FDIM 2012: A field strength meter built onto a Harbor Freight DMM

Continuing the series on neat projects I saw at the Four Days In May, here is a litle field strength meter built onto the side of a cheap Harbor Freight DMM by Dana Browne AD5VC.

ADC5VC field strength meter

Dana’s inspiration for the little field strength meter came when he was teaching a radio class to college students and wanted to show them the pattern of a Yagi-Uda antenna. He explained that cheap digital multimeters like this Harbor Freight model were readily available in the physics lab, so he built the field-strength circuit and  a little plug-in adapter using the DMM as the readout.

Though I don’t see much need for a field-strength meter in my shack, I admire Dana’s inventiveness in coming up with a solution that is cheap, convenient, and inexpensive.

An enclosure for the R2/T2 transceiver

After months of organizing parts, I have finally gotten back to the R2/T2 transceiver project. Don’t get me wrong, the cleaning and sorting is not done, but I felt the urge to do something a bit more… constructive.

While cleaning, I found a box of old electronics junk that had promising cases. Electronics enclosures are expensive. Salvage can be a good way to keep the cost down. I don’t know what this thing once was, but there are military-style circular connectors on the front and back, two fuse holders, a power inlet, and no visible controls.


Opening it up, I found this:

There’s a lot of empty space in there! It looks like it was some kind of power supply. Next to the weighty transformer and big blue filter cap, a circuit board carried 7805 and 7806 regulators, several current-sense resistors and an LM324 quad op amp. It also had a power transistor on board and connected to the big TO-3 transistor on the heat sink in the back.

The board on the other side had a couple of high-voltage film capacitors, some ten-turn pots with their positions set with nail polish, two LM324’s and one RCA 4151 voltage-to-frequency/frequency-to-voltage converter. Down in the lower-right corner, it also held a solid-state relay. I’m a little more stumped about what this board was for. Maybe it was more power-supply logic, or maybe some kind of controller.

Tracing out the wiring harness revealed that 120VAC is run to the front connector, with only a fuse between the connector and the power cord. That could get exciting quickly to anyone not expecting it.

I pulled apart the whole thing, salvaging only the transformer, two ICs, and some fasteners. I tried to salvage all the ICs, but some were corroded into their sockets and could not be extracted without breaking pins. I have not had that happen before.

That’s the final product. I left the fuse holders, the heat sink, and a common ground point in place. They might be useful when this box becomes a transceiver. The front and back panel are 0.125″ aluminum and slide out after a few screws are removed. It will be easy to replace them with new panels for the radio.

The only fixed surfaces in this box on which to mount things are a pair of narrow rails on each of the side extrusions. The bottom is removeable and isn’t set up well to hold circuit boards. I will have to either add a false bottom or come up with a way to mounting the boards at right angles from the sides. Putting the boards flat on the sides, like the original residents of this box, won’t give me enough room, and because the boards are not sized to fit the walls, I would have to improvise some kind of mounting panel or angled standoffs to hold them anyway.

In any event, that problem is solvable. It’s a nice case for what I hope will be a nice radio.

FDIM 2012: The YADI micropower digital interface

Next in our tour of projects from the Four Days In May event at the Dayton Hamvention is an innovative radio-to-computer interface.

The YADI rig-to-computer interface prototype

The unique feature of YADI, Yet Another Digital Interface, is a micropower VOX circuit. This lets it sip power while waiting for a signal, where other interfaces are less battery-friendly. The secret is that the VOX amplifier is biased in “class E”, according to its designer, Dana Browne AD5VC. (It is possible he meant class C, since class E amplifiers are tuned and a VOX amp is broadband.)  That means that the amp uses no bias current when there is no signal. Its biasing also causes it to rectify the input, eliminating the need for an additional rectifier. The quiescent current is less than the self-discharge current of a 9V battery. In other words, the battery loses more energy sitting on a shelf than this interface needs to keep idle.

Dana, with Jim Giammanco N5IB, designed it with an eye towards emergency communications, particularly after hurricanes in their home state of Louisiana. It is designed to interface any radio and to any computer sound card, to support the many digital modes available today.

A kit version of the interface is being produced by the Baton Rouge Amateur Radio Club. The kit version was also on display at FDIM, but unfortunately, my photos of it did not come out. Suffice it to say that the kit includes a good-looking PCB, features easy through-hole assembly, and fits in a mint tin. The price is $35.

Unfortunately, I can’t find a link for the kit, other than some copies of the manual on a file sharing site. The BRARC web site is a vacant placeholder, and Google turns up no other leads. I hope this worthy kit becomes available soon and gets the publicity it deserves.

FDIM 2012: N8WE’s CW transceiver

A few weeks ago, I went to the Four Days In May event at the Dayton Hamvention, and brought back some pictures of the cool projects I saw.

N8WE's 200 mW CW transceiver

Glenn Hazen, N8WE, brought his 20 meter transceiver project to show-and-tell night. The radio’s receiver uses a Softrock Lite II downconverter with a laptop running a software-defined-radio (SDR) application. The transmitter is a 200 mW Morse code transmitter. That’s the code key on the right.


FDIM 2012: HamOS

A few weeks ago, I went to the Dayton Hamvention and the accompanying QRP event, Four Days In May (FDIM). QRP-ARCI always puts on a good program for FDIM, with a full day conference on Thursday, the day before Hamvention opens, and evening events Thursday, Friday, and Saturday.

Among the evening events are a show-and-tell night and a building contest night. Show-and-tell is open to anyone to bring a QRP-related project to share, whether finished or work-in-progress. The building contest has had varying rules over the years, but this year it had several categories, including kit-built radios, scratch-built radios, and station accessories. Judging was by audience ballot.

I forgot about show-and-tell night and regretted it. The FPGA morse code keyer would have fit right in.

I took some photos and notes from the evening events and will be sharing them over the next few weeks.

HamOS demonstration

First up is HamOS. Rich Gordon KD0BJT and Brady AC0XR were demonstrating this Linux-based operating system and handing out CD-R’s as fast as they could burn them. HamOS is a GNU/Linux distribution focusing on ham radio, with a variety of radio applications pre-installed. I can’t find a download link for HamOS, but hopefully that will be rectified soon.

Rich was lucky enough to get the “BJT” (Bipolar Junction Transistor) acronym in his call — and it looks like it is sequentially assigned, not vanity!

Rich and Brady also produce the lowSWR podcast, to which I have just subscribed.

SMT Soldering – It’s Easier Than You Think

Front page of "SMT Soldering: It's Easier Than You Think"Soldering Is Easy was a great starter, and now here’s a sequel: SMT Soldering – It’s Easier Than You Think.  Greg Peek and Dave Roberts put together a great little comic book that teaches the art of soldering tiny SMT components. After reading it, you will be ready to tackle chip resistors, SOT-23’s, and SOIC’s with confidence.  (You might also like to take a look at Skywired’s own tutorial on soldering SMT ICs.)

The comic book is licensed CC-BY-SA (Creative Commons Attribution Share Alike), so everyone is free to pass it on.

(Via MightyOhm.)