How does this USART level shifter circuit work?
Consider what happens when the TX on the 3V side goes high and low.

If its High, Vbe &lt; 0V (emitter is at 3V and base is at the voltage of the 4k7 and 10k divider, ~2.2V), so no Base current, and the transistor is off. R8 pulls the other side to 5V.

When its Low, Vbe ~=0.7V, the transistor is on, and the TX is able to pull the other side low.