Converts the MIDI current loop into a logic‑level signal while maintaining galvanic isolation.

DIN input wiring

• DIN Pin 4 → 220 Ω resistor → 6N138 Pin 2
• DIN Pin 5 → 6N138 Pin 3
• DIN Pin 2 → Ground

This forms the standard MIDI current loop into the optocoupler.

Optocoupler power and bias

• 6N138 Pin 5 → Ground
• 6N138 Pin 8 → +5 V
• 6N138 Pin 7 (VB) → Leave unconnected for MIDI applications
  • (VB is used for CTR tuning in other applications, but not needed here)

Optocoupler output stage

• 6N138 Pin 6 (VO) → 10 kΩ pull‑up resistor → +5 V
• 6N138 Pin 6 (VO) → 470 Ω resistor → MPS6514 NPN Base
• MPS6514 NPN Emitter → Ground
• MPS6514 NPN Collector → +5 V through 10 kΩ pull‑up

This transistor stage strengthens the optocoupler output and ensures clean edges.

Logic output to 74HC14

• MPS6514 NPN Collector → 74HC14 Pin 1
• 74HC14 Pin 2 = cleaned, conditioned MIDI logic signal.

IN Activity LED

• MPS6514 NPN Collector → 1 kΩ resistor → LED anode
• LED cathode → Ground

The LED flashes on incoming MIDI activity.