Homebrew Desk Calculator

Back in the good old days of the early 1970s, small desktop calculators were made by several companies. However, they cost quite a lot of money. The average engineer didn't have that much cash lying around, but still wanted to relieve himself of the burdens of manual mathematical computations.

My twin brother married a lady whose father was just this sort of engineer. He took it upon himself to build a calculator from scratch c. 1972, using one of the then-new single-chip 4-function calculator ICs available at the time.

Back then, "single chip" meant that all the math was done on the chip, not that it could drive a display directly. So a lot of other parts were needed to build such a calculator. He had to design and fabricate several printed circuit boards, using the common method back them of drawing the circuit traces and pads with a Sharpie® pen (sold by Radio Shack at an inflated price as an "etch resist pen") and etching the copper-clad phenolic board using ferric chloride.

The case is made of hardwood, and the keyboard is a hand-lettered paper sheet sandwiched between a couple layers of Plexiglas® and a bunch of little pushbutton switches.

The display is a row of twelve tiny 7-segment vacuum-fluorescent display tubes. These tubes were common in calculators before LEDs had attained reasonable prices of a couple dollars a digit.

Note the wooden card guides to keep all the little PC boards in place.

This gizmo still works great!

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