Both signal paths validated with LTspice using built-in PC817
subcircuit model (Igain=1m). RX gives clean 0-3.13V at 3.3V VCC
with 5.8us rise time. TX confirms signal inversion and 0.17V bus
LOW with 4.66mA LED drive. 3.3V design works but is marginal vs
the original 5V Arduino circuit at worst-case CTR.
