MIDI Interface Manual

MIDI data carnival

Welcome to the construction manual for the MIDI IN + 4× MIDI THRU Interface, a compact little box whose entire purpose in life is to take one stream of MIDI data and multiply it like a well‑behaved, electrically isolated rabbit. This device is designed for builders who appreciate clean signal paths, modular wiring, and the deep personal satisfaction of knowing exactly where every resistor on the board lives — and why it’s there.

This project combines several classic ingredients of electronic music hardware:

A 6N138 optocoupler, whose job is to stare suspiciously at incoming MIDI data and only pass along the important bits.
A 74HC14 Schmitt inverter, which takes the optocoupler’s nervous whisper and turns it into a crisp, confident digital signal.
A 78L05 voltage regulator, which bravely converts your wildly unpredictable 9–15 V DC input into a calm, dependable 5 V supply.
And finally, a MatrixPortal tap point, because what’s the point of building a MIDI interface if you can’t also make it drive a lyric display, a robot drummer, or whatever other contraption you’ve decided to unleash upon the world.

All MIDI jacks, switches, and LEDs are panel‑mounted, because you deserve a device that doesn’t rip its connectors off the board the first time someone bumps a cable. Everything connects to the main board through pin headers, giving you modularity, serviceability, and the ability to unplug things dramatically when troubleshooting.

The circuit is built on a 2" × 3" perfboard, which is just large enough to be comfortable and just small enough to make you feel clever. The layout is intentionally simple, the wiring is intentionally logical, and the end result is intentionally robust — a reliable MIDI splitter that won’t add latency, won’t distort your signal, and won’t judge you for still owning that one synth with the sticky keys.

Whether you’re integrating this into a larger system, mounting it inside a custom enclosure, or simply building it because you enjoy the smell of flux in the morning, this manual will guide you through every step with clarity, precision, and the occasional moment of levity. After all, if you can’t have fun while soldering tiny metal legs onto a tiny green board, what are we even doing here.

Let’s begin.

始めましょう

Parts

This chapter provides a complete list of components required for the MIDI Interface build.
The parts are organized into three categories:

Voltage Regulation — components for generating and stabilizing the 5 V supply
Logic and Signal Components — the optocoupler, Schmitt trigger, transistor, and supporting resistors
Connectors and Indicators — MIDI jacks, LED, switch, and wiring harnesses

Use the subchapters below to view the detailed lists for each category.

Parts List (Grouped by Component Type)

🧩 Integrated Circuits

6N138 optocoupler
- Quantity: 1
74HC14 hex Schmitt‑trigger inverter
- Quantity: 1
78L05 voltage regulator (TO‑92 package)
- Quantity: 1
- Function: 5 V linear regulator, ~100 mA output
- Pinout (flat face toward you, leads downward):
  - Pin 1: VIN
  - Pin 2: GND
  - Pin 3: VOUT (+5 V)

🔩 Transistors

NPN transistor (2N3904, 2N2222, or equivalent)
- Quantity: 1

🛠️ Resistors

220 Ω resistors
- Quantity: 4
- Use: MIDI THRU current limiting
470 Ω resistor
- Quantity: 1
- Use: 6N138 VO → NPN base
10 kΩ resistor
- Quantity: 1
- Use: NPN collector pull‑up
1 kΩ resistor
- Quantity: 1
- Use: LED series resistor

🔋 Capacitors

Decoupling

0.33 µF capacitor (VIN → GND)
- Quantity: 1
- Type: Ceramic or electrolytic
0.1 µF capacitor (VOUT → GND)
- Quantity: 1
- Type: Ceramic

Bulk (Recommended)

10 µF electrolytic (input smoothing, VIN → GND)
- Quantity: 1
- Voltage rating: 16 V or higher
10 µF electrolytic (output smoothing, +5 V → GND)
- Quantity: 1
- Voltage rating: 10 V or higher

🔌 Connectors

DC input connector (2‑pin or barrel jack)
- Quantity: 1
MIDI IN connector (5‑pin DIN or harness)
- Quantity: 1
MIDI THRU connectors
- Quantity: 4

💡 Indicators

Standard LED (3 mm or 5 mm)
- Quantity: 1

🔧 Switches

SPST power switch
- Quantity: 1
- Function: DC+ → VIN

🪛 Wiring and Headers

Panel wiring for:
- MIDI IN
- 4× MIDI THRU
- LED
- Power switch
- DC input
Optional: pin headers for removable harnesses

Bill of materials

Category	Role	Component	Qty	Notes
Integrated Circuits	Logic and signal	6N138 Optocoupler	1	MIDI IN isolation
Integrated Circuits	Logic and signal	74HC14 Hex Schmitt Trigger	1	Signal conditioning (6 inverters)
Integrated Circuits	Power	78L05 Voltage Regulator (TO‑92)	1	5 V regulator, ~100 mA output, Pinout (flat face toward you, leads downward): Pin 1: VIN Pin 2: GND Pin 3: VOUT (+5 V)
Transistors	Logic and signal	NPN Transistor (2N3904, 2N2222, MPS6514 or similar)	1	General‑purpose NPN transistor used for buffering and driving the LED indicator, MIDI out/thru driver
Resistors	Logic and signal	220 Ω	4	MIDI THRU current limiting
Resistors	Logic and signal	470 Ω	1	6N138 VO → NPN base
Resistors	Logic and signal	10 kΩ	1	NPN collector pull‑up
Resistors	Logic and signal	1 kΩ	1	LED series resistor
Capacitors	Power	0.33 µF (ceramic or electrolytic)	1	Regulator input decoupling
Capacitors	Power	0.1 µF (ceramic)	1	Regulator output decoupling
Capacitors	Power	10 µF electrolytic (16 V+)	2	Input and output smoothing
Connectors	Power	DC Input Connector (2‑pin or barrel jack)	1	Power input
Connectors	Logic and signal	MIDI Connector (5‑pin DIN or harness)	5	Input port, parallel thru outputs
Indicators	Power	LED (3 mm or 5 mm)	1	Power or activity indicator
Switches	Power	SPST Power Switch	1	DC+ → VIN
Wiring & Headers	Wiring	Panel Wiring	As needed	MIDI IN, 4× THRU, LED, switch, DC input
Wiring & Headers	Wiring	Optional Pin Headers	As needed	For removable harnesses

Component pin-outs

6N138 Optocoupler

Pin 1 -- NC
Pin 2 -- LED Anode (input +)
Pin 3 -- LED Cathode (input -)
Pin 4 -- NC
Pin 5 -- GND
Pin 6 -- V (output)
Pin 7 -- V (base)
Pin 8 -- VCC (+5V)

74HC14 Schmitt‑Trigger Inverter (DIP‑14, notch up)

Pin 1 – Inverter 1 Input
Pin 2 – Inverter 1 Output
Pin 3 – Inverter 2 Input
Pin 4 – Inverter 2 Output
Pin 5 – Inverter 3 Input
Pin 6 – Inverter 3 Output
Pin 7 – GND
Pin 8 – Inverter 4 Input
Pin 9 – Inverter 4 Output
Pin 10 – Inverter 5 Input
Pin 11 – Inverter 5 Output
Pin 12 – Inverter 6 Input
Pin 13 – Inverter 6 Output
Pin 14 – VCC (+5 V)

MIDI 5-pin DIN

Pin 4 – Signal source
Pin 5 – Signal return
Pin 2 – Ground

Sub circuits

These are the circuits for the functional sub-sections of the interface.

Power regulation circuit

Provides a stable 5 V supply from a 9–15 V DC input source.

Regulator Pinout (front view)

Pin 1: VIN
Pin 2: GND
Pin 3: VOUT (5 V)

Input Side Components

DC jack → Power switch → 7805 Pin 1
0.33 µF ceramic capacitor from Pin 1 to Pin 2
10 µF electrolytic capacitor from Pin 1 to Pin 2

Output Side Components

7805 Pin 3 → +5 V rail
0.1 µF ceramic capacitor from Pin 3 to Pin 2
10 µF electrolytic capacitor from Pin 3 to Pin 2

Thermal Notes

At 15 V input, the regulator may dissipate up to 0.4 W depending on load. Mount the 7805 with airflow clearance or attach a small heatsink if desired.

MIDI in circuit

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.

MIDI THRU distribution circuit

Generates four identical, low‑latency copies of the cleaned MIDI signal.

Signal Conditioning

74HC14 Pin 1 → Pin 2 (first inverter)
Pin 2 fans out to:
- Pin 3
- Pin 5
- Pin 8
- Pin 10

Output Pins

THRU 1: Pin 4
THRU 2: Pin 6
THRU 3: Pin 9
THRU 4: Pin 11

Each output passes through a 220 Ω resistor before reaching the panel‑mounted THRU jack.

Assembly instructions

Overview

12 V → 5 V regulator
6N138 MIDI IN
74HC14 fan‑out
5 V power‑on LED
THRU/OUT activity LED
Pin‑header MIDI jacks
Full wire‑color guidance
No grid coordinates

This version is ideal for perfboard assembly where you place parts by eye and follow the wiring map.

🎛️ Wire Color Key

Red → +12 V input
Orange → +5 V regulated
Black → Ground
Yellow → MIDI signal lines (raw + clean)
Blue → Logic connections (inverter inputs/outputs)
Green → LED wiring
White → Misc short links / regulator wiring

🧱 FULL ASSEMBLY INSTRUCTIONS

1. Build the 12 V → 5 V Regulator Block

78L05 orientation

Flat face toward you:
Pin 1 = IN, Pin 2 = GND, Pin 3 = OUT

Wiring

Red wire: 12 V input → 1N4007 diode → 78L05 pin 1
- Stripe on diode faces the regulator
Black wire: 12 V ground → perfboard ground bus
Orange wire: 78L05 pin 3 → +5 V bus
Black wire: 78L05 pin 2 → ground bus

Capacitors

100 µF electrolytic: + to pin 1, – to ground
10 µF electrolytic: + to pin 3, – to ground
100 nF ceramic: between pin 3 and ground

2. Add the 5 V Power‑On LED

Green wire: +5 V bus → 1 kΩ resistor → LED anode
LED cathode → Black wire → ground bus

This LED lights whenever the regulator is producing 5 V.

3. Place and Power the 6N138 Optocoupler

Power

Orange wire: pin 8 → +5 V
Black wire: pin 5 → ground

Pull‑up

10 kΩ resistor: pin 6 → +5 V

MIDI IN protection diode 1N4007

Between pin 2 and pin 3
Stripe toward pin 2

4. Install the MIDI IN Header (3‑pin)

Header pin mapping:

Pin 1 = DIN pin 4
Pin 2 = DIN pin 5
Pin 3 = DIN pin 2 (GND)

Wiring

Yellow wire: header pin 1 → 220 Ω resistor → 6N138 pin 2
Yellow wire: header pin 2 → 6N138 pin 3
Black wire: header pin 3 → ground bus

5. Connect 6N138 Output to 74HC14 Input + MatrixPortal RX

6N138 output

Pin 6 = raw MIDI logic output

Wiring

Yellow wire: 6N138 pin 6 → 74HC14 pin 1 (first inverter input)
Blue wire: 74HC14 pin 2 (first inverter output) → MatrixPortal RX

Important

74HC14 pin 2 is the “clean MIDI” signal.
All THRU inputs will come from this pin.

6. Place and Power the 74HC14

Power

Orange wire: pin 14 → +5 V
Black wire: pin 7 → ground

7. Fan Out the Clean MIDI Signal to All THRU Inputs

All THRU inverters must receive the same input from pin 2.

Wiring (all blue)

Pin 2 → pin 3
Pin 2 → pin 5
Pin 2 → pin 9
Pin 2 → pin 11

This feeds the clean MIDI signal to all four THRU channels.

8. Add the THRU/OUT Activity LED

This LED shows the buffered MIDI signal.

Wiring

Green wire: 74HC14 pin 2 → 1 kΩ resistor → LED anode
LED cathode → Black wire → ground

9. Install the Four THRU Headers (3‑pin each)

Each header uses the same mapping:

Pin 1 = DIN pin 4 (from +5 V via 220 Ω)
Pin 2 = DIN pin 5 (from inverter output via 220 Ω)
Pin 3 = DIN pin 2 (GND)

10. Wire the THRU Outputs

THRU1 (inverter B)

Blue wire: pin 4 → 220 Ω → header pin 2
Orange wire: +5 V → 220 Ω → header pin 1
Black wire: header pin 3 → ground

THRU2 (inverter C)

Blue wire: pin 6 → 220 Ω → header pin 2
Orange wire: +5 V → 220 Ω → header pin 1
Black wire: header pin 3 → ground

THRU3 (inverter D)

Blue wire: pin 8 → 220 Ω → header pin 2
Orange wire: +5 V → 220 Ω → header pin 1
Black wire: header pin 3 → ground

THRU4 (inverter E)

Blue wire: pin 10 → 220 Ω → header pin 2
Orange wire: +5 V → 220 Ω → header pin 1
Black wire: header pin 3 → ground

11. Internal MIDI OUT (No DIN Jack)

Use the last inverter (F):

Wiring

Yellow wire: MatrixPortal TX → 74HC14 pin 13
Blue wire: 74HC14 pin 12 → internal OUT pad/header

This gives you a clean, buffered logic‑level MIDI OUT.

Breadboard layout

🎛️ Wire Color Key

Red → +12 V input
Orange → +5 V regulated
Black → Ground
Yellow → MIDI signal lines (raw + clean)
Blue → Logic connections (inverter inputs/outputs)
Green → LED wiring
White → Misc short links / regulator wiring

🧱 FULL STEP‑BY‑STEP ASSEMBLY

1. Build the 12 V → 5 V Regulator Block

Place the 78L05

Flat face toward you:
(1) IN — (2) GND — (3) OUT

Wire the input

Red wire: 12 V → 1N4007 → 78L05 pin 1
Black wire: 12 V ground → GND rail

Add capacitors

100 µF: 78L05 pin 1 → GND
10 µF: 78L05 pin 3 → GND
100 nF: 78L05 pin 3 → GND

Create rails

Orange wire: 78L05 pin 3 → +5 V rail
Black wire: 78L05 pin 2 → GND rail

2. Add the 5 V Power‑On LED

Place this near the regulator block.

Green wire: +5 V rail → 1 kΩ resistor → LED anode
LED cathode → Black wire → GND

This LED lights whenever the regulator is producing 5 V.

3. Place the ICs

6N138 left‑center, straddling the trench
74HC14 right‑center, straddling the trench

4. Power the ICs

Orange wire: 6N138 pin 8 → +5 V
Black wire: 6N138 pin 5 → GND
Orange wire: 74HC14 pin 14 → +5 V
Black wire: 74HC14 pin 7 → GND

Pull‑up

10 kΩ: 6N138 pin 6 → +5 V (orange)

5. Place and Wire the MIDI IN Header

Header pin mapping:

Pin 1 = DIN pin 4
Pin 2 = DIN pin 5
Pin 3 = DIN pin 2 (GND)

Wire it

Yellow wire: IN pin 1 → 220 Ω → 6N138 pin 2
Yellow wire: IN pin 2 → 6N138 pin 3
Black wire: IN pin 3 → GND

Protection diode

Between 6N138 pins 2 and 3
Stripe toward pin 2

6. Connect 6N138 Output to 74HC14 and MatrixPortal RX

Yellow wire: 6N138 pin 6 → 74HC14 pin 1
Blue wire: 74HC14 pin 2 → MatrixPortal RX

⭐ Important clarification:

74HC14 pin 2 is the “clean MIDI” signal.
All THRU inverter inputs will connect to this exact pin.

7. Connect ALL THRU Inverter Inputs to 74HC14 Pin 2

Use blue wires to tie each inverter input to the clean MIDI node at pin 2:

74HC14 pin 3 → pin 2
74HC14 pin 5 → pin 2
74HC14 pin 9 → pin 2
74HC14 pin 11 → pin 2

This means:

Pin 2 is the single source feeding all four THRU inverters.

8. Add THRU/OUT Activity LED

This LED shows the buffered MIDI signal feeding all THRUs and your internal OUT.

Green wire: 74HC14 pin 2 → 1 kΩ → LED anode
LED cathode → Black wire → GND

9. Place THRU Headers (THRU1–4)

Each header:

Pin 1 = DIN pin 4 (from +5 V via 220 Ω)
Pin 2 = DIN pin 5 (from inverter output via 220 Ω)
Pin 3 = DIN pin 2 (GND)

Place them along the bottom right of the breadboard.

10. Wire THRU Outputs

THRU1 (inverter B)

Blue wire: 74HC14 pin 4 → 220 Ω → THRU1 pin 2
Orange wire: +5 V → 220 Ω → THRU1 pin 1
Black wire: THRU1 pin 3 → GND

THRU2 (inverter C)

Blue wire: 74HC14 pin 6 → 220 Ω → THRU2 pin 2
Orange wire: +5 V → 220 Ω → THRU2 pin 1
Black wire: THRU2 pin 3 → GND

THRU3 (inverter D)

Blue wire: 74HC14 pin 8 → 220 Ω → THRU3 pin 2
Orange wire: +5 V → 220 Ω → THRU3 pin 1
Black wire: THRU3 pin 3 → GND

THRU4 (inverter E)

Blue wire: 74HC14 pin 10 → 220 Ω → THRU4 pin 2
Orange wire: +5 V → 220 Ω → THRU4 pin 1
Black wire: THRU4 pin 3 → GND

11. Internal MIDI OUT (No DIN Jack)

Use the last inverter (F):

Yellow wire: MatrixPortal TX → 74HC14 pin 13
Blue wire: 74HC14 pin 12 → internal OUT node (your choice)

This gives you a clean, buffered logic‑level MIDI OUT.

Perfboard layout

This chapter provides the complete physical layout for assembling the MIDI Interface on a 2" × 3.5" solderable half‑breadboard perfboard (ElectroCookie style).
It includes:

Exact placement of the 6N138, 74HC14, 78L05, NPN transistor, resistors, and capacitors
Placement of all panel headers (MIDI IN, 4× THRU, LED, switch, DC input)
Power‑rail usage and recommended routing strategy
A literal, row‑and‑column‑based layout for reproducibility

Perfboard overview

You are using a 2" × 3.5" solderable half‑breadboard perfboard with:

A center gap between rows E and F for DIP ICs
Power rails along the left and right edges
Rows labeled A–J (top to bottom)
Columns numbered 1–30 (left to right)

For consistency, this manual uses:

Row A = top edge
Row J = bottom edge
Column 1 = left edge
Column 30 = right edge

ICs straddle the E/F gap.

Note -- I filed the traces off of the bottom power rails so that each of those holes is individual and can be used for the headers.

IC orientation

6N138 placement

Place the 6n138 so that:

Pins 1-4 sit on E8–E11
Pins 5-8 sit on F11–F8

74HC14 Placement

Place the 74HC14 so that:

Pins 1–7 sit on E12–E18
Pins 8–14 sit on F18–F12

This keeps it to the right of the 6N138 with clean routing to the THRU resistors.

78L05 Regulator

Place 78L04 so that:

H26 = VIN
H27 = GND
H28 = +5 V

Component placement map

Below is the literal placement map for all components.

ICs

6N138: E8–E11 / F8–F11
74HC14: E12–E18 / F12–F18

Transistor

NPN (2N3904/2N2222):
- Collector → near F10/F11 region
- Base → near F9 (470 Ω resistor connects here)
- Emitter → ground rail

Recommended location: Rows G–H, Columns 9–11

Resistors

MIDI IN / Optocoupler

470 Ω: between 6N138 pin 6 (F10) and NPN base
10 kΩ: between NPN collector and +5 V rail
1 kΩ: LED series resistor near LED header

MIDI THRU

Place the four 220 Ω resistors in a vertical bank below the 74HC14:

R1: G13
R2: G14
R3: G15
R4: G16

Each connects from a 74HC14 output to its corresponding THRU header.

Capacitors

Regulator input side

0.33 µF: between H26 (VIN) and H27 (GND)
10 µF (16 V+): same nodes, placed nearby

Regulator output side

0.1 µF: between H28 (+5 V) and H27 (GND)
10 µF (10 V+): same nodes, placed nearby

Panel header placement

All panel headers are placed along the bottom edge (Row J) for clean wiring to the enclosure.

Left to Right:

MIDI IN Header (3‑pin)
- J5–J7
MIDI THRU Headers (4× 3‑pin)
- THRU1: J10–J12
- THRU2: J14–J16
- THRU3: J18–J20
- THRU4: J22–J24
LED Header (2‑pin)
- J26–J27
Power Switch Header (2‑pin)
- J28–J29
DC Input Header (2‑pin)
- J30–J31 (or nearest available)

Power rails

Use the built‑in rails:

Left rail: Ground
Right rail: +5 V

Connect:

78L05 output (H28) → +5 V rail
78L05 ground (H27) → ground rail
6N138 pin 8 (F8) → +5 V rail
6N138 pin 5 (F11) → ground rail
74HC14 pin 14 → +5 V rail
74HC14 pin 7 → ground rail

Recommended routing strategy

MIDI IN section

Keep the LED‑side wiring short
Route the 470 Ω resistor directly to the NPN base
Keep the 10 kΩ pull‑up close to the NPN collector

THRU section

Route each 74HC14 output straight down to its 220 Ω resistor
Then straight to the THRU header pins
Maintain clean, parallel lines for serviceability

Power section

Keep regulator capacitors tight to the 78L05
Use short, direct traces to the rails
Avoid crossing the logic section with VIN wiring

Final checkpoints

Before soldering the wiring:

Confirm 6N138 pin 8 is on F8 and goes to +5 V
Confirm 6N138 pin 5 is on F11 and goes to ground
Confirm 74HC14 pin 14 is powered and pin 7 is grounded
Confirm all THRU resistors are 220 Ω
Confirm the NPN emitter is grounded
Confirm the LED resistor is 1 kΩ
Confirm the regulator capacitors are oriented correctly

Panel wiring via pin headers

MIDI IN Header (3‑pin)

Pin 1 → DIN Pin 4
Pin 2 → DIN Pin 5
Pin 3 → Ground (DIN Pin 2)

THRU Headers (4 × 3‑pin)

Each THRU jack uses:

Pin 1 → +5 V (through 220 Ω) → DIN Pin 4
Pin 2 → 74HC14 output (through 220 Ω) → DIN Pin 5
Pin 3 → Ground → DIN Pin 2

Power Switch Header (2‑pin)

Pin 1 → DC jack +V
Pin 2 → 7805 VIN

IN LED Header (2‑pin)

Pin 1 → LED anode (via 1 kΩ)
Pin 2 → Ground

MatrixPortal Header (3‑pin)

GND
RX (from MPS6514 NPN collector)
+5 V

Testing and verification

Power Test

Apply 9–15 V DC
Verify 5 V output from the regulator
Check for excessive heat

MIDI IN Test

Connect a MIDI source
Confirm IN LED activity

THRU Output Test

Connect downstream devices
Verify all receive identical MIDI data

MatrixPortal Test

Connect RX and GND
Run firmware
Confirm MIDI events are received

Usage notes and expansion options

Usage Notes

All THRU outputs are electrically identical
The circuit supports long cable runs
The MatrixPortal tap does not affect timing

Expansion Options

Add additional THRU outputs using unused 74HC14 gates
Add a power‑on LED
Add a second LED for THRU activity
Replace the 7805 with a switching regulator for higher efficiency

MIDI Interface Manual

MIDI Interface Manual

Parts

🧩 Integrated Circuits

🔩 Transistors

🛠️ Resistors

🔋 Capacitors

Decoupling

Bulk (Recommended)

🔌 Connectors

💡 Indicators

🔧 Switches

🪛 Wiring and Headers

Component pin-outs

6N138 Optocoupler

74HC14 Schmitt‑Trigger Inverter (DIP‑14, notch up)

MIDI 5-pin DIN

Sub circuits

Power regulation circuit

Regulator Pinout (front view)

Input Side Components

Output Side Components

Thermal Notes

MIDI in circuit

DIN input wiring

Optocoupler power and bias

Optocoupler output stage

Logic output to 74HC14

IN Activity LED

MIDI THRU distribution circuit

Signal Conditioning

Output Pins

Assembly instructions

Overview

🎛️ Wire Color Key

🧱 FULL ASSEMBLY INSTRUCTIONS

1. Build the 12 V → 5 V Regulator Block

78L05 orientation

Wiring

Capacitors

2. Add the 5 V Power‑On LED

3. Place and Power the 6N138 Optocoupler

Power

Pull‑up

MIDI IN protection diode 1N4007

4. Install the MIDI IN Header (3‑pin)

Header pin mapping:

Wiring

5. Connect 6N138 Output to 74HC14 Input + MatrixPortal RX

6N138 output

Wiring

Important

6. Place and Power the 74HC14

Power

7. Fan Out the Clean MIDI Signal to All THRU Inputs

Wiring (all blue)

8. Add the THRU/OUT Activity LED

Wiring

9. Install the Four THRU Headers (3‑pin each)

10. Wire the THRU Outputs

THRU1 (inverter B)

THRU2 (inverter C)

THRU3 (inverter D)

THRU4 (inverter E)

11. Internal MIDI OUT (No DIN Jack)

Wiring

Breadboard layout

Perfboard layout

Perfboard overview

IC orientation

6N138 placement

74HC14 Placement

78L05 Regulator

Component placement map

ICs

Transistor

Resistors

MIDI IN / Optocoupler

MIDI THRU

Capacitors

1. Build the 12 V → 5 V Regulator Block

2. Add the 5 V Power‑On LED