectly wired), this device is currently functional therefore I met my goal!

This device generates large prime numbers (v) while keeping track of how many prime numbers have been identified (N). For example, the 5th prime number is 11. Therefore, at one time this device displayed N=5 and V=11. N and V are displayed on the LCD. In the photo the numbers mean the 16,521,486th prime is 305,257,039 (see for yourself!). The LCD had some history. In December, 2003 (6 years ago) I worked with this SAME display, and I even located the blog entry on November 25’th, 2003 where I mentioned I was thinking of buying the LCD (it was \$19 at the time). Funny stuff. Okay, fast forward to today. Primes (Ns and Vs) are displayed on the LCD.

In addition to the LCD, numbers are displayed in binary: Each row of LEDs represents a number. Each row of 30 LEDs allows me to represent numbers up to 2^31-1 (2,147,483,647, about 2.15 billion) in the binary numeral system. Since there’s no algorithm to simply generate prime numbers (especially the Nth prime), the only way to generate large Nth primes is to start small (2) and work up (to 2 billion) testing every number along the way for primeness. The number being tested is displayed on the middle row (Ntest). The last two digits of Ntest are shown on the top left. To test a number (Ntest) for primeness, it is divided by every number from 2 to the square root of Ntest. If any divisor divides evenly (with a remainder of zero) it’s assumed not to be prime, and Ntest is incremented. If it can’t be evenly divided by any number, it’s assumed to be prime and loaded into the top row. In the photo (with the last prime found over 305 million) the device is generating new primes every ~10 seconds.

I’d like to emphasize that this device is not so much technologically innovative as it is creative in its oddness and uniqueness. I made it because no one’s ever made one before. It’s not realistic, practical, or particularly useful. It’s just unique. The brain behind it is an ATMEL ATMega8 AVR microcontroller (What is a microcontroller?), the big 28-pin microchip near the center of the board. (Note: I usually work with ATTiny2313 chips, but for this project I went with the ATMega8 in case I wanted to do analog-to-digital conversions. The fact that the ATMega8 is the heart of the Arduino is coincidental, as I’m not a fan of Arduino for purposes I won’t go into here).

I’d like to thank my grandmother’s brother and his wife (my great uncle and aunt I guess) for getting me interested in microcontrollers almost 10 years ago when they gave me BASIC Stamp kit (similar to this one) for Christmas. I didn’t fully understand it or grasp its significance at the time, but every few years I broke it out and started working with it, until a few months ago when my working knowledge of circuitry let me plunge way into it. I quickly outgrew it and ventured into directly programming cheaper microcontrollers which were nearly disposable (at \$2 a pop, compared to \$70 for a BASIC stamp), but that stamp kit was instrumental in my transition from computer programming to microchip programming.

The microcontroller is currently running at 1 MHz, but can be clocked to run faster. The PC I’m writing this entry on is about 2,100 MHz (2.1 GHz) to put it in perspective. This microchip is on par with computers of the 70s that filled up entire rooms. I program it with the C language (a language designed in the 70s with those room-sized computers in mind, perfectly suited for these microchips) and load software onto it through the labeled wires two pictures up. The microcontroller uses my software to bit-bang data through a slew of daisy-chained shift registers (74hc595s, most of the 16-pin microchips), allowing me to control over 100 pin states (on/off) using only 3 pins of the microcontroller. There are also 2 4511-type CMOS chips which convert data from 4 pins (a binary number) into the appropriate signals to illuminate a 7-segment display. Add in a couple switches, buttons, and a speaker, and you’re ready to go!

I’ll post more pictures, videos, and the code behind this device when it’s a little more polished. For now it’s technically complete and functional, and I’m very pleased. I worked on it a little bit every day after work. From its conception on May 27th to completion July 5th (under a month and a half) I learned a heck of a lot, challenged my fine motor skills to complete an impressive and confusing soldering job, and had a lot of fun in the process.

### Summer’s End is Nearing

My favorite summer yet is reaching its end. With about a month and a half before I begin dental school, I pause to reflect on what I’ve done, and what I still plan to do. Unlike previous summers where my time was devoted to academic requirements, this summer involved a 9-5 job with time to do whatever I wanted after. I made great progress in the realm of microcontroller programming, and am nearing the completion of my prime number calculator. I’m very happy with its progress.

Most of the LEDs are working but I’m still missing a few shift registers. It’s not that they’re missing, so much as I broke them. (D’oh!) I have to wait for a dozen more to come in the mail so I can continue this project. Shift registers are also responsible for powering the binary-to-7-segment chips on the upper left, whose sockets are currently empty.

Since this project is on pause, I began work hacking a VFD I heard about at Skycraft. It’s a 20×2 character display (forgot to photograph the front) and if I can make it light up, it will be gorgeous.

Here’s a high resolution photo of the back panel of the VFD. I believe it used to belong to an old cash register, and it has some digital interfacing circuitry between the driver chips (the big OKI ones) and the 9-pin input connector. I think my best bet for being able to control this guy as much as I want is to attack those driver chips, with help from the Oki C1162A datasheet. It looks fairly straightforward. As long as I don’t screw up my surface-mount soldering, and assuming that I come up with 65 volts to power the thing (!) I think it’s a doable project.

### Prime Number Generator Schematics

Warning: This post is several years old and the author has marked it as poor quality (compared to more recent posts). It has been left intact for historical reasons, but but its content (and code) may be inaccurate or poorly written.

Here’s a schematic of the prime number calculator I’m working on. Last night I finished wiring all 12 shift registers for the primary display, so now it’s time to start working on software. Notice that we have a lot of pins free still. This will be advantageous if I decide to go crazy adding extraneous functionality, such as fancy displays (LCD?, 7-segment LEDs?, VFD?, all 3?!) or creative input systems (how about a numerical keypad?).

After feeling the stink of paying almost \$15 for 100′ of yellow, 24 gauge, solid-core wire from DigiKey I was relieved (and a little embarrassed) to find I could score 1,000′ of yellow, 24 gauge, threaded wire for \$10 at Skycraft! Anyway, here’s the current schematic:

### Flipping Bits in C

Bitwise programming techniques (manipulating the 1s and 0s of binary numbers) are simple, but hard to  remember if you don’t use them often. Recently I’ve needed to perform a lot of bitwise operations. If I’m storing true/false (1-bit) information in variables, it’s a waste of memory to assign a whole variable to the task (the smallest variable in C is a char, and it contains 8 bits). When cramming multiple values into individual variables, it’s nice to know how to manipulate each bit of a variable.

```y = (x >> n) & 1; // stores nth bit of x in y.  y becomes 0 or 1.

x &= ~(1 << n); // forces nth bit of x to be 0.  all other bits left alone.

x &= (1 << (n + 1)) - 1; // leaves lowest n bits of x; all higher bits set to 0.

x |= (1 << n); // forces nth bit of x to be 1.  all other bits left alone.

x ^= (1 << n); // toggles nth bit of x.  all other bits left alone.

x = ~x; // toggles ALL the bits in x.```

### Prime Prototype Construction

Warning: This post is several years old and the author has marked it as poor quality (compared to more recent posts). It has been left intact for historical reasons, but but its content (and code) may be inaccurate or poorly written.

Now that I’ve worked-out the software side of the microcontroller-powered prime number generator, it’s time to start working on the hardware. I want to make a prototype which is far smaller and simpler than the final version but lets me practice driving lots of LEDs (30). I expect the final version to have around 80. Also, the heart of this project is an ATTiny2313 microcontroller, and for the full version I’d like to use an ATMEega8. I picked up an unfinished wooden box with a magnetic latch from Michaels. It’s delicate and tends to chip when you drill it, but moving slowly I’m able to make nice evenly-spaced holes.

This is the circuit concept. The chip is an ATTiny2313, sourced with 5V, where the left pins control the columns (by providing current) and the right pins control the rows (by providing ground). The “holes” at the top of the circuit represent where I hook up my PC and external power for testing purposes.

Thoughts from Future Scott (10 years later, August, 2019)

A+ for enthusiasm and construction but your design is… just no!

Why are you using an external crystal?

The schematic for the crystal is wrong: those capacitors should be to ground not in series!

You made the circuit diagram in InkScape!

You shouldn’t drive current directly out of the microcontroller pins.

The majority of the microcontroller CPU cycles will go into managing multiplexing of the display (not calculating primes).

After a little more work I have a functional device and it looks better than I expected. There are a few more features I want to add, and I want to work on the code some more, but I hope to be done tomorrow. The coolest part is that I’ve included an internal button which drives a pause/resume and speed-controller menu based upon the length of button presses! There’s a lot of awesome stuff I want to write, but once again, I’ll save it for the completed project page.

I rendered the cover sticker wrong and all the LEDs are mislabled. The first LED should be 2^0 (1), and the second should be 2^1 (2), etc. Also, 2^22 and 2^23 are mislabeled – oops! But the thing really does generate, differentiate, and display [only[ prime numbers. Once again, videos (including demonstration of the menus and the programming options) and source code will be posted soon.

### Prime Number Generator Prototype

Warning: This post is several years old and the author has marked it as poor quality (compared to more recent posts). It has been left intact for historical reasons, but but its content (and code) may be inaccurate or poorly written.

In my quest to build a hardware-based prime number generator I built a rapid prototype to assess how quickly primes can be found with an 8-bit microcontroller. There is a lot of room for improvement, but the code works. Instead of messing with tons of little LEDs, this design displays numbers on an LCD. Interestingly the library to run the LCD takes up about 90% of the memory of the chip leaving only a handful of bytes to write the prime calculation code in!

```#define F_CPU 1E6
#include <stdlib.h>
#include <avr/io.h>
#include <math.h>
#include <util/delay.h>
#include "lcd.h"
#include "lcd.c"

const unsigned long int primeMax=pow(2,25);
unsigned long int primeLast=2;
unsigned long int primeTest=0;
unsigned int primeDivy=0;

void wait(void);
void init(void);
void updateDisplay(void);
char *toString(unsigned long int);

int main(void){
init();
short maybePrime;
unsigned int i;
for(primeTest=2;primeTest<sqrt(primeMax);primeTest++){
maybePrime=1;
for (i=2;i<=(sqrt(primeTest)+1);i++){
primeDivy=i;
updateDisplay();
if (primeTest%primeDivy==0){maybePrime=0;break;}
}
if (maybePrime==1){primeLast=primeTest;updateDisplay();}
}
return 0;
}

void updateDisplay(void){
lcd_gotoxy(12,0);lcd_puts(toString(primeLast));
lcd_gotoxy(5,1);lcd_puts(toString(primeTest));
lcd_gotoxy(16,1);lcd_puts(toString(primeDivy));
return;
}

void init(void){
lcd_init(LCD_DISP_ON);
lcd_puts("PRIME IDENTIFICATIONn");
_delay_ms(2000);
lcd_clrscr();
lcd_puts("LAST PRIME:n");
lcd_puts("TRY:");
lcd_gotoxy(14,1);lcd_puts("/");
return;
}

char *toString(unsigned long int x){
char s1[8];
ltoa(x,s1,10);
return s1;
}```