Summary

A FeTAp 611 rotary-dial telephone, the iconic Deutsche Bundespost desk phone - converted into a standard USB HID keypad. Plug it into any computer with USB 1.1, no additional driver needed, install no software: just dial a digit on the rotary disc and the host sees a normal HID keypad keystroke.

The conversion is non-destructive on the dialing mechanics: the original nsa (off-normal) and nsi (pulse) contacts are reused as inputs to a small PIC18F2550 running a four-state pulse-counting state machine. The MCU then emits a USB HID keyboard report.

Demo

In the video the USB HID Phone is plugged into a Windows notebook running sipgate softphone software. sipgate accepts ordinary keypad / keyboard input to enter the number to call, so to it the rotary dial just looks like a regular numeric keypad. I dial my own cell phone number on the FeTAp 611 - and my cell phone actually starts ringing. Fun fact: It’s a Motorola RAZR foldable GSM phone playing the “The Beginning” SID tune (Jeroen Tel, 1987, Maniacs of Noise — from the HVSC; play it in the browser) as ring tone.

Background story

I originally built a first version of this project back in 2008 as a quick hack. Jan and myself had a beer and then we wondered about a mostly useless but still nerdy electronic project. So the idea was born to build a rotary-dial phone acting as a USB HID keyboard. The phone itself was purchased from ebay. While I am not anymore do designs using through hole components, I decided to re-do schematic and PCB for you as a simple learning example. And after all these years, still in 2026 all parts still can be bought, from e.g. Reichelt. On my You Tube video people asked for a deeper explanation, so this is the long-overdue revisit:

• a freshly drawn schematic and PCB in KiCad,
• a clean rotary-dial pulse-counting FSM in C,
• and an interactive simulator of that FSM you can play with right in the browser.

How the rotary dial works

A FeTAp 611 rotary dial gives us two digital signals:

• nsa (off-normal contact, MCU pin RB6): goes high while the dial is active (away from rest).
• nsi (pulse contact, MCU pin RB7): pulses low once per pulse during the spring-driven return.

The encoding is the historical loop-disconnect rule:

One pulse means digit “1”, two pulses mean “2”, … and ten pulses mean “0”.

Mechanically the nsi cam actually generates N + 2 pulses for digit N. The two extra pulses, the so-called Leerlaufimpulse, happen after nsa has already deasserted, while the cam over-rotates past the rest position. Because the FSM only counts nsi falling edges while nsa is high, those Leerlaufimpulse fall outside the gate and are ignored, exactly the same trick the original analog telephone exchange used. No software -2 compensation needed.

The state machine

A minimal four-state FSM, sampled every 10 ms by a Timer0 ISR, is enough:

State	Meaning
STATE_INIT	Wait for the dial to start (↑nsa).
STATE_DIAL	Armed, waiting for the next pulse low.
STATE_PULSE_LOW	A pulse is currently active (nsi low).
STATE_DONE	A digit is finalized: main loop sends HID report.

On ↑nsa we arm and reset pulseCount. Each ↓nsi increments pulseCount. When ↓nsa arrives and pulseCount > 0, the digit is finalized and the main loop emits a HID keyboard report containing pulseCount + HID_KP_BASE, then drops back to STATE_INIT.

If you want to see and step through the FSM live in your browser, head over to:

• FSM simulation: interactive timing diagram, scenarios for every digit (including “0” = ten pulses) and a manual-driven mode.

Hardware

A through-hole, low-parts-count board around a PIC18F2550 with full USB peripheral. USB bus-powered, 20 MHz crystal.

Connection points:

• USB-B (through-hole connector)
• NSI contact (PCB solder holes)
• NSA contact (PCB solder holes)
• Enter input (PCB solder holes for an external push button)

Note: the new KiCad schematic and PCB are a redesign of an old, verified breadboard. The redesigned PCB itself has not been re-verified in real life yet.

• KiCad schematics and board layout: https://gitlab.com/vampire-bytes/usb-hid-phone

For the original FeTAp 611 dial wiring see the Wikipedia schematic of the FeAp 611.

Software

Firmware for the PIC18F2550, based on the classic Microchip mouse_keyboard USB HID example, with the rotary-dial FSM bolted on top of InterruptHandlerHigh() and a thin ProcessIO() that turns pulseCount into a HID keyboard report.

• Firmware repository will be released: link will appear here.

Why bother

Because a 1960s-era electromechanical input device sending modern, driver-less HID keystrokes is exactly the kind of thing this site is about. Also: it actually works, repeatably, every digit including “0”.

Modern successor

The same idea, ~17 years later, with current parts and CircuitPython instead of PIC C: Adafruit published a Rotary Phone Dial Keypad guide in 2025 that turns a Western Electric Model 500 rotary phone into a USB number pad using the KB2040 (RP2040) board. Same loop-disconnect pulse-counting trick, just on a modern microcontroller and toolchain, nice to see the approach is still alive and well.