fregacmols/RotaryCell: A portable, battery-powered cellular conversion for classic rotary telephones—fully reversible, with no modification to the original phone. · GitHub

RotaryCell converts a traditional rotary telephone into a self-contained, battery-powered, portable cellular telephone without modifying the original telephone.

The design continues to use the original handset, rotary dial, switch-hook, mechanical ringer, network block, and existing jacks. The added electronics mount reversibly inside the case; no original telephone parts need to be drilled, cut, or permanently altered.

The working prototype can be carried and operated away from a fixed telephone connection, making the original desk telephone usable at meetings, demonstrations, or anywhere compatible cellular service is available.

This is a working engineering archive rather than a finished construction release. The hand-wired prototype operates, while the first integrated PCBs are currently awaiting assembly and validation.

The complete point-to-point wiring and component reference is the primary starting point for recreating the proven hand-wired prototype:

Open or download the full-resolution printable PDF. It covers the LilyGO, protected 21700, passive audio components, AG1171, GPIO connections, and original Model 500 circuitry without using the new PCBs. A hand-wired installation fits inside the telephone, but arranging and insulating all of the loose components and wiring is challenging; expect repeated dry-fitting and careful routing.

This repository records the project as it stood on August 28, 2026:

• Firmware v0.10.4 is the current prototype-tested software.
• The Audio and Reset A4 PCB was ordered from JLCPCB on August 27, 2026.
• The AG1171 Carrier Through-Hole PCB was ordered from JLCPCB on August 28, 2026.
• Both ordered PCB packages are archived exactly as submitted and have not yet been validated as assembled boards.

See STATUS.md for the distinction between tested prototype behavior and hardware awaiting validation.

• A LilyGO T-A7670G-S3 Standard board supplies the ESP32-S3 controller, A7670 cellular modem, battery charging, and cellular audio interface.
• A Silvertel AG1171 subscriber-line interface operates the telephone line circuitry, senses the switch-hook, and drives the mechanical ringer.
• The Audio and Reset A4 PCB provides adjustable transmit/receive audio conditioning and a hardware power-cycle circuit for recovery when software-only modem reset is insufficient.
• A single protected 21700 cell connects to the LilyGO battery pads through a harness in place of the original 18650 holder and directly supplies the AG1171 carrier VPWR input.

The telephone’s RJ11 line jack is used only to deliver regulated 5 V to the LilyGO charging input on the designated pins. It does not power the AG1171 directly and is not used as a telephone-line interface.

For a hand-wired build, start with the complete prototype wiring reference. For the newer PCB implementation, continue with STATUS.md, current hardware wiring, the master BOM, the assembly guide, and BUILDING.md.

• Rotary pulse dialing and switch-hook detection
• Incoming and outgoing cellular calls
• Physical bell ringing through the AG1171
• North American dial, reorder, and receiver-off-hook warning tones
• Bidirectional handset audio with adjustable levels
• Battery monitoring
• USB diagnostics and a temporary maintenance Wi-Fi dashboard
• Browser/USB AT-command terminal and persistent event log
• Cellular-network clock synchronization and application OTA updates

Dial service code 0000 starts maintenance Wi-Fi. Service code 9999 performs modem diagnostics and software recovery, but it did not recover the field-observed modem lockup described in STATUS.md.

• Never connect prototype Tip/Ring wiring or the repurposed charging jack to the public telephone network or energized premises telephone wiring.
• Clearly label the charging jack and verify its regulated voltage, polarity, pin assignment, and protection before use.
• Lithium-ion cells require suitable protection, charging, fusing, insulation, and mechanical restraint.
• The maintenance access point uses the development password rotarycell. Change WIFI_AP_PASSWORD in Config.h before use around untrusted people.
• The August 2026 PCB files are as ordered, not yet production-tested. Create a new revision rather than silently replacing an as-ordered package.

This is a public engineering and development archive. It is intended to preserve a durable, reproducible baseline, make the working hand-wired prototype available to other builders, and document progress toward a more integrated implementation.

The repository should not be mistaken for a finished construction kit or production release. Files under hardware/audio-reset-a4 and hardware/ag1171-carrier-through-hole record the exact board candidates ordered in August 2026; their assembled operation has not yet been validated. Tested behavior, known failures, and remaining documentation gaps are tracked in STATUS.md.

Code and original documentation in this repository are licensed under the MIT License. Third-party datasheets, vendor names, trademarks, and historical telephone designs remain the property of their respective owners.