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Teensy 4.0 CW Keyer: Ultra-Low Latency Iambic A/B + Straight Key over USB Serial to C++ CW KEYER JACK App on Ubuntu 24.04.
See this 49-second YouTube demo showing iambic paddles and straight key connected directly to Teensy GPIOs, sending precise key-down/up events over USB serial to a companion C++ JACK audio connection kit CW keyer app (also shown with remote rig keying demo):
", "DAH:U", "S1", plus "KA" keepalives at 115200 baud. The host Qt/JACK C++ app processes events in real-time (48kHz, envelope LUTs for rise/fall), generates clean smooth Raised Cosine edged sinewave tones (WPM 5-100, pitch 300-1200Hz), and also is able to couple perfectly into a UDP Sockets over IP C++ Sender APP for remote rig CW keying on the PI/RIG udp sockets receiver, keying GPIO PIN 17 to key the rig's cw jack using a FET OPTO. Optimized even for QRQ with atomic RT params, event queue (4096 deep), and -O3 native tuning.
Latest improved code (12/20/25, by multiple AI-assisted refinements): https://u.pcloud.link/publink/show?code=XZDczx5ZLwHbdfYh85FA6rXk9ltNAuKb3QlV
(Contains: teensy_keyer.ino, main.cpp, cw_keyer.pro for qmake/make)
See this 49-second YouTube demo showing iambic paddles and straight key connected directly to Teensy GPIOs, sending precise key-down/up events over USB serial to a companion C++ JACK audio connection kit CW keyer app (also shown with remote rig keying demo):
Project Overview
This is a complete CW keyer system using Teensy 4.0 as a raw serial sensor board for iambic paddles (modes A/B) and straight key simultaneously—no switches to choose iAMBICs or ST KEY are needed. Teensy debounces inputs with Bounce2, sends events like "DIT", "DAH:U", "S1", plus "KA" keepalives at 115200 baud. The host Qt/JACK C++ app processes events in real-time (48kHz, envelope LUTs for rise/fall), generates clean smooth Raised Cosine edged sinewave tones (WPM 5-100, pitch 300-1200Hz), and also is able to couple perfectly into a UDP Sockets over IP C++ Sender APP for remote rig CW keying on the PI/RIG udp sockets receiver, keying GPIO PIN 17 to key the rig's cw jack using a FET OPTO. Optimized even for QRQ with atomic RT params, event queue (4096 deep), and -O3 native tuning.Latest improved code (12/20/25, by multiple AI-assisted refinements): https://u.pcloud.link/publink/show?code=XZDczx5ZLwHbdfYh85FA6rXk9ltNAuKb3QlV
(Contains: teensy_keyer.ino, main.cpp, cw_keyer.pro for qmake/make)
Hardware Setup
- Teensy 4.0 pins (all INPUT_PULLUP, counting from top-left USB side):
- Straight key (S1): GPIO 2 (4th pin down from USB-top, left side)
- DIT paddle: GPIO 4 (6th pin down from USB-top, left side)
- DAH paddle: GPIO 5 (7th pin down from USB-top, left side)
- Heartbeat LED: GPIO 13 (onboard, flickers on keepalives)
- Wire paddles/straight key to pins + GND. Plug Teensy USB into Linux host with JACK server running (48kHz recommended). No extra hardware needed.
Key Software Features
- Teensy side (Arduino sketch): High-precision elapsedMillis timing, immediate Serial.send_now() flushes, adaptive 1s keepalives.
cpp
if (b_dit.fell()) { send_event("DIT"); } // Precise paddle capture
if (idle_timer >= 1000) { Serial.println("KA"); Serial.send_now(); } - Host app (Qt/C++ JACK): RT-safe event queue, iambic state machine (IDLE/RISING/STEADY/FALLING/SPACE), GUI sliders for WPM/pitch/volume/rise(1-25ms)/fall/comp/iambic mode. Serial thread auto-detects Teensy (/dev/ttyACM*).
- JACK process() callback: sine gen with LUT envelopes, denorm protection.
- Build: qmake cw_keyer.pro && make (needs Qt Widgets, JACK, pthread, rt).
Usage & Builds
- Flash teensy_keyer.ino to Teensy 4.0 (115200 baud).
- Build/run C++ app on Linux: using qmake.pro file (included in the link)
- Connect paddles/key to Teensy GPIO pins —start keying immediately with ultra-low latency. GUI updates RT params atomically.
- Remote rigs: a different C++ App sends UDP keying (see prior videos for Pi interface ).