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Sinclair Executive

Size: 56 mm x 138 mm x 9 mm

Display: 8 digits, red LED

Power: 3.9V (3 x hearing-aid batteries)

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The Sinclair Executive was the world's first pocket calculator, launched in August 1972 at a price of £79.95 + VAT. The advert described it as being "as thick as a cigarette packet", reflecting Clive Sinclair's stated belief that "One must always bear a packet of cigarettes in mind as the ideal size"; it was a running joke at Radionics that Sinclair, who smoked 40 cigarettes a day at the time, designed everything to be the size of a packet of 20.

By modern standards, the Executive was astoundingly limited: it had an eight-digit display and could add, subtract, multiply and divide - no other functions. It had automatic squaring, reciprocals and a choice of fixed or floating decimal points. Inside the case were 22 transistors, 50 resistors, 17 capacitors and the piéce de resistance, the Texas Instruments GLS 1802 integrated circuit, which the advert described breathlessly as "an electronic marvel... the largest ever produced for commercial use".

The Executive was seen as a genuinely revolutionary advance (which in truth it was). Calculators prior to the Executive had been bulky, desk-bound affairs, often depending on mains power. The fundamental problem was the high power demands of the LED display: indeed, it was precisely that problem which drove the development of liquid crystal displays and ultimately killed off the LED-equipped calculators.

The advert for the Executive highlighted this problem, though it was a little disingenous about how Sinclair had resolved it:

But the real genius lies in the circuitry linking the brain, the batteries, the keyboard and the display. Circuitry soaks up power, which is why other pocket calculators have to use large batteries - and that, in turn, makes them bulky.

In the Executive, the Sinclair flair for miniaturisation has developed circuitry which absorbs virtually no power. Tiny hearing-aid batteries take up the minimum space and, used from time to time during the day, will last for several weeks.

In fact, what Sinclair had done was to exploit the persistence of the diode displays and the chip memory. It was immediately apparent that continuous demands on the batteries drained them very rapidly. Jim Westwood, Radionics' chief designer, discovered that if one disconnected the power and turned it on again fairly quickly, the display and the contents of the memory remained intact. With the transistors built into the circuit holding up to five seconds of charge, it was a simple matter to pulse the power on and off. It reduced the power demand from an unacceptable 350 milliwatts to a barely acceptable 30 milliwatts. New Scientist explained the process:

Sinclair's "Executive" uses a single metal oxide semiconductor (MOS) chip containing 7000 transistors. This type of integrated circuit normally consumes 350 milliwatts of power, provided by a fairly bulky rechargeable battery. Sinclair's miniaturisation trick depends on a special circuit which reduces the power requirement to about 20 milliwatts. This is accomplished by the elimination of all resisitors from the circuitry and with a clock that turns off the power most of the time. Three small deaf-aid mercury cells provide 20 hours of life - over four months normal usage.

Power is turned on only in 1.7 microsecond pulses, a figure determined by the storage time of a control transistor. The unit has an oscillator clock which operates at 200 kilohertz during calculations and drops to 15 kilohertz between each operation. Thus shut off time ranges from 3.3 microseconds during calculations to over 65 microseconds between. The device relies on the capacitance of the chips to store information during the shut off times, and 1.7 micorseconds is long enough for the chip to carry out a single change of state of the electronics. Any calculation can be done in 1000 such changes.

Even so, battery life was only a few hours and the machine could be physically dangerous. If it was left on, the batteries had a nasty habit of exploding and blowing the machine to bits. One day a telex arrived from Moscow. A senior Soviet diplomat had been carrying an Executive in his breast pocket when it exploded, convincing the unfortunate man and his entourage that he was suffering a massive heart attack. After the fuss had died down, investigators found that he had forgotten to switch the calculator off: the current drain was so high on the batteries that they grew hotter and hotter, finally exploding. Perhaps fortunately, the incident was accepted as an accident, rather than a capitalist provocation! Such were the joys of early 1970s electronics.

Such slight mishaps did not dent the Executive's deserved success, however. It was virtually the first attractively-styled calculator, its rivals and predecessors being utilitarian lumps of plastic. The neat design of the injection-moulded polycarbonate casing won its designer, Richard Torrens, the Design Council Award for Electronics in 1973. New Scientist described it as "not so much a professional calculator - more a piece of personal jewellery". Design magazine called it "at once a conversation piece, a rich man's plaything and a functional business machine". It was even exhibited at the Museum of Modern Art in New York.

The first Executive was a major success, earning Sinclair over £1.8m in profits, and was followed up at the end of 1973 with the Executive Memory. This had the eponymous extra feature and "a new black and white styling", at a far cheaper price of £24.95 + VAT.

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Chris Owen 1994-2003