Bring a vintage rotary phone back to life on a modern telephone exchange. ErdTone converts pulse dialing to DTMF tones in real time, running on a single ATtiny85 hidden inside the phone with no visible modifications.

Built around the Siemens W48 with full support for the Earth key (German: Erdtaste), it goes beyond basic pulse conversion: last-number redial, speed dial with pause support, * and # via hold gestures, auto-dial on pickup, and adjustable DTMF timing for compatibility with older exchanges. Everything is configured directly from the rotary dial - no computer needed after flashing.

This project is a fork of the rotarydial firmware, extended with auto-dial, Earth key redial, Erdtaste speed dial, and configurable DTMF and menu timing.

Needed build parts:

• 1x ATTINY85V
• 1x 78L05 5V regulator
• 1x 4 MHz crystal
• 1x 680 ohm resistor
• 1x 27 V 1 W zener diode
• 2x 22 pF ceramic capacitors
• 4x 0.1 uF (100 nF) ceramic capacitors
• 1x 4 position terminal
• 1x 1N4007 diode

A Fritzing layout is included with a compact footprint designed to fit inside the W48 housing without modifications. See ErdTone.fzz.

• Disconnect the rotary dial from the phone circuitry (no parallel connection).
• Connect it to the converter:
  • Pulse contact (normally closed, opens for each pulse) → PULSE
  • Dial contact (closes while dial is moving) → DIAL

• Disconnect Earth key from the phone circuitry.
• Connect it to the same DIAL line used by the rotary dial:
  • One side → DIAL
  • Other side → GND
• Pressing Earth key must pull DIAL to GND.

• Connect power after the hookswitch:
  • Line + → VCC
  • Line - → GND
• Device must be off when on-hook.
• Optional: add a diode (e.g. 1N4007) between line + (anode) and VCC (cathode) for protection.

Install tools:

sudo apt update && sudo apt install gcc-avr avr-libc binutils-avr avrdude make

Build, flash and set fuses:

make install              # default 4 MHz (external crystal)
make CLOCK_MODE=8 install # 8 MHz (internal oscillator, not accurate enough, only for testing)

Erase EEPROM (clears all stored numbers and settings):

make erase

Dial a number and release. The firmware sends the matching DTMF tone. If you pause for more than ~3 seconds between digits, a pause is automatically inserted into the redial memory at that point. If you wait more than ~6 seconds, two pauses are inserted. This allows redial to correctly replay numbers that require waiting for an automated phone menu or office switchboard to respond.

Earth key is a quick redial shortcut.

• Brief press and release: redial the last number
• Hold until first beep: dial the stored Earth key speed dial number
• Hold until second beep: program the Earth key speed dial number
• If pressed during startup, auto-dial calling is canceled for that cycle.

Use the rotary dial for special functions:

• Dial a digit and hold until the first beep, then release:
  • 1 = send *
  • 2 = send #
  • 3 = redial last number
  • 4 to 9, 0 = dial the stored speed-dial number
• Keep holding until the second beep:
  • 4 to 9, 0 = enter programming mode for that slot
  • 1 = cycle DTMF tone duration (80ms / 200ms).
  • 2 = cycle menu hold time (1s / 2s).
  • 3 = enter auto-dial setup

1. Dial the slot digit.
2. Keep holding until the second beep, then release.
3. Enter the number one digit at a time.
4. Release after each digit.
5. Hang up when done.

Notes:

• Each digit is written to EEPROM immediately.
• * and # or pause can be stored by dialing 1, 2 or 3, holding until the first beep, then releasing.
• The Earth key has its own dedicated speed dial slot programmed via Earth key hold to second beep.
• During programming, you can copy into the current slot: press Earth key to load the last dialed number, or dial a source speed-dial slot (4 to 9, 0) and hold until the first beep.
• Hold 0 until the second beep to clear the current slot (works for Earth key slot too).

Auto-dial is stored in one of the speed-dial slots and auto-dials after power-up if enabled.

Set it:

1. Dial 3.
2. Hold until the second beep, then release.
3. Dial the speed-dial slot, or press Earth key to use the Earth key speed dial number as the auto-dial.
4. The auto-dial is now saved with the default 5 second delay.
5. Optional: dial a delay digit from 0 to 9 to override it.

Clear it:

1. Dial 3.
2. Keep holding until the second beep, then release.
3. Hold 0 until the second beep, then release to clear it.

Pressing Earth key or starting to dial during startup cancels the auto-dial call for that cycle.

Service codes are entered during a normal call using the rotary hold menu for * and #.

Dial lock codes with a 3-digit PIN PPP using digits only.

• If no PIN is set, **PPP sets the PIN and locks programming (3 digits from 0-9).
• If no PIN is set, *#PPP sets the PIN and locks programming and digits 4–9 and 0 become direct speed-dial keys.
• If any lock is active, **PPP or *#PPP with the correct PIN permanently unlocks and clears the PIN.
• If any lock is active, ##PPP with the correct PIN temporarily unlocks the phone for the current call only.

When programming-locked:

• Normal dialing still works
• Programming, settings changes and factory reset are blocked

When fully locked:

• Normal dialing is blocked
• Digits 4–9 and 0 directly dial their assigned speed-dial slot (no hold required)
• Erdtaste directly dials the Erdtaste speed-dial slot
• Programming, settings changes and factory reset are blocked

To unlock from a fully locked phone:

Dial * or # twice via L1 hold (any combination: **, ##, #, #) as the very first two digits after pickup - this arms PIN entry mode Then dial the three PIN digits directly - these feed the unlock sequence without dialing any speed-dial slots Use **PPP or *#PPP to permanently unlock, or ##PPP to unlock for the current call only

Dial *#0#* to erase all stored numbers, speed dial slots, auto-dial config, and reset DTMF duration and menu hold time to defaults. Blocked when locked.

ErdTone builds on the work of Boris Cherkasskiy who created the original firmware in 2011, Arnie Weber who reworked the hardware in 2015, and Matthew Millman who cleaned up the implementation in 2018.