USB Serial Adapter Pin (RTS or DTR) │ └───[ 1kΩ Resistor ]───┐ │ Base ┌─┴─┐ (2N2222 or 2N3904 NPN) │ ↗ │ Emitter └─┬─┘ Collector │ │ │ └────> [ TIP of Jack ] (Radio Key Line) ▼ [ GND Pin ] ───────────┴─────────────> [ SLEEVE of Jack ] (Radio Ground) *NOTE: Do not connect the USB pins directly to your radio. Use this NPN isolation loop to protect your hardware from logic voltage differences.*