Amateur Radio Stuff

When I was getting ready for this year's California QSO Party, I realized I wanted my logging computer to do the CW keying for the radio instead of juggling paddles or hand-keying while logging. Programs like N1MM can handle CW generation beautifully — but only if the PC can key the rig. That sent me down the rabbit hole of simple keying interfaces. After some experiments, I landed on this straightforward two-transistor design that was cheap and quick to build.

Why this design?

• Simplicity: Just two NPN transistors and a few resistors/diodes.
• Compatibility: Mimics a straight key or PTT switch closure to ground.
• Safety: Clamp diodes protect the transistors from RS-232 swings.

How it works

• DTR (pin 4) → drives the CW transistor (Q1) through R1 (4.7 kΩ).
• RTS (pin 7) → drives the PTT transistor (Q2) through R2 (4.7 kΩ).
• Each base has:
• A 1N4148 diode to ground (protects against negative RS-232 swing).
• A 4.7 kΩ pull-down to ground (ensures “off” when idle).
• Each collector goes to the radio’s KEY/PTT input and is shunted by a 1N4148 diode for RF/transient protection.
• Each emitter ties to DB-9 ground (pin 5) and to the radio ground.

When N1MM asserts DTR or RTS, base current flows, saturating Q1/Q2 and shorting the radio line to ground — exactly what the rig expects.

Build Parts List

• 2 × 2N4401 (Q1/Q2)
• 4 × 1N4148 (D1–D4)
• 4 × 4.7 kΩ resistors (R1–R4)
• DB-9 connector (male/female to match your adapter)
• Jacks for KEY and PTT lines (often 3.5 mm mono plugs)
• Small perfboard + box enclosure (optional)

N1MM setup

• Config → Configure Ports → add your COM port.
• Assign CW keying to DTR (pin 4).
• Assign PTT to RTS (pin 7).
• Invert DTR/RTS if needed (depends on idle logic of your adapter)

Direct CW and Keying Circuit.pdf