My longish term goal is to build the LBS transceiver described by N6QW and KK6FUT (who I see is now AI6YR) and I am methodically working my way through that project.
However, I am not above taking a shortcut for instant gratification along the way. Remember the 80-meter Sudden Receiver variant I built a few weeks ago? Well, not much use for it in my current location. It occurred to me that I could gut the 80-meter parts of it and use the NE602/LM386 core as the rest of a very minimal direct conversion receiver for a more useful band, say 20-meters. So that’s what I did and long story short, it worked.
Over the last couple weekends, I put together an RF signal generator based on a AD9850 DDS module controlled by an ATmega328 microprocessor. In this entry, I describe both physical construction and the arduino sketch underlying its operation. The most recent versions of both code and schematic are archived on github.
On one hand, it probably doesn’t make too much sense to try to refine the MMM (the construction of which was described in an earlier post): it’s more an oscillator demo than a building block of any more complex radio, but there are a couple variables that I thought would be fun to explore: choice of transistor, supply voltage and emitter resistor value. Tables and pretty x-y graphs follow.
The 80m Madagascar Mighty Mite was suffering from “a tree falls in the forest but nobody hears it” syndrome. Eighty meters is a tall ask for Madagascar — there aren’t that many hams in the coverage area, and given local noise, I doubt any of them can hear well on 80m. It would be a long wait for a signal report about the on air performance of the MMM. Clearly, the thing to do was to create a mate for the MMM, the Madagascar Mighty Mite Mate (MMMM).
In keeping with the philosophy of back-to-basics rockbound simplicity, I decided to build an 80m version of the Sudden Receiver originally described by George Dobbs in SPRAT, and reprinted in 73 (October 1991, page 8, available online thanks to the Internet Archive).
I thought my 200mW Madagascar Mighty Mite (MMM) would benefit from some sort of afterburner, so I dusted off a project shelved in 2011: the Texas Topper amplifier. I had built based on a design by Chuck Carpenter and kitted by Rex Harper. I ran into a couple problems back then, including some difficulty getting the bias right on the mosfet at the heart of the amplifier. In another brilliant move, I managed to burn out said mosfet by grounding it while trying to get it and its heat sink to fit into a metal box.
Over the last few years, there have been a spate of postings from homebrewers taking inspiration from the Soldersmoke podcast to whip up various incarnations of the Michigan Mighty Mite, a very simple rock-bound QRPp transmitter. I’m a little late to the party, but here’s my story.
In an earlier post, I described an arduino-based clock that I made for my amateur radio station. I had to interrupt work on it for a business trip, but got back a few days ago. I’m now far enough past jet lag to operate heavy machinery, so I finished mounting the board on an aluminum faceplate and installed it in the station.
I’ve been meaning to add a clock with multiple time zone display to my radio operating position for a while. The time in UTC is displayed on my computer when I’m logging, but it’s small and it disappears when the screen sleeps. Also, I use the same room to take work-related conference calls, and it would be helpful to have a clock that displays the local time in Madagascar and the time on the US East Coast, where most of my overseas calls are scheduled. The fast, reasonable approach would be to wire up a microprocessor, a real-time clock chip, and an I2C-driven LCD display that could show all three zones at once. However, the only LCDs I have on hand have small fonts and aren’t that bright. On the other hand, I have a bunch of 4-digit x 7-segment LED displays that burn as bright as Sauron atop Barad-dûr. I went with those.
Continuing on the theme of power supplies and related, I thought I would try my hand at making a bench top variable power supply based on a universal laptop adapter. My rationale is that these adapters are made to run on 11-16V, so I could run it off whatever power is available (the 12V bus on my workbench, a battery, from mains by using another power adapter upstream, or even off the car); considering where I live and the reliability of electrical power, this seemed like a good idea. Also, these brick power supplies are, well, built like a brick, and are designed to tolerate abuse.
My plan was to do something similar, but I wanted to add the twist of being able to set a current limit above which power would be entirely cut to the load until reset. I anticipated using a microprocessor and thought the project wouldn’t be to complicated… but that turned out not to be the case. The power adapter is finicky about turning on into loads, doesn’t like being reset, and I haven’t managed to get more than 15W output from it. That said, I now have it working, but its capabilities fall short of what I had anticipated.
It’s been a chicken and egg race for the last couple months – I was working on power supply projects, but at the same time needed an electronic load so I could test the supplies at known current draws. I have had a working electronic load for a few months, but only got around to making it pretty and boxing it up in the last week. The design that I came up with is not very sophisticated, but it uses common parts and it seems pretty solid. The picture at right shows my recently built variable power supply working into the electronic load.