Sinclair Executive

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After the world's smallest radio receiver, Sir Clive Sinclair considered the development of the world's smallest calculator as his next goal. Due to the technology used, the current consumption of the available circuits was quite high, so the average pocket calculator could be operated with 4-6 dry cells or rechanrgeable batteries, and with that for only a few hours. After Texas Instruments declared that the solution presented by Sinclair, that the chip is powered not by direct voltage, but by pulsing voltage depending on the operating state, was not functional, the relatively expensive machine, which was only 9 millimeters thick, became a worldwide success. With 3 button cells, thanks to the innovation outlined above, it could work for 15 to 20 hours. The extremely simple designed Executive brought worldwide success to the Sinclair company, so later produced a version with memory, and then the Cambridge series, which could be operated with small penlight batteries, was also very successful. Competing manufacturers, targeting the less price-sensitive executive class, began to develop machines with similar dimensions.

The Executive itself was produced in several versions: the first was built with a Texas Instruments TMS1802 circuit, but later the notation was changed to TMS0103; it is easily recognizable based on its large six-digit display elements. The second version (including the one shown here) uses 4 button cells and is built with Bowmar's small Optostic display, with driver circuits instead of discrete transistors. The third version was built with General Instrument's especially cheap, algebraic logic chip (C550), the former constant switch was used here to change the brightness of the display, since the machine turns on the constant register in response to the K key. The decimal adjustment slide switch found on the back of the Texas versions was not included in the third version, since the chip only works in floating point mode. It is interesting that although the 2×6 digit display of the earlier versions could work for up to 12 digits, and Texas Instruments offered 10-digit chips since 1973, such machine was not produced ever.