The personal website of Scott W Harden

Minimal Radio Project Continues...

I got a big bag of fresh, new, copper clad PC board and I now wish I purchased a big pack months ago! Don’t laugh at me, but I was buying 4’‘x6’’ sheets of it at Radio Shack for about $5 a pop - ouch! I probably purchased 3 boards in my lifetime, but at that price you can imagine how careful I was not to use them. I soldered minimally to them, and only used them for the most important, established projects. Wake up Scott! If your experimental platform actually inhibits experimentation, there’s something fundamentally wrong. Anyway, I got a stack of the stuff and I no longer hesitate to grab a fresh board and start working. I made some progress today simplifying my ultra-minimalist functional radio project. Here’s what I came up with!

As you can see, it’s running on 9V batteries! The frequency counter has its own 9V battery and a spiffy new hand-me-down case (originally used for a power supply I think, before which it was a watch case!). The IC is a SA602, SA612, or NE602 (all about the same) direct conversion receiver (Gilbert cell mixer).

I now have a small battery powered handheld frequency counter. SWEET! I need to contrive a spectacular case for it. I can’t wait! It’s probably the most impressive thing I’ve ever made with respect to the “cool factor”. Does it look like a bomb? That probably makes it cooler! It just needs a big red on/off switch labeled “MISSILE LAUNCH”, then it’ll be the coolest thing on the planet! … moving on …

This is the receiver component. It’s about as simple as it gets. No antenna or headphone connector is attached, but doing this is trivial! A resonant front-end filter might make it more sensitive, and add some complexity, so comparisons are needed to get a feel for how much better it really is with one attached.

For this board, I added a buffer chip (74HC240) to take the pretty sine wave and turn it into a higher-power square wave…

The quality of the oscillator is reflected in the smoothness of the sine wave (purity?) and its amplitude (indicating high Q?), though more investigation/research is required to fully understand what makes a good oscillator circuit for this chip. My strategy has been to throw components in the air, let them fall randomly, and eventually something happens and the thing starts oscillating…

Frequency Counter Working!

I’m ecstatic! Finally I built something that worked the first time. Well… on the 3rd attempt! The goal was to develop a minimal-cost, minimal complexity frequency counter suitable for amateur radio. Although I think I can still cut cost by eliminating components and downgrading the microcontroller, I’m happy with my first working prototype.

__I haven’t tested it rigorously __ with anything other than square waves, but I imagine that anything over 1PPV is sufficient (the input is through a bypass capacitor, internally biased right at the trigger threshold). Counting is accomplished by a 74LV8154N (dual 16-bit counter configured as 32-bit) which displays the count as four selectable bytes presented on 8 parallel pins. The heart of the device is an ATMega16 which handles multiplexing of the display and has a continuously-running 16-bit timer which, upon overflowing, triggers a reset of the counter and measurement of the output. Software isn’t perfect (you can see the timing isn’t accurate) but I imagine its inaccuracy can be measured and is a function of frequency such that it can be corrected via software. Here are some photos…

A PCB is DESPERATELY needed. I’ll probably make one soon. Once it’s a PCB, the components are pretty much drop-in and go! No wires! It’ll be a breeze to assemble in 5 minutes. I wonder if it would make a fun kit? It would run on a 9V battery of course, but a calculator-like LCD (rather than LED) display would be ultra-low-current and might make a good counter for field operation (3xAAA batteries would last for months!)

UPDATE: I found out that the ATMega16 donation was from my friend Obulpathi, a fellow Gator Amateur Radio Club member! He also gave me a pair of ATMega32 chips. Thanks Obul!

Prototype Radio Receiver Improving Daily

This is the current state of my receiver. Unlike earlier designs this one uses NO VARIABLE CAPACITORS! This helps because (a) it reduces cost, (b) makes it easier to build for anyone (it’s hard to hunt down identical variable capacitors), and (c) allows it to be totally voltage controlled so microchip or PC control of frequency becomes trivial. Tuning over the entire 40m band is achieved with 3 LEDs reverse biased acting as varactors (wow!). The knobs are potentiometers. The whole circuit runs on 5v.

Home-Brew Transceiver Taking Shape!

In the spirit of furthering my knowledge of AC circuity, I’m trying to build a 100% homebrew transceiver. Yeah, QRSS and ultra-weak signal, ultra-narrowband communications is still fun, but it’s not the same thrill as actually engaging in real time communication with somebody! My goal is a transmitter / receiver in a box. The basic features I desire are (1) multiple bands (at least 40m, 30m, 20m), (2) FULL-band coverage, (3) direct conversion receiver, (4) 10W transmitter, (5) digital frequency display, (6) common standard components (nothing mechanical, no air variable capacitors, everything must be easily obtainable on sites like Mouser and DigiKey), (7) SMT capability, (8) inexpensive ($20 is my goal, but that’s a tough goal!). My designs are changing daily, so I’m not going to waste time posting schematics every time I write on this blog, but here are some photos and videos of the product in its current state.

(I just found that last video - it was one of my favorite songs as a teenager, performed live!)

UPDATE: I got a cool dual 16-bit counter IC made by TI, a SN74LV8154N - very cheap, and can be configured as a 32-bit counter. It seemed like a better option than multiple 8-bit counters, and this chip is about $0.60 so if I can make it work I’ll be happy! I breadboarded it up (see circuit diagram) and it seemed to work. I started wiring it on the perf board, but haven’t written software for it yet…

UPDATE - I just found this video on youtube I never posted on my blog, so this seems like an appropriate location for it:

A Taste of Culture

This blog has become too much about electronics. Yeah, electronics projects are good but there’s more to life than electronics! Right now I’m in dental school and… [pause] oh, yeah I don’t want to reflect on that. Let’s talk electronics! I keep planning to eventually write what dental school is like from an honest perspective for its archival significance, but the thought of spending time writing about dental school makes my gut churn. There are so many other things to do than to reflect on “that place”. My home is my escape, this website is my escape, and when I’m ready to talk, I’ll talk. This is a note to myself to, at least once before I graduate, write what it’s like to be in dental school. Back to tinkering. I leave you with a selection from Dvorak’s Piano Quintet Number 2 (opus 81). It’s not a wonderful recording, but I appreciate the performance.

EDIT: link updated in 2020 because original video is down. The referenced portion of the song is this video starting at 27:30.