Sinclair Executive
and
Sinclair Executive Memory
The distinctive Sinclair Executive hit the headlines when it was launched in 1972 since, for that time, it was incredibly thin and could easily fit in a shirt pocket without distorting it. Nothing quite like it had been seen before.
Page revised July 2026.
The Sinclair Executive was followed by the enhanced Sinclair Executive Memory.
Important notes: The Sinclair Executive was a very innovative calculator and employed several novel techniques in order to reduce the thickness of the casing to the minimum that was then possible. In the light of experience of some of its shortcomings and with developments in electronics, the design was improved in a number of different versions over the year or two after its launch.
Of all the calculators on this site the Sinclair Executive is the most difficult to describe due to variations in designs. The different versions of the electronics are poorly documented, and so this page is subject to change as further details become available. The information below is believed to be correct, but please get in touch if you can provide further information or correct any mistakes.
Be aware that the two halves of the plastic casing of the Executive models are welded together and it is not possible to see the circuit board inside without severely damaging the casing. The circuit boards shown in the photographs here were obtained without a casing or were removed from non-working calculators in poor condition where there was less concern in destroying the casing.
The Sinclair Executive was made in two keyboard configurations, as illustrated below.
Early Model Keyboard
Later Model Keyboard
Earlier models use a Texas Intruments calculator integrated circuit, with "Adding Machine" keyboard with '-=' and '+=' keys and a decimal point setting slide switch on one side.
Later models use a General Instruments Microelectronics (GIM) calculator integrated circuit, with modern calculator keyboard.
Sinclair Executive
Distinctive features:
Technical details:
There are two known types of casing & keyboard found on the Sinclair Executive with different keyboard functionality, as shown above:
The incredibly small thickness for this stage in calculator development was attained by using "calculator-on-a-chip" electronics, button cells as the power source, a minimalist keyboard where tiny buttons press sprung copper alloy fingers onto the circuit board, and tiny slide switches which use contacts on the circuit board.
Display is 8 digits, red LED.
4-function.
56 mm x 138 mm x 9 mm (2.2" x 5.5" x 0.4").
Introduced August 1972.
Made in England
Significance
The Sinclair Executive was one of the smallest calculators of the time and certainly the thinnest. It was also the first in a long line of innovative Sinclair calculators.
Clive Sinclair was already well known for his innovative, compact, and stylish electronics. This
calculator demonstrated his flair with an incredible calculator design for this time.
Three main versions of the Sinclair Executive are known:
General photographs of an Early Model Sinclair Executive
Side view, showing how thin the Sinclair Executive models are. The power switch is the small slider towards the right.
For its time it was a very small calculator.
With battery cover removed, showing the button cells.
The casing of this early model which uses a Texas Instruments integrated circuit has a decimal point slider switch along the right side with the decimal point settings — 6, 4, 2, F —
marked on the rear casing. The instructions recommend leaving it set to F (ie. Floating-point) unless specifically required otherwise.
Later calculators using the GIM integrated circuit do not have this switch and are
permanently set to use floating-point.
An advertisement for the earlier version Sinclair Executive, emphasising how slim it is.
It also explains "The secret of the Sinclair Executive: The Executive's "brain" is an electronic marvel – a 7,000-transistor integrated circuit (the largest ever produced for commercial use)"
An advertising brochure for the later version Sinclair Executive with the later type of keyboard (courtesy of Gary Walton). Click here for a larger version.
Various models of the Sinclair Executive calculator and its electronics
For this period in the development of electronic calculators the Sinclair Executive was incredibly thin, attained by using very innovative techniques. For instance, to reduce thickness conventional keyboard switches are not used but thin metal shim fingers which are pressed by tiny buttons onto the circuit board. Also, the power switch, on the side, presses spring contacts onto the circuit board and the decimal point selection switch slides contacts across the circuit board tracks.
Inevitably, shortcomings with some of the the innovations employed led to the need for modifications and so the design of the Executive evolved. Sinclair appear to have used the designations Mk I, Mk II, and Mk III for some versions, though it is not certain to which specific models these version numbers refer.
The low power consumption needed to employ button cells with their small electrical capacity was initially attained by pulsing the "calculator-on-a-chip" integrated circuit—not recommended by the manufacturer, but it worked. In later versions this method of operation was discontinued.
The journal "New Scientist" proclaimed in July 1972[6]:
"Just as the price war in electronic calculators was reaching its peak, a Huntingdon hi-fi maker has
stepped in with a smaller rather than cheaper unit. Sinclair Radionics Ltd has produced the first calculator that can comfortably fit in the user's pocket along with his wallet and cheque book. Sinclair's Executive is
only 3/8in thick, smaller than a pound note, and weighs 2.5 oz, which makes it less than one third the weight and volume of the Rapid Data Rapidman 800. The Executive also has
additional functions, including squares, reciprocals, and multiplication or division by a fixed constant.
At �79 [GBP, about US$200], the Executive is twice the price of the Rapidman 800. But Clive Sinclair argues
that the "executive toy" market is not particularly price sensitive; the firm has ordered components for 100,000 calculators.
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 resistors 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 microseconds 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."
Note that this says: "Three small deaf-aid mercury cells". It is not known how many Sinclair Executive calculators using three cells were sold, but they were soon redesigned to take four cells.
The journal "Electronics" gave more technical information[1]:
"Using the standard Texas Instrument 1802 MOS calculator chip ...
Engineers at Texas
Instruments Ltd. say they've not tested 1802 chips in the particular way that Sinclair is using them, but the only snag they can foresee is that some chips might need more power than Sinclair is giving them to work properly. To
guard against this, Sinclair is checking each chip for its working power threshold before use. Since he offers a five-year guarantee on a machine priced at $205 in Britain, he obviously has no qualms.
The seven-segment
gallium-arsenide-phosphide display is driven directly from the batteries and the chip, an approach which avoids the need for buffer amplifiers between chip and display."
A brief review in the journal "Wireless World" in August 1972[7] was somewhat critical and did not like the crude on/off and decimal-place switches, the keyboard, and the way that the electronics was pulsed. In a letter to the journal published in the September issue, Clive Sinclair rebutted the complaints. However, Richard Torrens, who was service manager at Sinclair at this time, reports[3] that soon after the release of the Sinclair Executive there were a large number of returns due to problems with the battery contacts, the on-off switch and the keyboard—much as the review in Wireless World had criticised. Richard says that he does not recall that there were problems with the battery switching—but the Sinclair Executive was soon redesigned and the redesign did not include the battery power switching feature, which then resulted in criticism of the short battery life.
Below is information about various versions of the Sinclair Executive that have come to light, in approximate order of their development.
Please get in touch if you can provide further information or correct any mistakes.
Important note: Unfortunately, the two halves of the casing of the Executive are welded together and so it is not possible to easily see the electronics inside without severely damaging the calculator.
Version 1 - Using 3 button cells and Texas Instruments calculator integrated circuit
I have not been able to find an example, or a photograph of an example, of a complete original Sinclair Executive when it was launched and actually showing three button cells, and may have been designated as the Mk I. However, here is an example of a Sinclair Executive circuit board which has a cutout with a size that is only sufficient for three button cells. It was obtained complete with the switch contacts and the clear plastic cover, which may indicate that the assembly is to illustrate this version.
Points to note:
Sinclair Executive circuit board with a cutout for only three button cells. Note the large two-part LED display across the whole width of the board and the driver transistors above and below it.
Sinclair Executive circuit board fitted with a special transparent top cover which allows a view of the components inside.
The key switch spring contacts have also been fitted. Each switch would be operated by a rubber push button located in the hole above it in the top cover.
With this rather crude arrangement the thickness of the keyboard was minimised.
Notes: Contemporary journals of 1972 report the integrated circuit simply as the 1802 or standard 1802[1]. Chris Curry in an interview on Youtube[2] says that it was TMS1802 integrated circuits that he brought back from Texas Instruments in the U.S.A. which led to the development of the Executive. He reiterates this on the website "Sinclair Radionics, October 1964 to April 1975"[3]. Rodney Dale in the book "The Sinclair Story" says that the integrated circuit used was the GLS1802[4], however the GLS prefix is not a published Texas Instruments code of the period[5].
The TMS1802 chip was quickly renumbered TMS0102 and became the first member of the TMS01xx calculator-on-chip family of devices - See the article "The Arrival of the 'Calculator-on-a-Chip'".
From this family Texas Instruments listed their "preferred types" which did not include the TMS0102 but did include the TMS0103 with the description "The TMS0103 provides eight digits, four operations, floating or fixed decimal point, constant or chain operation, automatic roundoff, overflow and underflow, leading zero suppression, and automatic power-up clear. This variation uses the arithmetic keyboard entry system, the same as standard business machines".
The TMS0103 may have just replaced the TMS1802 in this version of the Sinclair Executive, while retaining the large LED display and having space for just three button cells for the power supply.
Version 2) Using 4 button cells, Texas Instruments calculator integrated circuit, and the "Adding Machine" keyboard
This version appears to have been designated Sinclair Executive Mk II (Mk 2), and appears to have had a long production run.
There are several variations of this version with different types of LED display and either the Texas Instruments TMS1802NC (aka TMS0102NC) calculator integrated circuit or the TMS0103NC:
Version 2, Variety A) Large-digits display aligned centrally
This calculator has not been opened up so the version of integrated circuit inside is not known. It uses 4 button cells and has the earlier model keyboard with '+=' and '-=' keys and a switch on the right edge for selecting a floating decimal point or a fixed decimal point with 2, 4 or 6 decimal places.
Note that this version has the large LED numbers in the display.
The example in the photographs below has serial number 21550.
Sinclair Executive with display with large LED digits.
On the left of the display is a single LED digit to indicate "Overflow".
Close up of the display showing the locations of the large LED digits which are in two groups of four symmetrically about the centre line, with a separate single LED digit at far left for indicating "Overflow".
Compare with the version with the display with magnifiers, shown below.
Version 2, Variety B) Small-digits display aligned to right, Texas Instruments TMS1802NC integrated circuit
The integrated circuit is a Texas Instruments TMS1802NC, which is the first type used in the Sinclair Executive. This IC type was later renumbered TMS0102NC. The small LED display manufactured by Bowmar, 4 button cells, switch for floating decimal point or fixed point, '+=' and '-=' keys.
To improve the battery life the large LED display has been replaced by a Bowmar LED display with smaller characters and the addition of an array of plastic magnifying lenses in front to increase the perceived size, but which also reduced the viewing angle.
The example shown below which has been opened up (carried out on a non-working calculator in poor condition) has serial number 369396. The two SN75492N LED display driver chips are date coded 1972 week 23, so the calculator could not have been manufactured before that date, unless both of these chips had been replaced and a new casing used. It uses the Texas Instruments TMS1802 calculator integrated circuit, whereas the 'Variety C', below, has the same components but uses the TMS0103NC integrated circuit which replaced the the TMS1802NC.
Close up of the display showing the bubble magnifiers in front of the nine LED digits of the display, which is displaced towards the right.
Compare with the version with the display with larger LED digits and without magnifiers, shown above.
Sinclair Executive with small LED digits with magnifying lenses.
The front side of the circuit board showing the Texas Instruments TMS1802NC "calculator-on-a-chip", here date coded 1971 week 37. However, the calculator would have been manufactured much later than this, some time after 1972 week 23 printed on the two smaller SN75492N LED display driver chips.
The rear of the circuit board, with the manufacturer "Bowmar" printed on the rear of the LED display.
Version 2, Variety C) Small-digits display aligned to right, Texas Instruments TMS0103NC integrated circuit
Integrated circuit is Texas Instruments TMS0103NC, LED display manufactured by Bowmar[3] with small LEDs and a magnifying lens array, 4 button cells, switch for floating decimal point or fixed point, '+=' and '-=' keys.
To improve the battery life the large LED display was replaced by a Bowmar LED display with smaller characters and the addition of an array of plastic magnifying lenses in front to increase the perceived size, but which also reduced the viewing angle. The number of button cells was also increased from three to four.
The example shown in the photographs below has serial number 26646. The TMS0103NC integrated circuit is date coded 1973 week 7, so the calculator could not have been manufactured before that date, unless the chip had been replaced and a new casing used. Note that the 'Variety B', above, has the same components but uses the TMS1802NC integrated circuit which was replaced by the TMS0103NC.
Sinclair Executive circuit board and part of the casing from a calculator with casing & keyboard type 1 and Texas Instruments TMS0103NC chip. Distinctive features, from top to bottom:
Version 3) Using 4 button cells, General Instruments Microelectronic (GIM) C550 calculator integrated circuit, and modern calculator keyboard
This version appears to have been designated the Sinclair Executive Mk III (Mk 3), and also appears to have had a considerable production run.
Integrated circuit is GIM (General Instrument Microelectronics) C550, display manufactured by Bowmar with small LEDs[3] and magnifying lens array, 4 button cells.
The cut-away casing type 2 Sinclair Executive on display at the Science Museum in London, shown in the photograph below, uses an integrated circuit from a different manufacturer, the GIM (General Instrument Microelectronics) C550, rather than the Texas Instruments chips used in the casing type 1 models described above.
This casing type 2 model is known in two versions - with the high/low brightness switch the opposite way round, as shown in the photographs below.
Sinclair Executive with casing & keyboard type 2 and "L"/"H" (Low/High) display brightness switch.
Sinclair Executive with casing & keyboard type 2 and "H"/"L" (High/Low) display brightness switch.
A Sinclair Executive with casing & keyboard type 2, electronics version 5, cut to show inside. Photographed at the Science Museum, London.
Distinctive features are:
Circuit boards for Version 3 (Mk 3) of the Sinclair Executive calculator
Photograph of one half of a circuit board sheet showing 4 individual boards for manufacturing Sinclair Executive Mk 3 calculators (the version 3). The full board contains the basis for 8 calculators, but due to size constraints here only 4 of the individual Executive boards are shown. This large board would be chopped up into the individual boards used in the calculators. It is dated 2nd July 1973. Compare these bare boards with the populated boards in the photographs above and below.
Journalists' views of the Sinclair Executive
'Which?'
In December 1973 the British consumer journal "Which" tested a selection of small calculators, unusually including two types of Sinclair Executive calculator[8]; both a version 2 (which it refers as the Mk II) and a version 3 (which it refers as the Mk III).
During their tests both their "Sinclair Executives (Mk II and Mk III) broke down; further samples proved satisfactory." Sinclair was renowned for its "no-quibble" replacement policy if there were problems with its products, though problems were not uncommon but it did give a 5 year guarantee.
The Sinclair Executive was found to have "the narrowest viewing angle (only 20 degrees) of the models tested, and was the one our users found most difficult to read. It was also the only one you had to hold straight in front of you — a further restriction. It's clear that designing the Sinclair to be slim enough to fit unnoticed into a breast pocket has caused problems in other respects." A narrow viewing angle is a property of the lenses used in front of the LED characters to magnify the small LEDs, which are used in order to reduce power consumption. This is a feature of a lot of LED calculators of the time which use these lenses.
Their tests gave a battery life of 4� hours for the Mk II (version 2) and 9� hours for the Mk III (version 3).
'New Scientist'
Nicholas Val�ry in a postscript to an article in the journal New Scientist on choosing an electronic calculator gave a telling
description of his experience with the Sinclair Executive[9]:
"Nine months ago, I bought a Sinclair Executive and have carried it
everywhere. Being just ⅜ in thick and weighing only 2� oz, it is not only the smallest electronic calculator in the world but also the only true pocket-sized machine. I am now on my third model, the first two
being thoroughly unreliable. Sinclair, fortunately, gives a five year guarantee.
The chip on the latest model appears to have been replaced by a General Instrument design which allows full flow
arithmetic. The display has also been changed, for the worse, and the two, four and six decimal switch has been ditched. So, alas, has the invaluable last entry clear key. Admittedly the price has come down from
�79 to �59. Good value? Well hardly.... But then it's not so much a professional calculator but more a piece of personal jewellery. And as such it continues to delight and amaze people".
'Wireless World'
The comment above that the Executive "continues to delight and amaze people" is reinforced by a report in the journal Wireless World in August 1973 of a reversal in the normal direction of world trade[10]:
"Calculators for Japan. Sinclair Radionics are to export more than 80,000 "Executive" pocket
calculators to Japan. The order is valued at �750,000 [GBP, about US$1,875,000] — a remarkable feat."
Casing style 1
Casing style 2
Casing style 3
Sinclair Executive Memory
Similar to the Sinclair Executive above, but also has a memory.
Display is 8 digits, red LED.
4-function, memory.
5.4v (4x mercury button cells).
Main integrated circuit: TI TMS0132NC.
56 mm x 138 mm x 9 mm (2.2" x 5.5" x 0.4").
Introduced 1973.
Made in England.
Is found in at least three styles of casing, as shown above.
Click here to see an advertising brochure for the Sinclair Executive Memory (pdf format).
Side view showing how thin this machine is.
Sinclair Executive Memory circuit board and part of the casing. Distinctive features, from top to bottom:
Sinclair Executive Memory in presentation case, with soft pouch and instructions.
Black Sinclair Executive Memory
Frank Fr�hn has kindly provided this photograph of an all-black Executive Memory. The only marking on it is the logo, which appears to be that of CBS (Columbia Broadcasting System), so it was probably supplied by CBS as a promotional item.
Click here for more Sinclair calculators on the associated Vintage British calculators site.
Also see the article Clive Sinclair and the pocket calculator in the Collecting Calculators section of this site.
References:
Hand-held Calculators
Vintage Calculators
Text & photographs copyright, except where stated otherwise, © Nigel Tout 2000-2026.
