RotaryCell: turn a real rotary phone into a cellular handset
RotaryCell preserves a working rotary telephone while adding a cellular modem, subscriber-line interface, battery, and open firmware; the hand-wired prototype is reproducible, but the integrated PCB path remains unvalidated.
The pick
RotaryCell turns a traditional rotary telephone into a portable cellular phone. The original handset, dial, hookswitch, ringer, network block, and case stay in place; the added electronics sit inside the telephone and can be removed later. Hackaday published the project feature on September 5, 2026, inside this week's seven-day window. 1
Weekend verdict: RotaryCell is a strong advanced build for a careful hardware hacker who owns a suitable Model 500-type telephone and can tolerate point-to-point wiring. The working hand-wired prototype has demonstrated dialing, ringing, handset audio, and cellular calls. The cleaner two-PCB version is still waiting for assembly and validation, so a weekend builder should reproduce the proven hand-wired path rather than assume that the ordered boards are production-ready. 23
The project earns the pick because the old telephone remains a real telephone. RotaryCell supplies the missing subscriber line, modem, battery, audio conditioning, and control logic while preserving the original electromechanical interface. The result is a cellular handset that still rings a mechanical bell and places calls through a rotary dial. 12
Why it stands out
The project has two different hardware baselines, and the distinction matters before you order anything.
The working prototype is a hand-wired assembly built from a LILYGO T-A7670G-S3 Standard board, a Silvertel AG1171 subscriber-line interface, a protected 21700 lithium-ion cell, passive audio parts, and the original telephone circuitry. The repository supplies a full point-to-point wiring and component schematic for this version. 24
The newer integrated version uses an Audio and Reset A4 board plus an AG1171 Carrier Through-Hole board. The repository records the exact files sent for fabrication in August 2026, but both assembled boards were still awaiting validation at the documented baseline. 23
That split gives the project an unusually honest build path. A builder can start with the tested prototype wiring while still inspecting the PCB implementation. The tradeoff is packaging: loose components, insulated joints, battery restraint, and wiring around the dial and hookswitch take more time than the schematic suggests.
RotaryCell's feature list is substantial. The firmware handles rotary pulse dialing, switch-hook detection, incoming and outgoing calls, bell ringing, North American dial and warning tones, bidirectional handset audio, battery monitoring, USB diagnostics, a temporary maintenance Wi-Fi dashboard, an AT-command terminal, event logging, and OTA application updates. 2
The repository also records a real limitation. During an August 25 field test, the modem remained registered but stopped making and receiving calls after about an hour. Cycling the LILYGO board's physical power switch restored service. Firmware v0.10.4 did not yet drive the new hardware reset input intended to address this failure. 3
BOM and budget
The table below follows the project's August 28 master BOM. Prices are planning allowances for one prototype, not prices published by the project. The official LILYGO listing showed the T-SIM/T-A Standard Series at $56.34 when checked; the final cost still depends on the selected modem variant, shipping, cellular service, telephone, and the missing installation details. 56
Major assemblies
| Qty | Part or assembly | Specification | Role / status | Planning cost |
|---|---|---|---|---|
| 1 | Rotary telephone | Western Electric / AT&T Model 500-type set with working dial, hookswitch, network block, handset, and ringer | Existing phone; unmodified | $20–80 used |
| 1 | Controller and modem | LILYGO T-A7670G-S3 Standard with A7670G modem | Prototype-tested | $56–65 |
| 1 | Cellular antenna | Antenna compatible with the LILYGO modem and its feed | Usually supplied with the board; verify the package | $5–15 |
| 1 | SIM and service | Voice-capable SIM and service compatible with the modem and local network | Installation-specific recurring cost | $5–20 to start |
| 1 | Subscriber-line interface | Silvertel AG1171-S | Generates the telephone-line behavior, senses the hookswitch, and drives the ringer | $25–45 |
| 1 | Battery | Protected single 21700 Li-ion cell; Samsung 58E was used in the prototype | Feeds the LILYGO battery pads and AG1171 carrier power | $8–15 |
| 1 | Audio and Reset A4 PCB | Ordered PCB assembly from the repository's manufacturing package | Candidate hardware; awaiting validation | $5–20 assembled, excluding shipping |
| 1 | AG1171 Carrier Through-Hole PCB | Ordered carrier board from the repository's manufacturing package | Candidate hardware; awaiting validation | $5–20 assembled, excluding shipping |
| 1 | Charging source | Regulated 5 V input through the designated RJ11 charging path | Charges the LILYGO board; does not feed AG1171 power directly | $5–15 |
The project therefore lands around $134–295 before tools, shipping, and cellular service, with the broad range driven by the telephone, modem availability, and board-assembly choice. A hand-wired prototype can avoid ordering the two new PCBs, while a PCB build still needs a telephone-specific mechanical plan.
The LILYGO listing is a useful price anchor, but the repository's own BOM remains the purchase authority for the exact build. The repository leaves the final protected-cell choice, inline fuse and connector, regulated 5 V source, RJ11 charging harness, physical power-switch attachment, and printed mechanical carrier open. 56
Audio and reset A4 parts
The A4 assembly contains the following named parts. The group costs below are planning allowances for the component set; the board house and assembly quote are separate.
| Qty | Reference(s) | Part | Planning cost |
|---|---|---|---|
| 1 | C1 | 1 uF, 0603 capacitor | $0.05–0.20 |
| 1 | C2 | 10 nF, 0603 capacitor | $0.05–0.20 |
| 1 | C3 | 22 nF, 0603 capacitor | $0.05–0.20 |
| 1 | C4 | 220 nF, 0603 capacitor | $0.05–0.20 |
| 2 | C5, C7 | 100 nF, 0603 capacitors | $0.05–0.20 |
| 1 | C6 | 4.7 uF, 0805 capacitor | $0.05–0.30 |
| 1 | J1 | JST B4B-XH-A, 4-pin header | $0.10–0.50 |
| 1 | J2 | JST B3B-XH-A, 3-pin header | $0.10–0.50 |
| 1 | U2 | JST B3B-XH-A, 3-pin header | $0.10–0.50 |
| 1 | Q1 | DMP1012USS-13 P-channel MOSFET, SO-8 | $0.20–1 |
| 2 | R1, R4 | 10 kOhm, 0603 resistors | $0.05–0.20 |
| 1 | R2 | 3.3 kOhm, 0603 resistor | $0.02–0.10 |
| 1 | R3 | 1 kOhm, 0603 resistor | $0.02–0.10 |
| 1 | R5 | 820 kOhm, 0603 resistor | $0.02–0.10 |
| 1 | R6 | 10 kOhm, 0603 resistor | $0.02–0.10 |
| 2 | R7, R9 | 100 kOhm, 0603 resistors | $0.05–0.20 |
| 1 | R8 | 1 kOhm, 0603 resistor | $0.02–0.10 |
| 1 | U1 | SN74LVC1G123DCTT one-shot, SM8 | $0.20–1 |
| 2 | SPK_LVL, MIC_LVL | Bourns 3362P-1-103LF, 10 kOhm trimmers | $1–4 |
The external prototype wiring also calls for a 1 uF film or bipolar capacitor between the LILYGO
MIC- terminal and common ground. The working drawing labels this part as the microphone AC reference; the A4 BOM's C6 is a different 4.7 uF reset-timing capacitor. 45AG1171 carrier parts and harness
| Qty | Part | Specification / use | Planning cost |
|---|---|---|---|
| 1 | U1 | Silvertel AG1171-S | $25–45 |
| 1 | C1 | Nichicon UPW1C471MPD, 470 uF, 16 V | $0.20–1 |
| 1 | U4 | Vishay K104K10X7RF5UH5, 100 nF | $0.10–0.50 |
| 1 | D2 | 1N4148, DO-35 | $0.05–0.20 |
| 3 | R1–R3 | 1 kOhm axial DIN0207 resistors | $0.10–0.50 |
| 2 | CN1, CN2 | JST B2B-XH-A, 2-pin vertical headers | $0.20–1 |
| 1 | U2 | JST B3B-XH-A, 3-pin audio header | $0.10–0.50 |
| 1 | U3 | JST B4B-XH-A, 4-pin logic header | $0.10–0.50 |
| 2 | — | JST XHP-4 housings with crimp contacts | $1–4 |
| 3 | — | JST XHP-3 housings with crimp contacts | $1–4 |
| 2 | — | JST XHP-2 housings with crimp contacts | $1–3 |
| As required | — | Stranded hookup wire, heat-shrink, sleeving, strain relief | $5–15 |
| As required | — | Fuse / protection, insulated battery harness, connectors | $5–20 |
| As required | — | Nonconductive mounting hardware or printed carrier | $5–25 |
The connector quantities assume the straight-through harnesses described by the project. Check pin order against the PCB pin-1 marks before crimping. 5
Schematic and enclosure
The proven prototype has four electrical blocks:
- The LILYGO board supplies the ESP32-S3 controller, A7670 cellular modem, battery charging, and cellular audio interface.
- The AG1171 turns the added electronics into a telephone subscriber line. Its
TipandRingoutputs connect to the original telephone network block, while its control pins handle ringing, rotary pulse behavior, and switch-hook state. - The passive audio network conditions receive audio from the LILYGO before it reaches the AG1171, then conditions the AG1171 output before it returns to the LILYGO microphone input.
- The protected 21700 connects directly to the LILYGO battery pads and the AG1171 carrier's
VPWRinput. The regulated 5 V charging path goes to the LILYGO charging input and stays separate from AG1171VPWR. 47
The audio path is easy to miswire.
SPEK- stays floating because the LILYGO speaker output is bridge-tied. MIC- reaches common ground only through the external 1 uF film or bipolar capacitor. The receive path runs from SPEK+ through the passive network to AG1171 VIN; the transmit path runs from AG1171 VOUT through the passive network to LILYGO MIC+. 7The phone connection remains internal. AG1171
Ring(B) and Tip(A) connect to the Model 500 network block after the builder identifies the correct telephone-specific terminals. The repository explicitly leaves the exact network-block screw terminals for a later close-up and warns that the internal pair must never reach public telephone service or energized premises wiring. 7The enclosure problem is the original telephone case. Lay out the LILYGO board, battery, AG1171 interface, trimmers, and passive components before cutting wire. Keep every exposed joint insulated, give the battery and harness strain relief, and leave clearance for the dial, hookswitch, handset cord, and ringer. The prototype fits inside the telephone, but point-to-point wiring occupies more volume than the future PCBs. 4
Firmware and bring-up
The current firmware is v0.10.4, located at
firmware/current/ESP-RotaryCell_v0.10.4/ESP-RotaryCell_v0.10.4.ino. The documented toolchain is Arduino IDE or Arduino CLI with Espressif ESP32 Arduino core 3.3.11. Select ESP32S3 Dev Module, 16 MB flash, QIO flash mode, 240 MHz CPU, Hardware CDC and JTAG, USB CDC on boot enabled, PSRAM disabled, and the Minimal SPIFFS (Large APP with OTA) partition scheme. 8Use this order for the first bring-up:
- Build the sketch before connecting the telephone hardware. This separates an Arduino configuration problem from a wiring problem.
- Inspect the LILYGO board, AG1171 interface, battery harness, passive network, and every connector against the hand-wiring PDF.
- Verify battery polarity, insulation, fuse or protection, and the regulated 5 V charging path with a meter. The charging input goes to the LILYGO board; AG1171
VPWRreceives the protected cell voltage. - Power the controller and test the firmware's USB diagnostics before attaching the telephone's internal
TipandRingpair. - Check rotary pulse decoding, hookswitch state, dial tone, and bell ringing with the telephone open and the wiring visible.
- Set the receive and microphone trimmers with a call, then test incoming and outgoing calls.
- Run an endurance test. The documented field failure appeared after about an hour, so a short successful call is not enough to clear the modem path. 38
The prebuilt
.ino.bin file is an application-only OTA image. It is suitable for the project's web updater when a compatible OTA partition table already exists; it is not a complete factory-flash image. 8The maintenance Wi-Fi configuration includes a development credential in the public source. Replace that value in
Config.h before exposing the access point, and audit any fork or copied configuration before sharing it. The repository also documents service codes for maintenance and modem diagnostics; treat those functions as a development interface until the reset behavior has been tested on the hardware you built. 2Reproduction difficulty: advanced
Rating: Advanced. The firmware setup is manageable for an embedded developer. The hard part is combining an old telephone's unknown internal wiring with lithium-ion power, cellular service, analog audio, subscriber-line signaling, and a dense hand-wired package.
Five jobs drive the rating:
- Telephone identification: The build needs a Model 500-type telephone with a functional dial, hookswitch, ringer, handset, and network block. The exact Tip/Ring terminals still need to be established on the particular telephone.
- Power safety: The protected 21700 has to be restrained, insulated, fused or otherwise protected, and connected with the correct polarity. The AG1171 receives raw protected-cell voltage, while the LILYGO uses a separate 5 V charging path.
- Analog adjustment: The builder has to route bridge-tied audio correctly and adjust the receive and microphone levels without grounding the wrong LILYGO terminal.
- Mechanical packaging: The hand-wired version fits, but the builder must repeatedly dry-fit parts around moving telephone mechanisms and protect every loose joint.
- Incomplete recovery path: The project already documents a modem lockup and a reset circuit whose firmware control was still unfinished at the baseline date. A successful first call does not prove long-duration reliability. 37
The project is still weekend-sized when the goal is a working bench prototype. A realistic plan is one day for telephone inspection, wiring, firmware compilation, and power checks, followed by a second day for audio adjustment, call testing, and enclosure cleanup. Ordering and assembling the two candidate PCBs adds a separate manufacturing schedule.
Build or skip?
Build RotaryCell if you already have a suitable rotary telephone, access to a voice-capable cellular network, a meter and soldering tools, and patience for point-to-point wiring. Start with the repository's tested hand-wiring reference. Treat the A4 and AG1171 carrier files as an inspection and future packaging path until the boards have been assembled and validated. 23
Skip the build when the telephone will be used as an emergency-only lifeline, when the battery protection is unresolved, or when you need the device to work without a long-duration validation run. The original project itself frames the prototype as an engineering archive rather than a finished construction kit. 2
For this weekend, choose the hand-wired prototype, verify every power and telephone connection before inserting the AG1171, compile firmware v0.10.4 with the documented ESP32-S3 settings, and reserve time for an endurance test. The project gives you enough files to begin immediately, while its unvalidated PCB path and known modem failure give you equally clear reasons to keep the first build on the bench.
References
- 1Free Your Rotary Telephone From Its Wire
hackaday.com
- 2RotaryCell README
github.com
- 3RotaryCell project status
github.com
- 4RotaryCell complete prototype wiring
github.com
- 5RotaryCell master BOM
github.com
- 6LILYGO T-SIM / T-A Standard Series
lilygo.cc
- 7RotaryCell current hardware wiring
github.com
- 8RotaryCell build instructions
github.com
This story was produced automatically by a channel. One sentence is all it takes for Neodrop to keep producing for you.