Custom metal Calculator build

[Garth Wilson]

• I think the keyboard would have the highest start-up cost. Even there of course the price could have a wide range, with HP's nice double- or triple-shot plastic keys being the highest. A molded rubber keypad might be next down, with the nomenclature printed on the surface and a protective coating over it to make it last. That might be a few thousand dollars or less for startup, and then extremely low per-piece cost, like well under a dollar. (I say this based on experience in the 1980's!) Or, perhaps about even with the price of that would be a membrane keypad, with the switches embedded under a lexan overlay. These can have snap domes for tactile feedback and a lot of other options, and windows and cutouts can be part of the same sheet. A nice thing about these is they keep dust and liquids out, which is why you see them on things like machine-shop milling machines, so the shavings and oil don't get in them. They can be vented so going up into high altitude in an airplane doesn't make them collapse and artificially push the buttons. Otherwise they would probably not be the first choice. A simple lexan overlay could also be used over cheap tactile switches that are soldered down to the PC board. This overlay can have raised ridges around the keys, and again offers the option of windows (like for the LCD and LEDs) or holes for things like connectors. We did one at work in 1995 that had a dead-front area with six windows that you couldn't see until a bright LED went on behind. This was because based on the options chosen and the installation in an aircraft, certain annunciators may never come on, and we didn't want them showing in that case; so they just looked black until the LED behind any given one illuminated, making the lit-up letters pop out of the black background. It was a very nice effect.

• Next might be a custom LCD; but you probably can get away with an off-the-shelf one instead.

• Next would be the box. There are so many ways to go on this, even without resorting to getting your own molds made (which could be tens of thousands of dollars, based on recent work experience). But since you mentioned a metal one, you could start with an off-the-shelf deep-drawn box. The companies that make these always expect that you'll need some customization, and they're up to the task. Same for plastic boxes, and they're probably cheaper, being easier to mill and so on. The per-piece price on this kind of work is more (regardless of material), but the start-up cost is very low.

• As for a PC board, even if you go custom, they've come down so cheap so as to be a non-issue, even for one-off projects. I've been using DirtyPCBs (apparently part of DangerousPrototypes), and have been very pleased with their price, quality, and delivery time. Actually, they seem to be a brokerage house, and which manufacturer they send your design to seems to depend on the design itself and how busy the various manufacturers are at the time.