I’m posting this information to the world hoping that someone else in a position similar to mine can benefit from the experience I gained through trial and error when trying to rapidly design/develop professional-looking QSL cards for as little cost and risk as possible. I Googled around for this information, but didn’t find anything too helpful, so I figured I’d share my story! For those of you who don’t know, QSL cards are like postcards which amateur radio operators often mail to one another after making long distance contacts. In addition to providing tangible proof of the communication, they’re cool mementos to tote around to show people / remind yourself who you’ve made contact with over the years. QSL cards display information bout the contact (time, date, call sign, frequency, signal report, etc.) and sometimes contain extra pictures/graphics which make them unique and appealing.

Once I got a HF rig for my apartment (a Century 21 CW-only HF rig which puts out ~30 watts, pictured below), I started making contacts and getting QSL cards myself, so I wanted to send some nice ones in return. Being a poor college student (and a graduate student at that), I was extremely cash-limited, and didn’t want to sit around for weeks while my cards were professionally printed. I wanted a fast solution. This post describes how I created amazingly cool QSL cards in a few hours, and for less than $0.25 each!

My QSL card:

Step 1: Design the cards with PERFECT dimensions. Here’s the deal. The most cost-effective way to print nice digital images is my local Target (a store with a 1-hr photo lab which accepts JPEGs as the image source for $0.20 cents a picture), but the snag was that they only print 4” x 6”. QSL cards need to be 3.5” by 5.25”. I used Inkscape to create an image exactly 4” by 6”, and inside of it I drew a border 3.5” by 5.25”. Everything outside that border I made black. I designed my QSL card INSIDE that border, such that when the images would be printed I could trim-off the black border and have a perfect 3.5” by 5.25” QSL card.

This is how the image turned out:

This is what the 1-hr photos looked like:

Step 2: Print the reverse side on full-size label paper. All I needed was some framed boxes for QSL information, so I quickly sketched up the design in Inkscape and saved it in the same format as before (4” by 6”). I left a LOT of white space around the edges so it’s very forgiving down the line. I then printed the design on full-page label paper (full-sheet stickers, available at most office stores cheaply in the printer paper section), placing 4 “backs” per page.

Here’s what the adhesive paper looked like after printing:

Step 3: Attach backings to QSL cards. This part is easy if you have a paper cutter. I purchased mine ~5yrs ago and I *LOVE* it. It’s almost as useful as my soldering iron. Seriously, so convenient. I wouldn’t dream of doing this with scissors! Anyhow, roughly cut the sticker paper into quarters:

Next, peel and stick on the backs of cards. DONT WORRY ABOUT OVERHANG! We’ll take care of that later…

Step 4: Trim the edges. Make sure you do this step AFTER applying the sticker. This was the secret that I wish I realized a while ago. If you trim first, sticker placement is critical and difficult. If you place the sticker BEFORE you trim, you get *PERFECT* edges every time. Way more professional…

How nice does that look? If you did your math correctly, your new dimensions should be EXACTLY 3.5” by 5.25”. Let’s view the back…


Step 5: fill-out information. I decided to use a metallic Sharpie to write the name of the call sign I send this to on the front of my card. How cool does that look? This is what the front/back of this card looks like after filling it out (Hi W2BFE in Maine! You’re a celebrity!).

I hope this information helps you. If you print your own QSL cards using this (or a similar) method, let me know about it! I have to say, for ~5 / $1, these don’t look to bad. It’s especially useful if you only want to print a few cards! Good luck.
–Scott, AJ4VD


Additional Resources

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

It’s time for a lecture. I’ve been spending a lot of time creating a DIY dlectrocardiogram and it produces fairly noisy signals. I’ve spent some time and effort researching the best ways to clean-up these signals, and the results are incredibly useful! Therefore, I’ve decided to lightly document these results in a blog entry.

Here’s an example of my magic! I take a noisy recording and turn it into a beautiful trace. See the example figure with the blue traces. How is this possible? Well I’ll explain it for you. Mostly, it boils down to eliminating excess high-frequency sine waves which are in the original recording due to electromagnetic noise. A major source of noise can be from the alternating current passing through wires traveling through the walls of your house or building. My original ECG circuit was highly susceptible to this kind of interference, but my improved ECG circuit eliminates most of this noise. However, noise is still in the trace (see the figure to the left), and it needed to be removed.

The key is the FFT (Fast Fourier Transformation) algorithm which can get pretty intimidating to research at first! I’ll simplify this process. Let’s say you have a trace with repeating sine-wave-shaped noise. The output of the FFT transformation of the signal is the breakdown of the signal by frequency. Check out this FFT trace of a noisy signal from a few posts ago (top graph). High peaks represent frequencies which are common. See the enormous peak around 60 Hz? (Hz means “per second” by the way, so 60 Hz is a sine wave that repeats 60 times a second) That’s from my AC noise. Other peaks (shown in colored bands) are other electromagnetic noise sources, such as wireless networks, TVs, telephones, and your computer processor. The heart produces changes in electricity that are very slow (the heartbeat is about 1 Hz, or 1 beat per second), so if we can eliminate all of the sine waves with frequencies higher than what we want to isolate we can get a pretty clear trace. This is called a band-stop filter (we block-out certain bands of frequencies) A band-pass filter is the opposite, where we only allow frequencies which are below (low-pass) or above (high-pass) a given frequency. By eliminating each of the peaks in the colored regions (setting each value to 0), then performing an inverse fast Fourier transformation (going backwards from frequency back to time), the result is the signal trace (seen as light gray on the bottom graph) with those high-frequency sine waves removed! (the gray trace on the bottom graph). A little touch-up smoothing makes a great trace (black trace on the bottom graph).

Here’s some Python code to get you started in cleaning-up your noisy signals! The image below is the output of the Python code at the bottom of this entry. This python file requires that test.wav (~700kb) (an actual ECG recording of my heartbeat) be saved in the same folder. Brief descriptions of each portion of the graph will follow.

(A) The original signal we want to isolate. (IE: our actual heart signal)

(B) Some electrical noise. (3 sine waves of different amplitudes and periods)

(C) Electrical noise (what happens when you add those 3 sine waves together)

(D) Static (random noise generated by a random number generator)

(E) Signal (A) plus static (D)

(F) Signal (A) plus static (D) plus electrical noise (C)

(G) Total FFT trace of (F). Note the low frequency peak due to the signal and electrical noise (near 0) and the high frequency peak due to static (near 10,000)

(H) This is a zoomed-in region of (F) showing 4 peaks (one for the original signal and 3 for high frequency noise). By blocking-out (set it to 0) everything above 10Hz (red), we isolate the peak we want (signal). This is a low-pass filter.

(I) Performing an inverse FFT (iFFT) on the low-pass iFFT, we get a nice trace which is our original signal!

(J) Comparison of our iFFT with our original signal shows that the amplitude is kinda messed up. If we normalize each of these (set minimum to 0, maximum to 1) they line up. Awesome!

(K) How close were we? Graphing the difference of iFFT and the original signal shows that usually we’re not far off. The ends are a problem though, but if our data analysis trims off these ends then our center looks great.

Here’s the code I used to make the image:

 import numpy, scipy, pylab, random

 # This script demonstrates how to use band-pass (low-pass)
 # filtering to eliminate electrical noise and static
 # from signal data!


 xs=numpy.arange(1,100,.01) #generate Xs (0.00,0.01,0.02,0.03,...,100.0)
 signal = sin1=numpy.sin(xs*.3) #(A)
 sin1=numpy.sin(xs) # (B) sin1
 sin2=numpy.sin(xs*2.33)*.333 # (B) sin2
 sin3=numpy.sin(xs*2.77)*.777 # (B) sin3
 noise=sin1+sin2+sin3 # (C)
 static = (numpy.random.random_sample((len(xs)))-.5)*.2 # (D)
 sigstat=static+signal # (E)
 rawsignal=sigstat+noise # (F)
 fft=scipy.fft(rawsignal) # (G) and (H)
 for i in range(len(bp)): # (H-red)
     if i>=10:bp[i]=0
 ibp=scipy.ifft(bp) # (I), (J), (K) and (L)

 ### GRAPHING ###


 pylab.subplot(h,w,1);pylab.title("(A) Original Signal")

 pylab.subplot(h,w,3);pylab.title("(B) Electrical Noise Sources (3 Sine Waves)")

 pylab.subplot(h,w,5);pylab.title("(C) Electrical Noise (3 sine waves added together)")

 pylab.subplot(h,w,7);pylab.title("(D) Static (random noise)")

 pylab.subplot(h,w,9);pylab.title("(E) Signal + Static")

 pylab.subplot(h,w,11);pylab.title("(F) Recording (Signal + Static + Electrical Noise)")

 pylab.subplot(h,w,2);pylab.title("(G) FFT of Recording")

 pylab.subplot(h,w,4);pylab.title("(H) Low-Pass FFT")

 pylab.subplot(h,w,6);pylab.title("(I) Inverse FFT")

 pylab.subplot(h,w,8);pylab.title("(J) Signal vs. iFFT")

 pylab.subplot(h,w,10);pylab.title("(K) Normalized Signal vs. iFFT")

 pylab.subplot(h,w,12);pylab.title("(L) Difference / Error")


Additional Resources

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

Am I going to die? It’s unlikely. Upon analyzing ~20 minutes of heartbeat data (some of which is depicted in the previous entry) I found a peculiarity. Technically this could be some kind of noise (a ‘pop’ in the microphone signal due to the shuffling of wires or a momentary disconnect from the electrodes or perhaps even a static shock to my body from something), but because this peculiarity happened only once in 20 minutes I’m not ruling out the possibility that this is the first irregular heartbeat I captured with my DIY ECG. Note that single-beat irregularities are common, and that this does not alarm me so much as fascinates me. Below is the section of the data which contains this irregular beat.

In the spirit of improvement I wonder how much more interesting this project would be if I were to combine the already-designed ECG machine with a sensor to detect the physical effect of the heart’s beating on my vasculature. in other words, can I combine my electrical traces with physical traces? (Blood pressure or blood flow) I found an interesting site that shows how someone built a DIY blood flow meter using a piezo film pulse sensor. Pretty clever I must say… but I think I draw my limit at what I’ve done. Although blood flow would be interesting to analyze (does the murmur depicted above produce an alteration in normal blood flow?), it’s not worth the time, hassle or expense of building.

Additional Resources

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

No 3-day weekend would be complete without a project that’s, well, virtually useless. I present to you my new and improved ECG machine! Instead of using a single op-amp circuit like the previous entries which gave me decent but staticky traces, I decided to build a more advanced ECG circuit documented by Jason Nguyen which boasted 6 op amps! (I’d only been using one) Luckily I picked up a couple LM 324 quad op amp chips at radioshack for about $1.40 each, so I had everything I needed. I’ll skip to the results. In short, they’re gorgeous. Noise is almost nothing, so true details of the trace are visible. I can now clearly see the P-Q-R-S-T features in the wave (before the P was invisible). I’ll detail how I did this in a later entry. For now, here are some photos of the little device and a video I uploaded to YouTube. It’s not fancy.

UPDATE: Upon analyzing ~20 minutes of heartbeat data I found a peculiarity. Technically this could be some kind of noise (a ‘pop’ in the microphone signal due to the shuffling of wires or a momentary disconnect from the electrodes or perhaps even a static shock to my body from something), but because this peculiarity happened only once in 20 minutes I’m not ruling out the possibility that this is the first irregular heartbeat I captured with my DIY ECG. Note that single-beat irregularities are common, and that this does not alarm me so much as fascinates me. Below is the section of the data which contains this irregular beat.

Additional Resources

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

Last night I finished building my DIY ECG as a prototype (I finally got the circuit off the breadboard and onto a plastic sheet). This is a similar circuit to the one used to record data from the last entry (resister values are now identical to the crude circuit described several posts ago). I left-in the crude band-pass filter (made by grounding my primary electrode sensor through a 0.1µF capacitor) because it seemed to help a great deal, and wasn’t hard to implement. I picked up all of my parts (including the LF324 quad-op-amp microchip) at RadioShack. Of note, the quad-op-amp is overkill because I’m only using one of the 4 op-amps. Theoretically I could add 3 more electrodes to this circuit (which would allow for multi-sensor recording, similar to the electrodes placed at http://en.wikipedia.org/wiki/File:Limb_leads.svg) but this would require multiple microphone jacks, which isn’t very common. I guess I could use 2 microphone jacks, and differentiate right/left channels. Anyway, here are some photos.

I made the prototype by drilling holes in a small rectangular piece of a non-conductive plastic-fiberish material. (I picked up a stack of these rectangular sections for a quarter at a local electrical surplus store and they’re perfect for prototyping). The two green wires coming out the left side attach to a ~5v power supply (either a plugged in AC->DC transformer, 3 or 4 AA batteries, or even a 9V should work). The blue wires on the right attach to the electrodes I put on my chest. The black wires go to a headphone-jack which I plug into the microphone hole of my PC to record the signal.

This is the back of the device which shows my crummy soldering technique. Let it slide folks, I’m a molecular biology not an electrical engineer. Anyhow, basic point-to-point contacts, nothing special. The white/yellow wires correspond to the left/right channels of the microphone connector. I only use the left one (white), but attached the right channel (yellow) to the op-amp just in case I decide to add another sensor later – this is not required.

Here’s the full shabang! You can clearly see the circuit (note its small size – easy to mount inside of a tictac box or something) with the green wires leading to a power supply, black cable being the microphone connector, and the blue wires leading to electrodes made… from… fanta… cans…? Yes, in the spirit of ghetto-rigging electronics (my specialty) I found that surprisingly nice chest electrodes can be made from aluminum soda cans! If you go this route, cut them delicately so you don’t get metal shards in your skin like I did at first. Also, note that you have to firmly scrape each side of the aluminum to get the insulating waxy-plastic stuff off or it just won’t work. (I guess it’s coated with something to prevent the soda from tasting like metal.) Aluminum rapidly transfers heat, so it’s nearly impossible to solder leads onto these pads, so I wrapped a wire (tipped with a bead of solder) with the edge of the aluminum several times and crushed the heck out of it with pliers and it seems to stay on well and make a good connection. Also, before taping these onto your skin, you have to put a highly-conductive goo on it to make the connection better. I chose to use a copious squirt of my wife’s skin moisturizer on each electrode (shh, don’t tell her!) and taped the gooey electrode directly onto my chest.

I recorded ~20 minutes of data last night with this rig and it looked pretty good. I went to analyze it with Python and it kept crashing! The python script I gave you previously loads the entire wave file into an array of numbers, but with a 20 minute wave file (at over 40,000 samples a second) this is just too big for memory. I wrote an updated wave loader function which loads large wave files in parts which is much more efficient. It also performs the condensation method at load time. Basically, it loads 100 data points (or whatever deg is set to), averages them, and adds this value to a list. The result is a smoothed trace with a resolution of ~400Hz instead of ~40,000Hz. I’d apply it to my wave file I recorded last night but it’s on my laptop which is in the car and I’ve got to get back to work. Here’s that function:

 def loadWav(fname,deg=100):
     global hz
     nchannel, width, rate, length, comptype, compname = w.getparams()
     print "[%s]
 rate: %d Hz
 frames: %d
 length: %.02f sec" %
           (fname, rate, length, float(length)/rate)
     for i in range(chunks):
         if i%7777==0:
             print "processing chunk %d of %d (%0.02f%%)" %
         vals = struct.unpack("%sh" %deg,w.readframes(deg))
     print "complete!"
     return data