Similar to keying, PTT requires a transistor switch. Many K3NG schematics dedicate a separate pin (e.g., D13) for PTT. This allows the keyer to put the radio into transmit mode before the first dit—essential for QSK (full break-in) or semi-break-in operation.
Schematic Note: Some advanced versions include a PTT delay circuit using an RC network (resistor + capacitor) to prevent hot-switching of relays.
Modern K3NG schematics use I2C LCDs. This requires only 4 wires: k3ng keyer schematic
Older schematics (pre-2015) use parallel 4-bit mode, which eats up 6 pins (RS, E, D4, D5, D6, D7). If you see a schematic with a 16-pin LCD connector and a potentiometer (10kΩ for contrast), that is a legacy parallel design. Avoid it unless you have the pins to spare.
Symptom: The keyer sends random dots and dashes when you turn on your transmitter.
Cause: You forgot the 4.7k pull-up resistors on pins 2 & 3.
Fix: Add resistors physically or enable INTERNAL_PULLUP in the code (though external resistors are better for RF immunity). Similar to keying, PTT requires a transistor switch
The most critical part of the K3NG schematic is the input protection. The paddle inputs rely on pull-up resistors. Look closely at the schematic: you will see 4.7kΩ resistors connected from pins 2 and 3 to +5V.
Why are these required? Without pull-up resistors, when the paddle is open (not pressed), the pin floats. Electrical noise from your transmitter (RFI) will cause the keyer to trigger randomly. With the 4.7kΩ resistor, the pin stays at +5V until the paddle shorts it to ground. Older schematics (pre-2015) use parallel 4-bit mode, which
The Straight Key Trap: Some schematics show a 10kΩ resistor specifically for the straight key input (Pin 7). Straight keys are mechanical switches that bounce. The firmware handles debouncing, but the resistor ensures the logic level is valid.
Reverse Voltage Protection: Many advanced K3NG schematics include a 1N4148 diode in reverse parallel across the transistor (Cathode to +V, Anode to Collector). This protects the transistor from the "flyback voltage" created by the relay coils inside older tube radios.