Busicom "HANDY-LE" LE-120A

When Nippon Calculating Machine Corp of Japan, better known under their brand Busicom, explored their options of using LSI (Large Scale Integration) PMOS (p-channel Metal–oxide Semiconductor) technology for their electronic calculators, they teamed up with two American Semiconductor companies:

The cooperation with Intel resulted in the famous 4004, the World's first microprocessor, while Mostek's MK6010 is recognized as the World's first "single-chip calculator circuit".

Busicom's earliest electronic desktop calculators like the Model 161 (July 1966) and Model 141 (November 1967) were using a mix of SSI (Small Scale Integration) DTL (Diode-Transistor Logic), discrete transistors and diodes plus a magnetic core memory and it was the Model 120-DA introduced in April 1969 paving the way for the MK6010 design. The Model 120-DA was Busicom's first design using JMOS technology, mostly simple SSI devices for the logic but a few MSI (Medium Scale Integration) chips sprinkled in for both the Serial BCD (Binary Coded Decimal) Adder and the 48-bit Shift Registers for number storage. The successor of the Model 120-DA - introduced in January 1970 and consequently named Model 120-DB - changed the decimal point switch from [0-1-2-3] to [0-2-3-4]. Within a few months, Busicom was able to squeeze the electronics of the Model 120-DB into a housing with roughly half the volume and the Model 120-DM, also known as Busicom Junior, was born. Its main electronics were distributed on two large printed circuit boards (PCBs), containing 22 Integrated Circuits (ICs) in JMOS technology from Mitsubishi, NEC and Toshiba, complemented by around 220 discrete diodes and more than 100 resistors. It was the very same logic design that Busicom handed in Summer 1970 over to Mostek in Dallas, Texas; tasking the young company to integrate the circuitry into a single chip manufactured in high-threshold, p-channel MOS technology, operating from −12 Volts and −24 Volts.
While Mostek originally estimated to be able to realize conversion of the circuitry from a mix of JMOS and Diode-Resistor logic to PMOS technology and provide the layout of the chip within a couple of weeks, the actual work took about three months and the first working samples of the MK6010 were manufactured in November 1970. The resulting chip is integrating more than 2,100 transistors in 360 gates plus 160 flip-flops and measures around 185 mils * 170 mils / 4.7 mm * 4.3 mm.

When Busicom finally introduced early in 1971 the second generation of the portable desktop calculator Model 120-DM or Junior, it looked at first glance identical to its predecessor but its housing is actually 0.5” (12 mm) slimmer due to the reduction from two large Logic-PCBs and the smaller Display-PCB to just one Main-PCB containing the MK6010 "single-chip calculator chip", the discrete keyboard interface, 12 low-voltage VF-Display tubes and the necessary transistors for the display. Neither the keyboard nor the power supply changed between the two iterations of the calculator.

Busicom realized the potential of applying Mostek's ion-implantation process to the chip design and the resulting MK6010L "low-voltage, low-power" version of the MK6010 allowed the design of this incredible LE-120A calculator.

The Busicom LE-120A is today credited with many "Firsts":

Unfortunately, the many "Firsts" did not immediately translate into a massive sales success as observed with the later Hewlett Packard HP-35 using the same playbook. The LE-120A with its unique metal-cast body and its 12-digit LED display was very expensive and when Busicom started lowering the manufacturing costs by switching to a plastic body (LE-120S), changing the display size to 10-digits (LE-100A) and even 8-digits (LE-80S, LE-80A, LE-80B); competition was already up and running and leading to the start of the "Calculator War" – with Busicom being in 1974 the first casualty of a Japanese calculator manufacturer.

As of May 2026, we have identified 7 members of the Handy series manufactured by Nippon Calculating Machine Corp. and sold under its Busicom brand, as well as white-labeled versions for National Cash Register (NCR) and Privileg (Quelle):

In April 2026, Javier Orbaiceta shared with the Datamath Calculator Museum photographs of a fully working Busicom LE-120A HANDY that his father had purchased in Tokyo in 1971. Thank you!

Dismantling the featured Busicom "LE HANDY" LE-120A calculator manufactured in June 1971 in Japan reveals a masterpiece of engineering. The calculator's two-piece housing and detachable battery compartment are made of die-cast metal and contain three double-sided printed circuit boards (PCBs):

Calculating Unit: Sprague Electric Company helped move ion-implantation from experimental research into practical semiconductor manufacturing. In collaboration with Mostek, the process was refined for large-scale production of integrated circuits. Ion-implantation provided major advantages over older diffusion techniques by allowing precise control of dopants within silicon wafers. This precision enabled lower transistor threshold levels and reduced operating voltages, leading to lower power consumption and improved reliability. Nippon Calculating Machine Corp., on the other hand, had developed its Busicom Model 120-DM using JMOS technology and requested that the MK6010 be designed with a high-threshold process so the existing power supply and display circuitry could be reused.

The MK6010L used in the Busicom LE-120A is a variant of Mostek's MK6010, the world's first single-chip calculator circuit, manufactured with an ion-implantation process to reduce the required supply voltages from −12 V and −24 V to −9 V and −16 V:

Display: The featured Busicom LE-120A calculator, manufactured in June 1971, utilizes a 12-digit LED display assembly composed of twelve Monsanto MAN-3A seven-segment LED displays housed in flat-pack packages and manually soldered onto the double-sided Display-PCB. In addition to the numeric displays, the Display-PCB incorporates two indicator LEDs for negative values and low-battery status, as well as the associated current-limiting resistors. The Display-PCB is connected to the Main-PCB of the LE-120A by 24 short wires that are soldered directly between the two boards.

Display Driver: The MK6010L single-chip calculator circuit and its companion, the MK6010, originated from the architecture of the Busicom Model 120-DM desktop calculator, better known as the Busicom Junior. This calculator was built from 22 JMOS integrated circuits and featured low-voltage vacuum fluorescent displays (VFDs). To facilitate display interfacing, the designers chose a negative-logic configuration for the digit and segment outputs, enabling straightforward drive circuitry based on discrete transistors. This design philosophy was carried over to the MK6010 and MK6010L. Consequently, both devices provide negative-logic digit and segment outputs, requiring PNP bipolar junction transistors (BJTs) to drive the anodes and cathodes of a 12-digit LED display. The necessary discrete transistors are arranged on the Main-PCB of the LE-120A calculator.

Clock: The MK6010L single-chip calculator circuit used in the Busicom LE-120A operates from a two-phase clock running at approximately 20 to 25 kHz. The clock signal is generated by an astable multivibrator consisting of four BJTs, which is located on the calculator's Main-PCB.

Power Supply: The Busicom LE-120A calculator is powered by four disposable AA-size 1.5 V alkaline batteries. A simple DC/DC converter generates two additional negative supply voltages required by the MK6010L calculator chip:

Calculating the power consumption at 6 Volts for the Busicom LE-120A results in about 500 mW displaying a '0' and about 1,000 mW with all segments illuminated.

Keyboard: The keyboard assembly of the Busicom LE-120A uses 17 plastic keys pushing individual metal fingers soldered on a double-sided PCB against large, gold-plated contacts mounted on the PCB and a 3-position sliding switch to select the position of the decimal point. The compact Keyboard-PCB incorporates 13 discrete diodes to convert the numeric key inputs into the BCD-encoding required by the MK6010L calculator chip. In addition, the board contains a number of discrete resistors and capacitors used for the keyboard interface circuitry.

Following the bankruptcy of Busicom in Japan in 1974, the rights to the Busicom trade name were acquired by the British business machine company Broughtons & Co. (Bristol) Ltd. The company subsequently continued to market and service business machines under the Busicom brand in the United Kingdom.

If you have additions to the above article please email: joerg@datamath.org.

© Joerg Woerner, May 26, 2026. No reprints without written permission.