Clive Sinclair started his first company, Sinclair Radionics, in 1961 when he was 21. The first products were low cost audio amplifiers and miniature radios, mostly sold in kit form. In 1972 Radionics produced the Sinclair Executive pocket calculator, the smallest and one of the cheapest available at the time. Several other successful models of calculator followed through to 1978.
By the late 1970s though competition meant that the price of calculators had fallen to the extent that there was hardly any profit margin in them and Radionics was losing money. The British National Enterprise Board bought a stake in Radionics and put in some money to keep the company going but this meant Clive Sinclair had less and less control over his business.
In 1979 Sinclair left Radionics to concentrate on a new company he had set up, called Science of Cambridge. Their first product was a very simple computer trainer called the MK 14 which consisted of a single printed circuit board with a microprocessor, a small amount of memory, a 20 key keyboard and a calculator display. Even though all this kit could really be used for was learning the rudiments of microprocessor programming it nonetheless sold in reasonable numbers.
In 1980 Science of Cambridge launched their first true computer, the ZX80. This included a full alphabetic keyboard, though using a flat touch sensitive membrane, used a television as the display and could be programmed in BASIC. The ZX80 was very limited by not being capable of floating point arithmetic, only having 1 kilobyte of RAM and having a text only display, but it cost a mere £100 which made it far cheaper than any other 'home computer' available at the time. Over 70,000 ZX80s were sold, an exceptional number for the computer industry in 1980.
In 1981 the company, now known as Sinclair Research Ltd, released its most influential computer, the ZX81. This was similar to the ZX80 but with a much improved version of BASIC, including full floating point arithmetic, and with cheaper memory expansion. This computer was capable of genuinely useful tasks, the price had been lowered to £80 and suddenly everyone wanted a ZX81. Including licensed clones almost one million ZX81s were sold.
Sinclair research continued its run of introducing a new computer every year with the ZX Spectrum in 1982. This added colour graphics, sound, a much larger memory, and an improved keyboard to the ZX81 and still the starting price was only £125. A huge amount of (mostly games) software was produced for the Spectrum and, including later derivatives, about 3 million Spectrums were sold, making it the most successful British-designed home computer.
Sales of the ZX81 and Spectrum had allowed Sinclair Research to grow to be worth over £100 million on the stock market, despite being a relatively small company. (All manufacturing was subcontracted.) Their next computer, in 1984, was the QL or 'Quantum Leap' and was Sinclair's attempt to move upmarket into business computers, which were beginning to be very lucrative. The QL was certainly ambitious for Sinclair Research, boasting a fast Motorola 68008 processor, up to 640 kilobytes of RAM and an advanced operating system capable of multitasking, but unfortunately it was much delayed and had several faults, including non-standard Sinclair 'Microdrives' instead of floppy disks and a poor quality keyboard. It never appealed to the business market, nor at £400 was it hugely popular as a home computer. About 100,000 QLs were sold, a small number compared to the ZX81 and Spectrum.
Sinclair's computer sales started to flag, despite an improved version of the Spectrum called the Spectrum 128, and the business was losing money. More money was lost on Clive Sinclair's ill-fated C5 electric 'car' project. In April 1986 the Spectrum design and the Sinclair computer brand name were sold to Alan Sugar's Amstrad electronics company. Amstrad repackaged the Spectrum 128 to aim it specifically at home games players and it continued to sell for several years, but essentially this was the end of the Sinclair line of computers.
However it was not the end for Sinclair Research or Clive Sinclair. In 1987 he introduced the Z88 portable computer, essentially a compact battery powered word processor and address book, which achieved modest sales. More recently Clive Sinclair has sold electric bicycles (the Zike) and electric drives to fit to existing bikes, and an ultra compact radio.
Clive Sinclair's 'ZX' range was probably the main reason for the home computer boom in the UK in the 1980s, and the subsequent computer games industry. For a more detailed history read The Story of Sinclair Computers.