Building a Paper Tape Reader To Read Bytes
David Lovett has been steadily developing his custom 1-bit vacuum tube-based computer, known as the UEVTC, at Usagi Electric. This project, inspired by the Motorola MC14500B 1-bit industrial control unit (ICU), has made significant progress over the years, surpassing the original in many ways. However, one major challenge remains: finding a faster method to input data into the computer than manually toggling switches. David decided that a punched paper tape reader would be an ideal solution, but creating one proved to be more complex than expected.
Punched paper tape, a historical data storage medium, was commonly used alongside punched cards and magnetic tape. Despite its low bit density, it has the advantage of longevity if acid-free paper or mylar is used. The key challenge in building a tape reader lies in accurately detecting the holes in the tape. Options include mechanical or optical detection, but mechanical readers were dismissed due to the potential for tape wear. Instead, David turned to the optical approach, drawing inspiration from the photodiodes used in the Bendix G-15’s tape reader, a 1950s computer he happens to own.
The photodiodes in the Bendix G-15 are 1.8 mm in diameter and align perfectly with the holes in the paper tape, making them a suitable choice for David’s design. After experimenting with various approaches using photodetector tubes, he settled on a setup that uses these small photodiodes. He then built a proof-of-concept on a breadboard, incorporating incandescent bulbs and a single photodiode to confirm that a valid logic signal could be produced when the tape holes were correctly detected.
This successful test has paved the way for the construction of a full-scale tape reader, which David plans to feature in his upcoming videos. The project highlights the complexity and ingenuity required to integrate vintage technology with modern electronics, demonstrating how even seemingly simple tasks can evolve into sophisticated engineering challenges.
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