### Precision Temperature Measurement

In an effort to resume previous work [A, B, C, D] on developing a crystal oven for radio frequency transmitter / receiver stabilization purposes, the first step for me was to create a device to accurately measure and log temperature. I did this with common, cheap components, and the output is saved to the computer (over 1,000 readings a second). Briefly, I use a LM335 precision temperature sensor (\$0.70 on mouser) which outputs voltage with respect to temperature. It acts like a Zener diode where the breakdown voltage relates to temperature. 2.95V is 295K (Kelvin), which is 22ºC / 71ºF. Note that Kelvin is just ºC + 273.15 (the difference between freezing and absolute zero). My goal was to use the ADC of a microcontroller to measure the output. The problem is that my ADC (one of 6 built into the ATMEL ATMega8 microcontroller) has 10-bit resolution, reporting steps from 0-5V as values from 0-1024. Thus, each step represents 0.0049V (0.49ºC / 0.882ºF). While ~1ºF resolution might be acceptable for some temperature measurement or control applications, I want to see fractions of a degree because radio frequency crystal temperature stabilization is critical. Here’s a video overview.

This is the circuit came up with. My goal was to make it cheaply and what I had on hand. It could certainly be better (more stable, more precise, etc.) but this seems to be working nicely. The idea is that you set the gain (the ratio of R2/R1) to increase your desired resolution (so your 5V of ADC recording spans over just several ºF you’re interested in), then set your “base offset” temperature that will produce 0V. In my design, I adjusted so 0V was room temperature, and 5V (maximum) was body temperature. This way when I touched the sensor, I’d watch temperature rise and fall when I let go.  Component values are very non-critical. LM324 is powered 0V GND and +5V Vcc. I chose to keep things simple and use a single rail power supply. It is worth noting that I ended-up using a 3.5V Zener diode for the positive end of the potentiometer rather than 5V.  If your power supply is well regulated 5V will be no problem, but as I was powering this with USB I decided to go for some extra stability by using a Zener reference.

On the microcontroller side, analog-to-digital measurement is summed-up pretty well in the datasheet. There is a lot of good documentation on the internet about how to get reliable, stable measurements. Decoupling capacitors, reference voltages, etc etc. That’s outside the scope of today’s topic. In my case, the output of the ADC went into the ATMega8 ADC5 (PC5, pin 28). Decoupling capacitors were placed at ARef and AVcc, according to the datasheet. Microcontroller code is at the bottom of this post.

To get the values to the computer, I used the USART capability of my microcontroller and sent ADC readings (at a rate over 1,000 a second) over a USB adapter based on an FTDI FT232 chip. I got e-bay knock-off FTDI evaluation boards which come with a USB cable too (they’re about \$6, free shipping). Yeah, I could have done it cheaper, but this works effortlessly. I don’t use a crystal. I set fuse settings so the MCU runs at 8MHz, and thanks to the nifty online baud rate calculator determined I can use a variety of transfer speeds (up to 38400). At 1MHz (if DIV8 fuse bit is enabled) I’m limited to 4800 baud. Here’s the result, it’s me touching the sensor with my finger (heating it), then letting go.

I spent a while considering fancy ways to send the data (checksums, frame headers, error correction, etc.) but ended-up just sending it old fashioned ASCII characters. I used to care more about speed, but even sending ASCII it can send over a thousand ADC readings a second, which is plenty for me. I ended-up throttling down the output to 10/second because it was just too much to log comfortable for long recordings (like 24 hours). In retrospect, it would have made sense to catch all those numbers and do averaging on the on the PC side.

On the receive side, I have nifty Python with PySerial ready to catch data coming from the microcontroller. It’s decoded, turned to values, and every 1000 receives saves a numpy array as a NPY binary file. I run the project out of my google drive folder, so while I’m at work I can run the plotting program and it loads the NPY file and shows it – today it allowed me to realize that my roomate turned off the air conditioning after I left, because I saw the temperature rising mid-day. The above graph is temperature in my house for the last ~24 hours. That’s about it! Here’s some of the technical stuff.

AVR ATMega8 microcontroller code:

#define F_CPU 8000000UL
#include <avr/io.h>
#include <util/delay.h>
#include <avr/interrupt.h>

/*
8MHZ: 300,600,1200,2400,4800,9600,14400,19200,38400
1MHZ: 300,600,1200,2400,4800
*/
#define USART_BAUDRATE 38400
#define BAUD_PRESCALE (((F_CPU / (USART_BAUDRATE * 16UL))) - 1)

/*
{
PORTD^=255;
}
*/

void USART_Init(void){
UBRRL = BAUD_PRESCALE;
UBRRH = (BAUD_PRESCALE >> 8);
UCSRB = (1<<TXEN);
UCSRC = (1<<URSEL)|(1<<UCSZ1)|(1<<UCSZ0); // 9N1
}

void USART_Transmit( unsigned char data ){
while ( !( UCSRA & (1<<UDRE)) );
UDR = data;
}

void sendNum(long unsigned int byte){
if (byte==0){
USART_Transmit(48);
}
while (byte){
USART_Transmit(byte%10+48);
byte-=byte%10;
byte/=10;
}

}

}

}

int main(void){
//DDRD=255;
USART_Init();
for(;;){
USART_Transmit('n');
_delay_ms(100);
}
}

Here is the Python code to receive the data and log it to disk:

import serial, time
import numpy
ser = serial.Serial("COM15", 38400, timeout=100)

t1=time.time()
lines=0

data=[]

while True:

if "," in line:
line=line.split(",")
for i in range(len(line)):
line[i]=line[i][::-1]
else:
line=[line[::-1]]
temp=int(line[0])
lines+=1
data.append(temp)
print "#",
if lines%1000==999:
numpy.save("DATA.npy",data)
print
print line
print "%d lines in %.02f sec (%.02f vals/sec)"%(lines,
time.time()-t1,lines/(time.time()-t1))

Here is the Python code to plot the data that has been saved:

import numpy
import pylab

print data
data=data*.008 #convert to F
xs=numpy.arange(len(data))/9.95  #vals/sec
xs=xs/60.0# minutes
xs=xs/60.0# hours

pylab.plot(xs,data)
pylab.grid(alpha=.5)
pylab.axis([None,None,0*.008,1024*.008])
pylab.ylabel(r'\$Delta\$ Fahrenheit')
pylab.xlabel("hours")
pylab.show()

If you recreate this project, or have any questions, feel free to email me!

### Wireless Microcontroller / PC Interface for \$3.21

Here I demonstrate a dirt-cheap method of transmitting data from any microchip to any PC using \$3.21 in parts.  I’ve had this idea for a while, but finally got it working tonight. On the transmit side, I’m having a an ATMEL AVR microcontroller (ATMega48) transmit data (every number from 0 to 200 over and over) wirelessly using 433mhz wireless modules. The PC receives the data through the microphone port of a sound card, and a cross-platform Python script I wrote decodes the data from the audio and graphs it on the screen. I did something similar back in 2011, but it wasn’t wireless, and the software wasn’t nearly as robust as it is now.

This is a proof-of-concept demonstration, and part of a larger project. I think there’s a need for this type of thing though! It’s unnecessarily hard to transfer data from a MCU to a PC as it is. There’s USB (For AVR V-USB is a nightmare and requires a precise, specific clock speed, DIP chips don’t have native USB, and some PIC DIP chips do but then you have to go through driver hell), USART RS-232 over serial port works (but who has serial ports these days?), or USART over USB RS-232 interface chips (like FTDI FT-232, but surface mount only), but both also require precise, specific clock speeds. Pretend I want to just measure temperature once a minute. Do I really want to etch circuit boards and solder SMT components? Well, kinda, but I don’t like feeling forced to. Some times you just want a no-nonsense way to get some numbers from your microchip to your computer. This project is a funky out-of-the-box alternative to traditional methods, and one that I hope will raise a few eyebrows.

Ultimately, I designed this project to eventually allow multiple “bursting” data transmitters to transmit on the same frequency routinely, thanks to syncing and forced-sync-loss (read on). It’s part of what I’m tongue-in-cheek calling the Scott Harden RF Protocol (SH-RFP). In my goal application, I wish to have about 5 wireless temperature sensors all transmitting data to my PC.  The receive side has some error checking in that it makes sure pulse sizes are intelligent and symmetrical (unlike random noise), and since each number is sent twice (with the second time being in reverse), there’s another layer of error-detection.  This is *NOT* a robust and accurate method to send critical data. It’s a cheap way to send data. It is very range limited, and only is intended to work over a distance of ten or twenty feet. First, let’s see it in action!

The RF modules are pretty simple. At 1.56 on ebay (with free shipping), they’re cheap too! I won’t go into detail documenting the ins and out of these things (that’s done well elsewhere). Briefly, you give them +5V (VCC), 0V (GND), and flip their data pin (ATAD) on and off on the transmitter module, and the receiver module’s DATA pin reflects the same state. The receiver uses a gain circuit which continuously increases gain until signal is detected, so if you’re not transmitting it WILL decode noise and start flipping its output pin. Note that persistent high or low states are prone to noise too, so any protocol you use these things for should have rapid state transitions. It’s also suggested that you maintain an average 50% duty cycle. These modules utilize amplitude shift keying (ASK) to transmit data wirelessly. The graphic below shows what that looks like at the RF level. Transmit and receive is improved by adding a quarter-wavelength vertical antenna to the “ANT” solder pad. At 433MHz, that is about 17cm, so I’m using a 17cm copper wire as an antenna.

Transmitting from the microcontroller is easy as pie! It’s just a matter of copying-in a few lines of C.  It doesn’t rely on USART, SPI, I2C, or any other protocol. Part of why I developed this method is because I often use ATTiny44A which doesn’t have USART for serial interfacing. The “SH-RFP” is easy to implement just by adding a few lines of code. I can handle that.  How does it work? I can define it simply by a few rules:

• Pulses can be one of 3 lengths: A (0), B (1), or C (break).
• Each pulse represents high, then low of that length.

To send a packet:

• prime synchronization by sending ten ABCs
• indicate we’re starting data by sending C.
• for each number you want to send:
• send your number bit by bit (A=0, B=1)
• send your number bit by bit (A=1, B=0)
• indicate number end by sending C.
• tell PC to release the signal by sending ten Cs.

Decoding is the same thing in reverse. I use an eBay sound card at \$1.29 (with free shipping) to get the signal into the PC. Synchronization is required to allow the PC to know that real data (not noise) is starting. Sending the same number twice (once with reversed bit polarity) is a proof-checking mechanisms that lets us throw-out data that isn’t accurate.

From a software side, I’m using PyAudio to collect data from the sound card, and the PythonXY distribution to handle analysis with numpy, scipy, and plotting with QwtPlot, and general GUI functionality with PyQt. I think that’s about everything.

The demonstration interface is pretty self-explanatory. The top-right shows a sample piece of data. The top left is a histogram of the number of samples of each pulse width. A clean signal should have 3 pulses (A=0, B=1, C=break). Note that you’re supposed to look at the peaks to determine the best lengths to tell the software to use to distinguish A, B, and C. This was intentionally not hard-coded because I want to rapidly switch from one microcontroller platform to another which may be operating at a different clock speed, and if all the sudden it’s running 3 times slower it will be no problem to decide on the PC side. Slick, huh? The bottom-left shows data values coming in. The bottom-right graphs those values. Rate reporting lets us know that I’m receiving over 700 good data points a second. That’s pretty cool, especially considering I’m recording at 44,100 Hz.

All source code (C files for an ATMega48 and Python scripts for the GUI) can be viewed here: SHRFP project on GitHub

If you use these concepts, hardware, or ideas in your project, let me know about it! Send me an email showing me your project – I’d love to see it. Good luck!

## I got everything running on Windows 7 x64 with the following:

Here’s the “hello world” of microchip programs (it simply blinks an LED). I’ll assume the audience of this page knows the basics of microcontroller programming, so I won’t go into the details. Just note that I’m using an ATMega48 and the LED is on pin 9 (PB6). This file is named “blink.c”.

#define F_CPU 1000000UL
#include <avr/io.h>
#include <util/delay.h>

int main (void)
{
DDRB = 255;
while(1)
{
PORTB ^= 255;
_delay_ms(500);
}
}

Here’s how I compiled the code:

In reality, it is useful to put these commands in a text file and call them “compile.bat”

Here’s how I program the AVR. I used AVRDudess! I’ve been using raw AVRDude for years. It’s a little rough around the edges, but this GUI interface is pretty convenient. I don’t even feel the need to include the command to program it from the command line! If I encourage nothing else by this post, I encourage (a) people to use and support AVRDudess, and (b) AVRDudess to continue developing itself as a product nearly all hobby AVR programmers will use. Thank you 21-year-old Zak Kemble.

And finally, the result. A blinking LED. Up and running programming AVR microcontrollers in 64-bit Windows 7 with an unofficial programmer, and never needing to install bloated AVR Studio software.

### Tenma 72-7750 Multimeter Excellent for RF Engineering

I recently got my hands on a Tenma 72-7750 multimeter. Tenma has a pretty large collection of test equipment and measurement products, including several varieties of hand-held multimeters. The 72-7750 multimeter has the standard measurement modes you’d expect (voltage, current capacitance, resistance, conductivity), but stood out to me because it also measures frequency, temperature, and has RS232 PC connectivity. Currently it’s sale from Newark for under fifty bucks! This is what mine arrived with:

The obvious stuff worked as expected. Auto ranging, (5 ranges of voltage and resistance, 3 of current, 7 of capacitance), accurate measurement, etc. I was, however, impressed with the extra set of test leads they provided – little short ones with gator clips! These are perfect for measuring capacitance, or for clipping onto wires coming out of a breadboard. So many times with my current multimeters I end-up gator-clipping wires to my probes and taking them to what I’m measuring. I’m already in love with the gator clip leads, and know I’ll have a set of these at my bench for the rest of my life.

I was impressed by the frequency measuring ability of this little multimeter! When I read that it could measure up to 60MHz, I was impressed, but also suspected it might be a little flakey. This was not at all the case – the frequency measurement was dead-on at several ranges! With so many of the projects I work on being RF-involved (radio transmitters, radio receivers, modulators, mixers, you name it), I sided with this meter because unlike some of its siblings this one is rated beyond 50Mz. I hooked it up to the frequency synthesizer I built based around an ad9850 direct digital synthesizer and played around. When the synthesizer was set to various frequencies, the multimeter followed it to the digit! Check out the pics of it in action, comparing the LCD screen frequency with that being displayed on the meter:

I also took a closer look at the PC interface. When I looked closely, I noticed it wasn’t an electrical connection – it was an optical one! It has a phototransistor on one end, and a serial connection on the other. I’m no stranger to tossing data around with light (I made something that did this here, which was later featured on Hack-A-Day here). I wondered what the format of the data was, when to my surprise I saw it spelled out in the product manual! (Go Tenma!)  It specifically says “Baud Rate 19230, Start Bit 1 (always 0), Stop bit 1 (always 1), Data bits (7), Parity 1 (odd)”. Although they have their own windows-only software to display/graph readings over time, I’d consider writing Python-based logging software. It should be trivial with python, pySerial, numpy, and matplotlib. Clearly I’m no stranger to graphing things in python 🙂

How does the photo-transistor work without power? I attached my o-scope to the pins and saw nothing when RS232 mode was activated on the multimeter. Presumably, the phototransistor requires a voltage source (albeit low current) to operate. With a little digging on the internet, I realized that the serial port can source power. I probably previously overlooked this because serial devices were a little before my time, but consider serial mice: they must have been supplied power! Joseph Sullivan has a cool write-up on a project which allowed him to achieve bidirectional optical (laser) communication over (and completely powered by) a serial port. With a little testing, I applied 0V to pin 5 (GND), +5V to pin 6 (DSR, data set ready), and looked at the output on pin 3 (PC RX). Sure enough, there were bursts of easy-to-decode RS232 data. Here’s the scheme Joseph came up with to power his laser communication system, which presumably is similar to the one in the multi-meter. (Note, that the cable is missing its “TX” light, but the meter has an “RX” phototransistor. I wonder if this would allow optically-loaded firmware?)

There were a couple other things I found useful. Although I didn’t appreciate it at first, after a few days the backlight grew on me. I’ve been doing experiments with photosensors which require me to turn out the lights in the room, and the backlight saved the day! Also, the meter came with a thermocouple for temperature measurement. It has it’s own “ºC” setting on the dial, and displays human-readable temperature right on the screen. I used to do this with LM334-type thermosensitive current sources but it was always a pain (especially if I had one which output temperature in Kelvin!) I’m not sure exactly what’s inside the one that came with this meter, but the datasheet suggests it can measure -40 through 1,000 C, which certainly will do for my experiments!

All in all, I’m happy with this little guy, and am looking forward to hacking into it a little bit. There may be enough room in the case to add a hacked-together high frequency divider (a decade counter would be fantastic, divided by ten would allow measurement through 500MHz), but I might be over-reaching a bit. Alternatively, a high gain preamplifier would be a neat way to allow the sort probe to serve as an antenna to measure frequency wirelessly, rater than requiring contact. Finally, I’m looking forward to writing software to interface the RS232 output. The ability to measure, record, and display changes in voltage or temperature over time is an important part of designing controller systems. For example, an improved crystal oven is on my list of projects to make. What a perfect way to monitor the temperature and stability of the completed project! Straight out of the box, this multimeter is an excellent tool.

### Simple DIY ECG + Pulse Oximeter (version 2)

UPDATE: An improved ECG design was posted in August, 2016.
Check out: https://www.swharden.com/wp/2016-08-08-diy-ecg-with-1-op-amp/

Of the hundreds of projects I’ve shared over the years, none has attracted more attention than my DIY ECG machine on the cheap posted almost 4 years ago. This weekend I re-visited the project and made something I’m excited to share!  The original project was immensely popular, my first featured article on Hack-A-Day, and today “ECG” still represents the second most searched term by people who land on my site. My gmail account also has had 194 incoming emails from people asking details about the project. A lot of it was by frustrated students trying to recreate the project running into trouble because it was somewhat poorly documented. Clearly, it’s a project that a wide range of people are interested in, and I’m happy to revisit it bringing new knowledge and insight to the project. I will do my best to document it thoroughly so anyone can recreate it!

The goal of this project is to collect heartbeat information on a computer with minimal cost and minimal complexity.  I accomplished this with fewer than a dozen components (all of which can be purchased at RadioShack). It serves both as a light-based heartbeat monitor (similar to a pulse oximeter, though it’s not designed to quantitatively measure blood oxygen saturation), and an electrocardiogram (ECG) to visualize electrical activity generated by heart while it contracts. Let’s jump right to the good part – this is what comes out of the machine:

That’s my actual heartbeat. Cool, right? Before I go into how the circuit works, let’s touch on how we measure heartbeat with ECG vs. light (like a pulse oximeter).  To form a heartbeat, the pacemaker region of the heart (called the SA node, which is near the upper right of the heart) begins to fire and the atria (the two top chambers of the heart) contract. The SA node generates a little electrical shock which stimulated a synchronized contraction. This is exactly what defibrillators do when a heart has stopped beating. When a heart attack is occurring and a patient is undergoing ventricular fibrillation, it means that heart muscle cells are contracting randomly and not in unison, so the heart quivers instead of pumping as an organ. Defibrillators synchronize the heart beat with a sudden rush of current over the heart to reset all of the cells to begin firing at the same time (thanks Ron for requesting a more technical description).  If a current is run over the muscle, the cells (cardiomyocytes) all contract at the same time, and blood moves. The AV node (closer to the center of the heart) in combination with a slow conducting pathway (called the bundle of His) control contraction of the ventricles (the really large chambers at the bottom of the heart), which produce the really large spikes we see on an ECG.  To measure ECG, optimally we’d place electrodes on the surface of the heart. Since that would be painful, we do the best we can by measuring voltage changes (often in the mV range) on the surface of the skin. If we amplify it enough, we can visualize it. Depending on where the pads are placed, we can see different regions of the heart contract by their unique electrophysiological signature. ECG requires sticky pads on your chest and is extremely sensitive to small fluctuations in voltage. Alternatively, a pulse oximeter measures blood oxygenation and can monitor heartbeat by clipping onto a finger tip. It does this by shining light through your finger and measuring how much light is absorbed. This goes up and down as blood is pumped through your finger. If you look at the relationship between absorbency in the red vs. infrared wavelengths, you can infer the oxygenation state of the blood. I’m not doing that today because I’m mostly interested in detecting heart beats.

For operation as a pulse oximeter-type optical heartbeat detector (a photoplethysmograph which produces a photoplethysmogram), I use a bright red LED to shine light through my finger and be detected by a phototransistor (bottom left of the diagram). I talk about how this works in more detail in a previous post. Basically the phototransistor acts like a variable resistor which conducts different amounts of current depending on how much light it sees. This changes the voltage above it in a way that changes with heartbeats. If this small signal is used as the input, this device acts like a pulse oximeter.

For operation as an electrocardiograph (ECG), I attach the (in) directly to a lead on my chest. One of them is grounded (it doesn’t matter which for this circuit – if they’re switched the ECG just looks upside down), and the other is recording. In my original article, I used pennies with wires soldered to them taped to my chest as leads. Today, I’m using fancier sticky pads which are a little more conductive. In either case, one lead goes in the center of your chest, and the other goes to your left side under your arm pit. I like these sticky pads because they stick to my skin better than pennies taped on with electrical tape. I got 100 Nikomed Nikotabs EKG Electrodes 0315 on eBay for \$5.51 with free shipping (score!). Just gator clip to them and you’re good to go!

In both cases, I need to build a device to amplify small signals. This is accomplished with the following circuit. The core of the circuit is an LM324 quad operational amplifier.  These chips are everywhere, and extremely cheap. It looks like Thai Shine sells 10 for \$2.86 (with free shipping). That’s about a quarter each. Nice!  A lot of ECG projects use instrumentation amplifiers like the AD620 (which I have used with fantastic results), but these are expensive (about \$5.00 each). The main difference is that instrumentation amplifiers amplify the difference between two points (which reduces noise and probably makes for a better ECG machine), but for today an operational amplifier will do a good enough job amplifying a small signal with respect to ground. I get around the noise issue by some simple filtering techniques. Let’s take a look at the circuit.

This project utilizes one of the op-amps as a virtual ground. One complaint of using op-amps in simple projects is that they often need + and – voltages. Yeah, this could be done with two 9V batteries to generate +9V and -9V, but I think it’s easier to use a single power source (+ and GND). A way to get around that is to use one of the op-amps as a current source and feed it half of the power supply voltage (VCC), and use the output as a virtual ground (allowing VCC to be your + and 0V GND to be your -). For a good description of how to do this intelligently, read the single supply op amps web page. The caveat is that your signals should remain around VCC/2, which can be done if it is decoupled by feeding it through a series capacitor. The project works at 12V or 5V, but was designed for (and has much better output) at 12V. The remaining 3 op-amps of the LM324 serve three unique functions:

STAGE 1: High gain amplifier. The input signals from either the ECG or pulse oximeter are fed into a chain of 3 opamp stages. The first is a preamplifier. The output is decoupled through a series capacitor to place it near VCC/2, and amplified greatly thanks to the 1.8Mohm negative feedback resistor. Changing this value changes initial gain.

STAGE 2: active low-pass filter. The 10kOhm variable resistor lets you adjust the frequency cutoff. The opamp serves as a unity gain current source / voltage follower that has high input impedance when measuring the output f the low-pass filter and reproduces its voltage with a low impedance output. There’s some more information about active filtering on this page. It’s best to look at the output of this stage and adjust the potentiometer until the 60Hz noise (caused by the AC wiring in the walls) is most reduced while the lower-frequency component of your heartbeat is retained. With the oximeter, virtually no noise gets through. Because the ECG signal is much smaller, this filter has to be less aggressive, and this noise is filtered-out by software (more on this later).

STAGE 3: final amplifier with low-pass filter. It has a gain of ~20 (determined by the ratio of the 1.8kOhm to 100Ohm resistors) and lowpass filtering components are provided by the 22uF capacitor across the negative feedback resistor. If you try to run this circuit at 5V and want more gain (more voltage swing), consider increasing the value of the 1.8kOhm resistor (wit the capacitor removed). Once you have a good gain, add different capacitor values until your signal is left but the noise reduced. For 12V, these values work fine. Let’s see it in action!

Now for the second half – getting it into the computer. The cheapest and easiest way to do this is to simply feed the output into a sound card! A sound card is an analog-to-digital converter (ADC) that everybody has and can sample up to 48 thousand samples a second! (overkill for this application) The first thing you should do is add an output potentiometer to allow you to drop the voltage down if it’s too big for the sound card (in the case of the oximeter) but but also allow full-volume in the case of sensitive measurements (like ECG). Then open-up sound editing software (I like GoldWave for Windows or Audacity for Linux, both of which are free) and record the input. You can do filtering (low-pass filter at 40Hz with a sharp cutoff) to further eliminate any noise that may have sneaked through. Re-sample at 1,000 Hz (1kHz) and save the output as a text file and you’re ready to graph it! Check it out.

Here are the results of some actual data recorded and processed with the method shown in the video. let’s look at the pulse oximeter first.

That looks pretty good, certainly enough for heartbeat detection. There’s obvious room for improvement, but as a proof of concept it’s clearly working. Let’s switch gears and look at the ECG. It’s much more challenging because it’s signal is a couple orders of magnitude smaller than the pulse oximeter, so a lot more noise gets through. Filtering it out offers dramatic improvements!

Here’s the code I used to generate the graphs from the text files that GoldWave saves. It requires Python, Matplotlib (pylab), and Numpy. In my case, I’m using 32-bit 2.6 versions of everything.

# DIY Sound Card ECG/Pulse Oximeter
# by Scott Harden (2013) http://www.SWHarden.com

import pylab
import numpy

f=open("light.txt")
f.close()

data = numpy.array(raw,dtype=float)
data = data-min(data) #make all points positive
data = data/max(data)*100.0 #normalize
times = numpy.array(range(len(data)))/1000.0
pylab.figure(figsize=(15,5))
pylab.plot(times,data)
pylab.xlabel("Time Elapsed (seconds)")
pylab.ylabel("Amplitude (% max)")
pylab.title("Pulse Oximeter - filtered")
pylab.show()

Future directions involve several projects I hope to work on soon. First, it would be cool to miniaturize everything with surface mount technology (SMT) to bring these things down to the size of a postage stamp. Second, improved finger, toe, or ear clips (or even taped-on sensors) over long duration would provide a pretty interesting way to analyze heart rate variability or modulation in response to stress, sleep apnea, etc. Instead of feeding the signal into a computer, one could send it to a micro-controller for processing. I’ve made some darn-good progress making multi-channel cross-platform USB option for getting physiology data into a computer, but have some work still to do. Alternatively, this data could be graphed on a graphical LCD for an all-in-one little device that doesn’t require a computer. Yep, lots of possible projects can use this as a starting point.

LET ME KNOW WHAT YOU THINK! If you make this, I’m especially interested to see how it came out. Take pictures of your projects and send them my way! If you make improvements, or take this project further, I’d be happy to link to it on this page. I hope this page describes the project well enough that anyone can recreate it, regardless of electronics experience. Finally, I hope that people are inspired by the cool things that can be done with surprisingly simple electronics. Get out there, be creative, and go build something cool!

### AVR Programming in Linux

It is not difficult to program ATMEL AVR microcontrollers with linux, and I almost exclusively do this because various unofficial (inexpensive) USB AVR programmers are incompatible with modern versions of windows (namely Windows Vista and Windows 7). I am just now setting-up a new computer station in my electronics room (running Ubuntu Linux 12.04), and to make it easy for myself in the future I will document everything I do when I set-up a Linux computer to program microcontrollers.

Install necessary software

sudo apt-get install gcc-avr avr-libc uisp avrdude

Connect the AVR programmer
This should be intuitive for anyone who has programmed AVRs before. Visit the datasheet of your MCU, identify pins for VCC (+), GND (-), MOSI, MISO, SCK, and RESET, then connect them to the appropriate pins of your programmer.

Write a simple program in C
I made a file “main.c” and put the following inside. It’s the simplest-case code necessary to make every pin on PORTD (PD0, PD1, …, PD7) turn on and off repeatedly, sufficient to blink an LED.

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

int main (void)
{
DDRD = 255; // MAKE ALL PORT D PINS OUTPUTS

while(1) {
PORTD = 255;_delay_ms(100); // LED ON
PORTD = 0;  _delay_ms(100); // LED OFF
}

return 0;
}

Compile the code (generate a HEX file)

avr-gcc -w -Os -DF_CPU=2000000UL -mmcu=atmega8 -c -o main.o main.c
avr-gcc -w -mmcu=atmega8 main.o -o main
avr-objcopy -O ihex -R .eeprom main main.hex

note that the arguments define CPU speed and chip model – this will need to be customized for your application

Program the HEX firmware onto the AVR

sudo avrdude -F -V -c avrispmkII -p ATmega8 -P usb -U flash:w:main.hex

note that this line us customized based on my connection (-P flag, USB in my case) and programmer type (-c flag, AVR ISP mkII in my case)

When this is run, you will see something like this:

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.01s

avrdude: Device signature = 0x1e9307
avrdude: NOTE: FLASH memory has been specified, an erase cycle will be performed
To disable this feature, specify the -D option.
avrdude: erasing chip
avrdude: input file main.hex auto detected as Intel Hex
avrdude: writing flash (94 bytes):

Writing | ################################################## | 100% 0.04s

avrdude: 94 bytes of flash written

TIP 1: I’m using a clone AVRISP mkII from Fun4DIY.com which is only \$11 (shipped!). I glued it to a perf-board with a socket so I can jumper its control pins to any DIP AVR (80 pins or less). I also glued a breadboard for my convenience, and added LEDs (with ground on one of their pins) for easy jumpering to test programs. You can also build your own USB AVR ISP (free schematics and source code) from the USBtinyISP project website.

TIP 2: Make a shell script to run your compiling / flashing commands with a single command. Name them according to architecture. i.e., “build-m8” or “build-t2313”. Make the first line delete all .hex files in the directory, so it stalls before it loads old .hex files if the compile is unsuccessful. Make similar shell scripts to program fuses. i.e., “fuse-m8-IntClock-8mhz”, “fuse-m8-IntClock-1mhz”, “fuse-m8-ExtCrystal”.

TIP 3: Use a nice text editor well suited for programming. I love Geany.

### My DIY Bench Power Supply

Another thing everybody needs (and has probably built) is a simple laboratory bench power supply. A lot of people use things like modified PC power supplies but I wasn’t in favor of this because I wanted something smaller, lower current, and cleaner (from an RF perspective). My needs are nothing particularly high power, just something to provide a few common voltages for digital logic and small RF circuits. This is what I came up with!

In the image above you can see an ordinary LED being powered directly from the a 5V hook-up. There is no current limiting resistor, so a lot of current is travelling through the LED, burning it up as I photographed it. The ammeter (blue number) shows it’s drawing 410 mA – whoa! The layout is pretty simple. Each red banana plug hook-up supplies a voltage (5, 5, 12, and variable respectively). Black hook-ups are ground. The black hook-up on the top left is a current-sensing ground, and current travelling through it will be displayed on the blue dial. The right dial shows the voltage of the variable voltage supply, and can go from about 3.5 – 30.5 V depending on where the potentiometer is set. All voltage outputs are designed to put-out approximately 1A of current.

I built this using a lot of (eBay) components I had on hand. I often save money where I can by stocking my workbench with components I buy in bulk. Here’s what I used:

• 4.5-3.0V DC volt meter – \$2.08 (shipped) eBay
• 0-9.99 A ampere meter – \$4.44 (shipped) eBay
• L7805 5V voltage regulator – 10 for \$3.51 (\$.35 ea) (shipped) eBay
• L7812 12V voltage regulator – 20 for \$3.87 (\$.19 ea) (shipped) eBay
• LM317 variable voltage regulator – 20 for \$6.15 (\$0.30 ea) (shipped) eBay
• 10k linear potentiometer – 10 for 4.00 (\$.40 ea) (shipped) eBay
• banana plug hook-ups – 20 for \$3.98 (\$.20 ea) (shipped) eBay
• aluminum enclosure – \$3.49 (radioshack)

TOTAL: \$13.60

Does the variable voltage actually work? Is the voltmeter accurate? Let’s check it out.

I’d say it’s working nicely! I now have a new took on my workbench.

A note about the yellow color: The enclosure I got was originally silver aluminum. I sanded it (to roughen the surface), then sprayed it with a yellow rustoleum spray paint. I figured it was intended to go on metal, so I might as well give it a shot. I sprayed it once, then gave it a second coat 20 minutes later, then let it dry overnight. In the future I think I would try a lacquer finish, because it’s a bit easy to scratch off. However, it looks pretty cool, and I’m going to have to start spray-painting more of my enclosures in the future.

A note about smoothing capacitors. Virtually all diagrams of linear voltage regulators like the LM7805 show decoupling capacitors before and after the regulator. I added a few different values of capacitors on the input (you can see them in the circuit), but I intentionally did not include smoothing capacitors on the output. The reason was that I always put smoothing capacitors in my breadboards and in my projects, closer to the actual circuitry. If I included (and relied) on output capacitors at the level of the power supply, I would be picking-up 60Hz (and other garbage) RF noise in the cables coming from the power supply to my board. In short, no capacitors on the output, so good design must always be employed and decoupling capacitors added to whatever circuits are being built.

The input of this circuit is a 48V printer power supply from an archaic inkjet printer. It’s been attached to an RCA jack to allow easy plugging and unplugging.

### UPDATE: Check out the improved circuit here: https://www.swharden.com/wp/2013-04-14-simple-diy-ecg-pulse-oximeter-version-2/

I want to create a microcontroller application which will utilize information obtained from a home-brew pulse oximeter. Everybody and their cousin seems to have their own slant how to make DIY pulse detectors, but I might as well share my experience. Traditionally, pulse oximeters calculate blood oxygen saturation by comparing absorbance of blood to different wavelengths of light. In the graph below (from Dildy et al., 1996 that deoxygenated blood (dark line) absorbs light differently than oxygenated blood (thin line), especially at 660nm (red) and 920nm (infrared). Therefore, the ratio of the difference of absorption at 660nm vs 920nm is an indication of blood oxygenation. Fancy (or at least well-designed) pulse oximeters continuously look at the ratio of these two wavelengths. Analog devices has a nice pulse oximeter design using an ADuC7024 microconverter. A more hackerish version was made and described on this non-english forum. A fail-at-the-end page of a simpler project is also shown here, but not well documented IMO.

That’s not how mine works. I only use a single illumination source (~660nm) and watch it change with respect to time. Variability is due to a recombination effect of blood volume changes and blood oxygen saturation changes as blood pulses through my finger. Although it’s not quite as good, it’s a bit simpler, and it definitely works. Embedded-lab has a similar project but the output is only a pulsing LED (not what I want) and a voltage output that only varies by a few mV (not what I want).

Here’s what the device looks like assembled in a breadboard:

I made a sensor by drilling appropriately-sized holes in a clothespin for the emitter (LED) and sensor (phototransistor). I had to bend the metal spring to make it more comfortable to wear. Light pressure is better than firm pressure, not only because it doesn’t hurt as much, but because a firm pinch restricts blood flow considerably.

An obvious next step is microcontroller + LCD (or computer) digitization, but for now all you can do is check it out on my old-school analog oscilloscope. Vertical squares represent 1V (nice!). You can see the pulse provides a solid 2V spike.

Here’s some video of it in action:

Out of principal, I’m holding-back the circuit diagram until I work through it a little more. I don’t want to mislead people by having them re-create ill-conceived ideas on how to create analog amplifiers. I’ll post more as I develop it.

### Geek Spin – ATTiny44 Project Prototype

Some days you feel like working on projects to benefit humanity. The day I made this clearly wasn’t one of those days. A little over a year ago, I got into a troll war with my friend Mike Seese. The joke, similar to that of rick rolling, was to get each other to unexpectedly click a link to the Hatsune Miku version of the leekspin song. After several weeks of becoming beyond annoying, I decided to make an actual Hatsune Miku which would spin her leek and bobble her head to the techno version of the Levan Polka for his birthday.

The goal was to create a minature Miku which would perform perfectly in sync with audio coming from a portable music player (iPod or something) and NOT require a computer connection. I accomplished it by sending some creative control beeps out of the left channel of the stereo signal. Although I didn’t finish the project, I got pretty far with the prototype, so I decided to dig it out of the archives and share it with the world because it’s pretty entertaining!

(look how close I came to replicating the original!)

How did I do it? First off, I used servos. If you’re not familiar with them, I suggest you look up how servos work. Perhaps check out how to control servos with AVR microcontrollers. Basically, their position along a rotational axis is determined by the width of a pulse on a 20ms time window. Anyhow, if I only had 1 servo to control (i.e., leek only), I’d have controlled the servo directly with PWM signals in the left channel – no microcontroller needed, easy as pie, problem solved. However, since I needed to control two servos, I had to come up with something a bit more creative. Although I could have probably done this ten different ways, the way I chose to do it was using a series of pre-encoded leek spin and head bobble motions, triggered by control beeps in the left channel of the audio cable. (The right channel was patched through to the speakers.) Below is a diagram of what I believe I did, although I didn’t thoroughly document it at the time, so you might have to use your imagination if you decide to re-create this project.

The idea is that by sending bursts of sine waves, the circuit can rectify them and smooth them out to have them look to a microcontroller like a brief “high” signal. Each signal would tell the microcontroller to proceed to the next pre-programmed (and carefully timed) routine. With enough practice listening, watching, and tweaking the code, I was able to make a final version which worked pretty darn well!

LISTEN TO:MUSIC WITH CONTROL BEEPS (it’s a surprisingly fun listen)

A few technical details are that I used an ATTiny44a microcontroller (it may have been an ATTiny2313, I can’t remember for sure, but they’re so similar it’s virtually negligable). The servos I used were cheap (maybe \$4?) from eBay. They looked like the one pictured below. The servo position was controlled by PWM, but I manually sent the pulses and didn’t actually use the integrated PWM in the microcontroller. I can’t remember why I did it this way – perhaps because it was so simple to use the _delay_us() and _delay_ms() functions? I also used an operational amplifier (if I remember, it was a LM741) to boost the left channel control signals rather than rectifying/assessing the left channel directly.

This is the video which I mimiced to create my prototype (note how the leek in her arm and her head move exactly the same as the prototype I made – score!)

For good measure, here’s the original song:

And how did I find out about this song? I actually saw it on the video below which was hosted on wimp.com. I thought the song was catchy, looked it up, and the rest was history. It’s worth noting that (perhaps to avoid copyright issues?) the key was shifted two half-steps up. I get a kick out of the way the girl waves her arm in the beginning, mimicking the leek 🙂

Here are some of the images I made which I printed, glued to foam board, and cut out with a razor blade. I’m not sure how useful they are, but they’re provided just in case.

… but sometimes Japan takes it a bit too far and things get awkward …

Below is the code I used. Note that PWM that controls the servos isn’t the integrated PWM, but rather a couple pins I manually pulse on and off to control the arm and head positions. Also notice how, in the main routine, I wait for the control beeps before continuing the next sequences.

// leek spin code - designed for ATTiny
// by Scott Harden, www.SWHarden.com

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

void go_high(){
// sets the arm to the highest position
for (char i=0;i<5;i++){
PORTA|=(1<<PA0);
_delay_us(1400);
PORTA&=~(1<<PA0);
_delay_us(20000-1200);
}
}

void go_low(){
// sets the leek to the middle position
for (char i=0;i<5;i++){
PORTA|=(1<<PA0);
_delay_us(1900);
PORTA&=~(1<<PA0);
_delay_us(20000-1900);
}
}

void go_lowest(){
// sets the leek to the lowest position
for (char i=0;i<5;i++){ // takes 100ms total
PORTA|=(1<<PA0);
_delay_us(2300);
PORTA&=~(1<<PA0);
_delay_us(20000-2500);
}
}

void go_slow(char times){
// does one slow leek down/up
// beat is 500ms
for (char i=0;i<times;i++){
go_low();
_delay_ms(10);
go_high();
_delay_ms(290);
PORTA^=(1<<PA2);
PORTA^=(1<<PA3);
}
}

void go_fast(char times){
// does one fast leek down/up
// beat is 250ms
for (char i=0;i<times;i++){
go_low();
_delay_ms(10);
go_high();
_delay_ms(15);
PORTA^=(1<<PA2);
PORTA^=(1<<PA3);
}
}
// tilts the head to the left
for (char i=0;i<5;i++){
PORTA|=(1<<PA1);
_delay_us(1330);
PORTA&=~(1<<PA1);
_delay_us(20000-1200);
}
}

// tilts the head to the right
for (char i=0;i<5;i++){
PORTA|=(1<<PA1);
_delay_us(1500);
PORTA&=~(1<<PA1);
_delay_us(20000-1200);
}
}

for (char i=0;i<5;i++){
PORTA|=(1<<PA1);
_delay_us(1400);
PORTA&=~(1<<PA1);
_delay_us(20000-1200);
}
}

// rocks the head back and forth once
for (char i=0;i<(times-1);i++){
_delay_ms(400);
PORTA^=(1<<PA2);
PORTA^=(1<<PA3);
_delay_ms(400);
PORTA^=(1<<PA2);
PORTA^=(1<<PA3);
}
_delay_ms(400);
PORTA^=(1<<PA2);
PORTA^=(1<<PA3);
}

int main(void) {
while (1){
DDRA=255; // set port A (servos) as outputs
DDRB=0; // set port B (listening pins) as inputs

while ((PINB & _BV(PB0))){} // wait for beep que
PORTA=(1<<PA3);
go_high();_delay_ms(1000);
while ((PINB & _BV(PB0))){} // wait for beep que
go_slow(31); // tilt leek slowly 31 times
while ((PINB & _BV(PB0))){} // wait for beep que
go_slow(31); // tilt leek slowly 31 times

while ((PINB & _BV(PB0))){} // wait for beep que
_delay_ms(200);
while ((PINB & _BV(PB0))){} // wait for beep que
go_fast(68); // tilt leek rapidly 68 times
while ((PINB & _BV(PB0))){} // wait for beep que
go_slow(24); // tilt leek slowly 24 times
while ((PINB & _BV(PB0))){} // wait for beep que
go_fast(17); // tilt leek rapidly 17 times
while ((PINB & _BV(PB0))){} // wait for beep que
go_slow(31); // tilt leek slowly 31 times
while ((PINB & _BV(PB0))){} // wait for beep que
go_slow(31); // tilt leek slowly 31 times

while ((PINB & _BV(PB0))){} // wait for beep que
_delay_ms(200);
go_lowest(); // reset position
PORTA=0;
}
return 0;
}

Finally, I’d like to take a moment to indicate one of the reasons this project is special to me. My wife, Angelina Harden, died one year ago today. This project was the last one she worked on with me. She died a few days after the video was taken, and in the process of moving out of our apartment I threw away almost everything (including this project). Although I never finished it, I remember working on it with Angelina – we went to wal-mart together to buy the foam board I used to make it, and she told me that I should make her head rock back and forth rather than just move her arm. I remember that, once it was all done, I let her sit in the chair in front of it and played it through, and she laughed nearly the whole time 🙂 I’ll always miss her.

### Introduction to PIC Programming from an AVR Guru

I’m not ashamed to say it: I’m a bit of an ATMEL guy. AVR microcontrollers are virtually exclusively what I utilize when creating hobby-level projects. Wile I’d like to claim to be an expert in the field since I live and breathe ATMEL datasheets and have used many intricate features of these microchips, the reality is that I have little experience with other platforms, and have likely been leaning on AVR out of habit and personal convention rather than a tangible reason. Although I was initially drawn to the AVR line of microcontrollers because of its open-source nature (The primary compiler is the free AVR-GCC) and longstanding ability to be programmed from non-Windows operating systems (like Linux), Microchip’s PIC has caught my eye over the years because it’s often a few cents cheaper, has considerably large professional documentation, and offers advanced integrated peripherals (such as native USB functionality in a DIP package) more so than the current line of ATTiny and ATMega microcontrollers. From a hobby standpoint, I know that ATMEL is popular (think Arduino), but from a professional standpoint I usually hear about commercial products utilizing PIC microcontrollers. One potential drawback to PIC (and the primary reason I stayed away from it) is that full-featured C compilers are often not free, and as a student in the medical field learning electrical engineering as a hobby, I’m simply not willing to pay for software at this stage in my life.

I decided to take the plunge and start gaining some experience with the PIC platform. I ordered some PIC chips (a couple bucks a piece), a PIC programmer (a Chinese knock-off clone of the Pic Kit 2 which is <\$20 shipped on eBay), and shelved it for over a year before I got around to figuring it out today. My ultimate goal is to utilize its native USB functionality (something at ATMEL doesn’t currently offer in DIP packages). I’ve previously used bit-banging libraries like V-USB to hack together a USB interface on AVR microcontrollers, but it felt unnecessarily complex. PIC is commonly used and a bit of an industry standard, so I’m doing myself a disservice by not exploring it. My goal is USB functionality, but I have to start somewhere: blinking a LED.

Here’s my blinking LED. It’s a bit anticlimactic, but it represents a successful program design from circuit to writing the code to programming the microchip.

Based on my limited experience, it seems you need 4 things to program a PIC microcontroller with C:

The first thing I did was familiarize myself with the pin diagram of my PIC from its datasheet. I’m playing with an 18F2450 and the datasheet is quite complete. If you look at the pin diagram, you can find pins labeled MCLR (reset), VDD (+5V), VSS (GND), PGC (clock), and PGD (data). These pins should be connected to their respective counterparts on the programmer. To test connectivity, install and run the PICkit2 installer software and it will let you read/verify the firmware on the chip, letting you know connectivity is solid. Once you’re there, you’re ready to start coding!

I wish I were friends with someone who programmed PIC, such that in 5 minutes I could be shown what took a couple hours to figure out. There are quite a few tutorials out there – borderline too many, and they all seem to be a bit different. To quickly get acquainted with the PIC programming environment, I followed the “Hello World” Program in C tutorial on PIC18F.com. Unfortunately, it didn’t work as posted, likely because their example code was based on a PIC 18F4550 and mine is an 18F2450, but I still don’t understand why such a small difference caused such a big problem. The problem was in their use of LATDbits and TRISDbits (which I tried to replace with LATBbits and TRISBbits). I got around it by manually addressing TRISB and LATB. Anyway, this is what I came up with:

#include <p18f2450.h> // load pin names
#include <delays.h>   // load delay library

#pragma config WDT = OFF // disable watchdog timer
#pragma config FOSC = INTOSCIO_EC // use internal clock

void main() // this is the main program
{
TRISB=0B00000000; // set all pins on port B as output
while(1) // execute the following code block forever
{
LATB = 0b11111111; // turn all port B pins ON
Delay10KTCYx(1);   // pause 1 second
LATB = 0b00000000; // turn all port B pins OFF
Delay10KTCYx(1);   // pause 1 second
}
}

A couple notes about the code: the WDT=OFF disables the watchdog timer, which if left unchecked would continuously reboot the microcontroller. The FOSC=INTOSCIO_EC section tells the microcontroller to use its internal oscillator, allowing it to execute code without necessitating an external crystal or other clock source. As to what TRIS and LAT do, I’ll refer you to basic I/O operations with PIC.

Here is what the MPLAB IDE looked like after I successfully loaded the code onto the microcontroller. At this time, the LED began blinking about once per second. I guess that about wraps it up! This afternoon I pulled a PIC out of my junk box and, having never programmed a PIC before, successfully loaded the software, got my programmer up and running, and have a little functioning circuit. I know it isn’t that big of a deal, but it’s a step in the right direction, and I’m glad I’ve taken it.