In the mid-1970s, as pocket calculators began transitioning from luxury items to everyday tools, the Commodore Minuteman 3 Metric calculator stood out for its blend of practicality and thoughtful design. Manufactured in England in 1974, this calculator was built for durability and precision, packaged in a compact, palm-sized white plastic case with a matte finish. The upper half features a sleek black metallic panel adorned with silver-printed conversion guides and a raised CBM logo, while the red, slightly tilted LED display filter provides a crisp, bright readout despite lacking a lens. The keyboard’s red, white, and blue keys, squishy but positive enough to press, are framed by a black plastic panel that ties the design together. A conversion guide printed on the back panel can be used for quick reference.
At approximately 75mm wide, 105mm tall, and 27mm deep, and weighing just 96g (without batteries), the Minuteman 3 was portable yet sturdy. It ran on a 6.0V DC rechargeable battery, compatible with an external adapter via a top-mounted 3mm positive-center socket. The 8-digit LED display, supplemented by a ninth digit for error flags and negative signs, prioritized clarity over complexity. Functionally, it offered basic arithmetic operations, percentage calculations, a four-function memory, and specialized unit conversions—a feature that, while innovative, came with a learning curve due to the unconventional use of the (CE) key.
Here is another version of the Minuteman 3 Metric calculator:
Logic
- (C) cancels a numerical entry; pressing it twice clears the entire calculation.
- (CE) (Conversion Entry) acts as a function key to toggle conversion modes:
- Imperial to metric: Enter a value (e.g.,
10), press (CE)(0) to convert inches to centimeters. - Metric to imperial: Enter a value (e.g.,
3048), press (CE)(M)(1) to convert meters to feet. - Areas: Enter a value (e.g.,
100), press (CE)(=)(7) to convert square miles to square kilometers. - Volumes: Enter a value (e.g.,
1000), press (CE)(M)(-)(4) to convert cubic meters to cubic yards.
Under the hood, the Minuteman 3 relied on a MOS MCS 2521A006 CPU (dated week 1 of 1974) alongside Texas Instruments and ITT display driver chips. Its internals included a nine-digit red LED display, a transformer, and a mix of diodes, resistors, and capacitors. The main CPU board, soldered to the keyboard assembly, connected to the display via a 17-way ribbon cable. Disassembly requires removing two screws on the back panel, revealing a floating battery pack and components secured to the front casing.
While the calculator’s conversion features were ambitious, the non-intuitive (CE) key and layered input sequences could frustrate casual users. Yet, its robust construction, clear display, and niche focus on unit conversions made it a practical tool for engineers, students, or professionals needing quick metric-imperial translations.
