I’ve been unbelievably busy over the past couple weeks. Last Thursday my boss approached me and asked if I could work over the weekend. He wanted to complete and submit a grant by the deadline (Monday at 5pm). Scientific research grants are not easy to prepare. ~40 pages of forms to fill out, biographies to design for each of the lab workers, budget appropriation plans, animal handling protocol approval forms, plus 12 pages of scientific dialogue explaining the experiment. Now, I know that it may not sound like much, but trust me – it’s a very dense (11 and 9pt Arial font, single spaced) paper which takes about 2.5 hours for me to read from start to finish. Being the only native English speaker in the laboratory, I’m expected to assist in the polishing of its dialogue, as well as thoroughly evaluate its scientific content and offer suggestions. To make a long story short, I worked really hard (really long days) on Friday, Saturday, Sunday, and Monday to accomplish this. Monday afternoon when it was done (at about 4pm) I went home and collapsed in exhaustion. I don’t know how my boss does it! He worked on it far more than I did, and over that weekend he didn’t sleep much at all. Anyway, in exchange for my over-weekend work I got Tuesday and Wednesday off.

So, what did you do on your mid-week weekend? Well, I knew in advance that I’d have two days to bask in freedom and to do whatever I wanted. I prepared ahead of time by ordering a small handful (I think 4?) of ATMEL AVR type ATTiny2313 chips from Digi-Key at $2.26 per chip. I know I could have gotten better deals elsewhere “Like $1.88 per chip from RoboticsWorld) but the truth is that I had a random assortment of items I wanted to buy, and although all these different companies offered them, I feel I saved in the long run by ordering everything from a single source, cutting down on shipping costs. I also was impressed by the delivery time! I got my boxes in the mail on Monday. Way better (and cheaper) than ordering from the UK or Hong Kong.

The first thing I did with these chips was try to program them. Unlike the embarrassingly-easy PICAXE chips which can be programmed a form of BASIC cod from 2 wires of a serial port, the AVR series of chips are usually programmed from assembly-level code. Thankfully, C code can be converted to equivalent ASM (thanks to AVR-GCC) and loaded onto these chips. The result is a much faster and more powerful coding platform than the PICAXE chips. PICAXE chips are great for rapid development or for a starting platform for microcontroller education, but I feel that I’m ready to tackle something new. Anyway, I built a straight-through parallel programmer for my ATTiny2313 chips. It was based upon the dapa configuration and connects to the appropriate pins of the ATTiny 2313. However I would recommend that you be safe (protect your parallel port and microcontrollers) by installing the proper resisters (~1k?) between the devices, but I didn’t do this. No no. I live on the wild side. I eat danger for breakfast.

I decided to dive right in to the world of digital RF transmission and should probably go to jail for it. I blatantly violated FCC regulations and simply wired my microcontroller to be able to control the power level given to a 3.579545 MHz oscillator. Check out the circuit. The antenna is the copper wire sticking vertically out of the breadboard.

These crystals release wide bands of RF not only near the primary frequency (F), but also on the harmonic frequencies (F*n where n=1,2,3…). I was able to pick up the signal on my (uber-old) scanner at its 9th harmonic (32.215905 MHz). Supposedly the harmonic output power is inversely proportional to n. Therefore the frequency I’m listening to represents only a fraction of the RF power the crystal is putting out at its primary frequency. Unfortunately the only listening device I have (currently) is the old scanner, which can only listen above 30 mhz.

Remember when I talked about the illegal part? Yeah, I detected harmonic signals being emitted way up into the high 100s of MHz. I don’t think it’s a big deal because I doubt the signal is getting very far, but I’m always concerned about irritating people (Are people trying to use Morse code at one of the frequencies? Am I jamming my neighbors’ TV reception?) so I don’t keep it on long. Once I get some more time, I’ll build the appropriate receiver circuits (I have another matched crystal) and install a low-pass filter (to eliminate harmonics) and maybe even get a more appropriate radio license (I’m still only technician). But for now, this is a proof-of-concept, and it works. Check out the output of the scanner.

Something I struggled with for half an hour was how to produce a tone with a microcontroller and the oscillator. Simply supplying power to the oscillator produces a strong RF signal, but there is no sound to it. It’s just full quieting when it’s on, and static noise when it’s off. To produce an AM tone, I needed amplitude modulation. I activated the oscillator by supplying power from the microcontroller with one pin (to get it oscillating), and fed it extra juice in the form of timer output from another pin. The fluctuation in power to the oscillator (without power-loss) produced a very strong, loud, clear signal (horizontal lines). I wrote code to make it beep. Frequency can be adjusted by modifying the timer output properties. The code in the screenshot is very primitive, and not current (doesn’t use timers to control AM frequency), but it worked. I’m sure I’ll write more about it later.

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I successfully created a speaker/microphone/transmit button circuit for the puxing 777 which probably works for all puxing radios. Instead of simply using circuits found on other websites (always for other radios), I decided to reverse-engineering an earphone/microphone headset that came with the radio to determine how it worked. I can’t claim that I’m an expert in electronics theory, but I can say that I faithfully rebuilt the circuitry within the factory-shipped headset and it worked. The result allows me to leave my handheld radio in its charger while casually listening/transmitting with a button that I made instead of having to reach around and awkwardly squeeze the transmit button on the side of the radio. Once again, I built this circuit and it was successful for me, but there may still be a better way to do it.

The microphone is a 20-cent electret microphone with no special modifications. The speaker I used is a standard 8ohm loudspeaker with no special modifications. The switch is a keyboard-style (push-to-talk) switch, and the capacitor I used is good for 10nF.

If you have any ideas for improvements, let me know! I’ll post some photos once I have my completed little “base station” set up. My ultimate goal is to turn an el-cheapo handheld VHF radio into a decent desktop transceiver by combining it with a nice antenna (located on a balcony at 30ft) and a convenient, easy-to-use switches/buttons/microphone/speaker/etc on a desktop panel.

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Over the last couple weeks whenever I had the time I’d work on creating a little DIY morse code keyer. After 6 designs (whoa!) I came up with the winner. The youtube video describes it all I guess. Basically it just uses a bar of aluminum which rocks on a metal pin. Thumb-screws on each side of the balance point (fulcrum?) can be adjusted to modulate the distance the paddle has to go down to be activated, and how high the paddle goes up when released. A couple springs (one pull-type and one push-type) help give it a good bounce between keys. Two potentiometers (knobs) control volume and frequency. I especially like the ability to control the frequency! A capacitor inline with the speaker helps smooth the output a bit too. It’s not professional, but hey – for a couple bucks of parts I made a functional keyer and had a blast doing it. Now I guess I should learn Morse code…

And some photos…

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Yes I’ve taken yet another plunge into the geek world by becoming a licensed amateur radio operator. My wife and I both took our technician exam last week (and passed), and this morning I discovered that our call signs have been processed. I’m KJ4LDF, she’s KJ4LDG. I’m a little disappointed that my call sign has an “F” in it. On the air, “F” and “S” sound similar, so I’m more likely to have people asking me to repeat it. The phonetics are Kilo, Juliet, Four, Lima, Delta, Foxtrot. Foxtrot! How lame is that? [sighs] Then let’s go to acronyms. LDF… “long distance fun”? Catchy and clever, but very gay sounding. [sighs again]. Either way, I’m glad I’ve been added to the database, and am now legally able to begin broadcasting on VHF/UHF. Beacon stuff (like I wrote about in the last post) would best involve lower frequencies, which would mean I have to take another exam to get a higher license class. Anyway, I’d better get back to work. Just wanted to share.

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I, Scott Harden, in my infinite restlessness and my limited sanity, hereby declare my next [potential] project. The idea is still in the earliest stages of development, and I have much to research (for example, I don’t even know if it’s legal) but it’s a cool idea and I want to try it. I know I’ll learn a lot from the project, and that’s what’s important, right? So, here’s the idea. I want to build an incredibly simple, low power radio transmitter that broadcasts data on a fixed frequency. Data is provided by (you guessed it) a picaxe chip! What data will it transmit? I’ll tell you! It could transmit… uh… err… um… okay it doesn’t really matter and I don’t even know, I just want to do this project! Maybe temperature and light intensity data or something. Who cares. Anyway, I want to put this whole deelibopper (temperature and light sensors, picaxe microcontroller, and transmitter) into a drybox (pictured). Once properly closed, this box will keep everything in pristine working condition by protecting against rain, heat, snow (not that we get much of that in Orlando), hurricanes, and perhaps even Florida panthers and bears (oh my). I’d make a glass (or plexiglas) window on the top so that light could get in, hitting solar panels, which trickle-charges the battery housed in the device as well. Pretty clever, huh?

My idea is to keep construction costs to a minimum because I’m throwing this away as soon as I make it. That’s right! Throwing it away. Parting with it – perhaps forever. My goal is to make it work so I can toss it in some random location (I’m thinking hidden on the roof of some building somewhere) and see how long it will run. Days? Weeks? Months? Years? How cool would it be go go to dental school, come back ~5 years from now, and have that transmitter still transmitting data. Super-awesome if you ask me. I’ve been poking around and I found someone who did something similar. They built a 40mW 10m picaxe-powered beacon (see photos and circuit diagram).

See the PICAXE chip in the center there? Remember, my ultimate goal is to learn from this project. I understand the basics of radio theory and amplitude and frequency modulation (AM and FM), but I’ve never actually built anything that does this. Yeah, I know, I could build a SoftRock radio like everyone says to do, but my educational grounding is in molecular biology. I know little about circuit-level electronics, electrical engineering, and radio theory… so my plan is to start small. This project is small enough to attack and understand, with a fun enough end result to motivate me throughout the process.