BOM
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2N4401 NPN transistor
RS-232/TTL Level Shifter
Male DE9 connector

Connections
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Switch is from collector to emitter, governed by base
4401 Collector - DE9 pin 2 (+5v)
4401 Emitter - DE9 pin 7 (PB0*)
4401 Base - Level Shifter out

Level Shifter GND - DE9 pin 3 (GND) and External Power ground
Level Shifter out - 4401 Base
Level Shifter Vcc - (External Power)

DE9 pin 3 (GND) - Level Shifter GND
DE9 pin 7 (PB0*) - 4402 Emitter
DE9 pin 2 (+5v) - 4402 Collector

* Consider using PB1 instead of PB0; PB0 held high is like holding down the open-apple key, turns ctrl-reset into ctrl-openapple-reset all the time