I've been playing around with the Arduino development platform for the last couple months and decided I need some sort of real project to really get my head around its capabilities. I decided to make a multifunctional accessory for QRP amateur radio operation. I don't have a formal design spec, but some of my ideas are jotted down here:


The photos and video clip here are of the prototype of the automatic antenna tuner unit. I realize that for purposes of miniaturization, stability, good RF ground, etc., I'll need a custom PC board at some point, but for now, I roughed it out on a stone age vector board (not even a vector board, as I went with a cheap Rat Shack prototyping board - probably not the best move, as I'd prefer a glass-epoxy substrate for rework, but didn't have one on hand).

The board has lot of blinken lights so I can see what state the relays are in. The production model won't need these at all, but I wanted them at this prototype stage so I could have a visual check on what the hardware is doing as I muck about in the firmware.

The video shows the test program running - the relays are zeroed out - that is, the capacitors are ungrounded and the inductors are shorted. Then, each relay is fired at half second intervals to run all the relays through their paces. 

In the upper left corner, four PNP transistors control power to the relay "rows", one to turn on the upper row of relays, one to turn it off, and similarly for the bottow row. All of the relays are controlled through one high power shift register. So, the control hardware costs about $4.00 without any volume discount. The relays themselves cost about a buck each, obtained from my friends at eBay.

It would have been much easier to use regular rather than latching relays, but since this unit is meant to operate in the field on battery power, latching relays were the only way to go. Not needing to power the relays after getting them in the right configuration is worth the hassle of the hardware needed to control the relay latches.
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