Bowmar 901D / TEN

Bowmar 901D / TEN

The 901D, also known as TEN, was an early vintage calculator manufactured by the Bowmar Corporation in 1971. It had basic functions to add, subtract, divide, and multiply, and memory facility to hold a constant.

Due to the immense success of this model, the firm managed to sell many re-branded versions of this calculator such as Commodore C110, and Craig 4501.

It was a significant leap in engineering technology resulting in full-size capability within a pocket-sized unit.

The circuit design is based around the Large Scale Integration (LSI) calculator chip TMS 0107 by Texas Instruments. It used the most advanced components of that time, which were, Light Emitting Diode (LED) display, and click-action keyboard.

To drive the 10-digit LED display, the design utilised a pair of SN75491 chips, and a pair of SN75492 chips. The display had a full-floating decimal point, with additional display indicators to denote power ON, low battery, overflow, minus, and error.

Six nickel-cadmium rechargeable batteries permitted five hours continuous use even when AC mains power was not available. A time-out feature to extend the battery operation involved blacking out the display after15 seconds from last key press.

An Executive’s Calculator

901D Calculator View

The “Bowmar” was one of the first pocket calculators for the discerning businessperson. We nicknamed this one Warren Buffett's calculator, as it is very likely, he bought the first pocket calculator. We figured that with all the money he has, he is bound to get one with ten digits!

The keyboard has a very nice subtle colour scheme of teal and blue which is hard to capture on a camera, but this image captures it just right.

An Item of Engineering Significance

Vintage electronic calculator designs are very interesting, and I love looking at the 70s digital electronics and the engineering that went behind them.

Texas Instrument pioneered some of the best logic chips, which are excellent for learning purposes even today. The LED display drivers are very interesting to see as I have used nearly all of the 74 series of TTL logic chips. Students in education often do not realise that all of these early logic chips had commercial applications.

Klixon Switch Technology

Klixon Switch Technology

The keys are generously large, and produce a clicking sound as well as providing a positive feel to the click. Modern calculators with their rubber membrane keyboards compare poorly to the design of this unit.

These switches are one of the most reliable components ever used on a pocket calculator, and when they were building the Space Shuttle cockpit control, they used variants of these.


Manufactured in1971
Charge Time7 hours
Adapter voltage115 V / 230 V
Adapter50 Hz / 60 Hz, 8 W
Adapter Out14 V AC, 140 mA
Adapter ModelB901-077-4
Low battery IndicatorL
Overflow IndicatorO
Error IndicatorE
K SwitchConstant value
Temperature0 °C to 50 °C
Relative Humidity95 %
Weight10.2 ounces
Size5.3 inch × 3.1 inch × 1.4 inch
Cost in 1971£120.00

This Article Continues...

Bowmar 901D / TEN
Bowmar 901D / TEN Inside View
Bowmar 901D PCB & Components
Schematic & TMS0107 NC Calculator Chip
Bowmar LED Display Unit
Klixon Keyboard
Keyboard Matrix
Bowmar 901D / TEN Power Supply
Bowmar 901D Adapter & Accessories
Contact Pete
Bowmar 901D / TEN Manual