The HP-35 - First Pocket Scientific Calculator

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The HP-35 - First Pocket Scientific Calculator



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The HP-35

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The HP 35 Printed-Circuit Boards


The HP 35 Circuitry

The HP-35 is assembled on two printed circuit boards (see picture above). The large board contains the display, drivers and keyboard. The smaller board has all the MOS logic, clock driver, and the power supply components. Metal "humps" on the larger board are pressed down by the keys to make contact with printed traces underneath.

The HP-35 contains five MOS/LSI (metal-oxide-semiconductor, large-scale-integration) circuits : three read-only-memories (ROMs), an arithmetic and register circuit (A&R), and a control and timing circuit (C&T). The logic design was done by HP and the circuits were developed and manufactured by two outside vendors. Three custom bipolar circuits are manufactured by HP's Santa Clara Division: a two-phase clock driver, an LED anode driver/clock generator, and an LED cathode driver. ( See block diagram of the calculator on picture below ).

Block Diagram of the HP 35, from the Hewlett Packard Journal, June 1972
Courtesy of the Hewlett Packard Company


The HP 35 from the Hewlett Packard Journal, June 1972
Courtesy of the Hewlett Packard Company

Packaging the Pocket Calculator


The industrial design of the HP-35 was very unusual for Hewlett-Packard. No other product previously designed had to be Pocket-Sized.

The industrial design began with an investigation of keyboard, packaging, and overall shape concepts. Several basic form factors were studied using sketches and simple three-dimensional models.

Only a general idea of the electronic design was known at this point. Designing and packaging all necessary electrical and mechanical components into the tiny product became a tremendous challenge for electrical, mechanical, and industrial designers alike.

Every descision had to be made without any reference or comparison to previously build devices. For example, the keyboard was the most critical area of the design. The problem was to place thirty-five keys in an area approximately 2.5 inches by 4.5 inches and retain the ability to operate the keys without striking more than one at a time.

The external package was developed from a human-engineering approach, with aesthetic appeal of major importance. The sculptural wedge shape permits the calculator to be comfortably held in the palm of one hand. It also allows the product to slide easily into a pocket. The keyboard and display slope upwards for a better viewing angle in desk-top use. The texture on the case provides a non-slip surface, important when the calculator is being hand-held.



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