Sinclair's Micro Kit was formalised in an exercise book dated 19 June 1958 three weeks before his
A-levels. Sinclair drew a radio circuit, Model Mark I, with a
components list: cost per set 9/11 (49½p), plus coloured wire and solder, nuts and bolts, plus celluloid chassis (drilled) for nine shillings (45p). Also in the book are advertisement rates for Radio Constructor (9d (3¾p)/word, minimum 6/- (30p)) and Practical Wireless (5/6 (27½p) per line or part line).
Sinclair estimated producing 1,000 a month, placing orders with suppliers for 10,000 of each component to be delivered.
Sinclair wrote a book for Bernard's Publishing, Practical transistor receivers Book 1, which appeared in January 1959. It was re-printed late that year and nine times subsequently. His practical stereo handbook was published in June 1959 and reprinted seven times over 14 years. The last book Sinclair wrote as an employee of Bernard's was Modern Transistor Circuits for Beginners, published in May 1962.
Bernard Babani he produced 13 constructors' books.
In 1961 Sinclair registered Sinclair Radionics Ltd. His original choice, Sinclair Electronics, was taken; Sinclair Radio was available but did not sound right. Sinclair Radionics was formed on 25 July 1961.
Sinclair made two attempts to raise
startup capital to advertise his inventions and buy components. He designed
PCB kits and licensed some technology. Then he took his design for a miniature
transistor pocket radio and sought a backer for its production in kit form. Eventually he found someone who agreed to buy 55% of his company for £3,000 but the deal did not go through.
Sinclair, unable to find capital, joined United Trade Press (UTP) as technical editor of Instrument Practice. Sinclair appeared in the publication as an assistant editor in March 1962. Sinclair described making
silicon planar transistors, their properties and applications and hoped they might be available by the end of 1962. Sinclair's obsession with miniaturisation became more obvious as his career progressed. Sinclair undertook a survey for Instrument Practice of
semiconductor devices, which appeared in four sections between September 1962 and January 1963.
His last appearance as assistant editor was in April 1969. Through UTP, Sinclair had access to thousands of devices from 36 manufacturers. He contacted
Semiconductors Ltd (who at that time sold semiconductors made by
 and ordered rejects to repair. He produced a design for a miniature radio powered by a couple of
hearing aid cells and made a deal with Semiconductors to buy its
micro-alloy transistors at 6d (2½p) each in boxes of 10,000. He then carried out his own quality control tests, and marketed his renamed MAT 100 and 120 at 7s 9d (38¾p) and 101 and 121 at 8s 6d (42½p).
Science of Cambridge
Sinclair formed another company, initially called Ablesdeal Ltd, in 1973. This changed name several times, eventually becoming Science of Cambridge Ltd in July 1977.
In June 1978 Science of Cambridge launched a
microcomputer kit, the
MK14, based on the National
SC/MP chip. By July 1978, a personal computer project was under way. When Sinclair learned the
NewBrain could not be sold at below £100 as he envisaged, he turned to a simpler computer. In May 1979
Jim Westwood started the
ZX80 project at Science of Cambridge; it was launched in February 1980 at £79.95 in kit form and £99.95 ready-built. In November, Science of Cambridge was renamed Sinclair Computers Ltd.
In March 1981, Sinclair Computers was renamed again as
Sinclair Research Ltd and the
Sinclair ZX81 was launched at £49.95 in kit form and £69.95 ready-built, by mail order. In February 1982
Timex obtained a licence to manufacture and market Sinclair's computers in the United States under the name
Timex Sinclair. In April the
ZX Spectrum was launched at £125 for the 16 kB RAM version and £175 for the 48 kB version. In March 1982 the company made an £8.55 million profit on turnover of £27.17 million, including £383,000 government grants for the
TV80 flat-screen portable television.
In 1982 Sinclair converted the Barker & Wadsworth mineral water bottling factory into the company's headquarters. (This was sold to Cambridgeshire County Council in December 1985 owing to Sinclair's financial troubles.) The following year, he received a
 and formed
Sinclair Vehicles Ltd. to develop electric vehicles, which resulted in the unsuccessful
Sinclair C5 in 1985.
In 1984, Sinclair launched the
Sinclair QL computer, intended for professional users. Development of the ZX Spectrum continued with the enhanced ZX Spectrum 128 in 1985.
In April 1986, Sinclair Research sold the Sinclair trademark and computer business to
Amstrad for £5 million.
 Sinclair Research Ltd. was reduced to an R&D business and holding company, with shareholdings in several spin-off companies, formed to exploit technologies developed by the company. These included Anamartic Ltd. (
wafer-scale integration), Shaye Communications Ltd. (
CT2 mobile telephony) and Cambridge Computer Ltd. (
Z88 portable computer and
satellite TV receivers).
By 1990, Sinclair Research consisted of Sinclair and two other employees,
 and its activities have since concentrated on personal transport, the
Zike electric bicycle, Zeta bicycle motor and the
A-bike folding bicycle.
Sinclair received an Honorary Doctorate from
Heriot-Watt University in 1983