# SN-L00 KiCad Project

## Schematic Overview

```
                            +12V (from Eurorack)
                              │
                              ▼
┌─────────────────────────────────────────────────────────────────┐
│  POWER SECTION                                                   │
│                                                                  │
│   J1 (2x5)        D1 (SS14)       U1 (AMS1117-3.3)              │
│   Eurorack ──────►Schottky──┬────►LDO────────────► +3.3V        │
│   Header         protection │     │                              │
│                            C1    C2,C3                           │
│                           10uF   10uF+100nF                      │
│                             │     │                              │
│                            GND   GND                             │
└─────────────────────────────────────────────────────────────────┘

┌─────────────────────────────────────────────────────────────────┐
│  MCU & DISPLAY                                                   │
│                                                                  │
│   MOD1 (RP2040-Zero)              MOD2 (OLED 128x32)            │
│   ┌─────────────┐                 ┌──────────┐                   │
│   │ 3V3     5V  │                 │ GND      │                   │
│   │ GND     GND │                 │ VCC      │◄── +3.3V          │
│   │ GP0     GP9 │──SDA───────────►│ SDA      │                   │
│   │ GP1     GP8 │──SCL───────────►│ SCL      │                   │
│   │ GP2     GP7 │──TRIG_OUT       └──────────┘                   │
│   │ GP3     GP6 │──RETURN_IN                                     │
│   │ GP4     GP5 │──BTN                                           │
│   │ GP29    3V3 │                                                │
│   │ GND     GND │                                                │
│   └─────────────┘                                                │
└─────────────────────────────────────────────────────────────────┘

┌─────────────────────────────────────────────────────────────────┐
│  TRIGGER OUTPUT                                                  │
│                                                                  │
│   GP2 ────► U2 (74LVC1G17) ────► R3 (100Ω) ────► J2 (TRIG OUT)  │
│            Schmitt buffer        current limit   3.5mm jack      │
│                 │                                                │
│                C4 (100nF) to GND                                 │
└─────────────────────────────────────────────────────────────────┘

┌─────────────────────────────────────────────────────────────────┐
│  RETURN INPUT                                                    │
│                                                                  │
│   J3 (RETURN) ──► R4 (10K) ──┬──► R5/R6 divider ──► U3 ──► GP3  │
│   3.5mm jack     protection  │   (10K/22K = 0.69x)   MCP6001     │
│                              │                       buffer      │
│                              ├── D3 to +3.3V (clamp high)        │
│                              └── D4 to GND (clamp low)           │
│                                  BAT54 Schottky                  │
└─────────────────────────────────────────────────────────────────┘

┌─────────────────────────────────────────────────────────────────┐
│  BUTTON                                                          │
│                                                                  │
│   +3.3V ──► R1 (10K) ──┬──► GP4                                 │
│             pull-up    │                                         │
│                       SW1 ──► GND                                │
│                       tactile                                    │
└─────────────────────────────────────────────────────────────────┘

┌─────────────────────────────────────────────────────────────────┐
│  POWER LED                                                       │
│                                                                  │
│   +3.3V ──► R2 (1K) ──► D2 (Green LED) ──► GND                  │
└─────────────────────────────────────────────────────────────────┘
```

## RP2040-Zero Pinout Reference

```
        ┌─────────────────┐
   5V ──┤ 5V          3V3 ├── 3.3V
  GND ──┤ GND         GND ├── GND
 GP29 ──┤ GP29        GP0 ├── I2C SDA → OLED
  GP28 ──┤ GP28        GP1 ├── I2C SCL → OLED
  GP27 ──┤ GP27        GP2 ├── TRIG_OUT → 74LVC1G17
  GP26 ──┤ GP26        GP3 ├── RETURN_IN ← MCP6001
 GP15 ──┤ GP15        GP4 ├── BTN (active low)
  GP14 ──┤ GP14        GP5 ├── (spare)
        └─────────────────┘
          USB-C on bottom
```

## Files

- `SN-L00.kicad_pro` - KiCad project file
- `SN-L00.kicad_sch` - Schematic (open in KiCad 8+)
- `SN-L00.kicad_pcb` - PCB layout
- `PCB_LAYOUT.md` - Component placement guide

## Custom Libraries

### Symbol Library (`SN-L00.lib/`)

| Symbol | Description |
|--------|-------------|
| `RP2040-Zero` | Waveshare RP2040-Zero module (18 pins) |
| `OLED_128x32_I2C` | 0.91" OLED display (4 pins: GND, VCC, SCL, SDA) |

### Footprint Library (`SN-L00.pretty/`)

| Footprint | Description |
|-----------|-------------|
| `RP2040-Zero` | 2×9 pin through-hole, 15.24mm row spacing |
| `OLED_128x32_I2C` | 38×12mm module with 4-pin header |

Libraries are auto-loaded via `fp-lib-table` and `sym-lib-table`.

### RP2040-Zero Pinout (Footprint)

```
Pin  Name     │ Pin  Name
─────────────┼─────────────
 1   5V      │ 10   3V3
 2   GND     │ 11   GND
 3   3V3     │ 12   GP0/SDA
 4   GP29    │ 13   GP1/SCL
 5   GP28    │ 14   GP2
 6   GP27    │ 15   GP3
 7   GP26    │ 16   GP4
 8   GP15    │ 17   GP5
 9   GP14    │ 18   GND
```

### OLED Pinout (Footprint)

```
Pin  Name
───────────
 1   GND
 2   VCC
 3   SCL
 4   SDA
```

## Design Notes

### Power
- Single +12V rail used (no -12V needed)
- AMS1117-3.3 LDO provides 3.3V for MCU and peripherals
- SS14 Schottky for reverse polarity protection
- Total current draw ~20-30mA

### Trigger Output
- 74LVC1G17 provides clean Schmitt-trigger buffered output
- 100Ω series resistor for short-circuit protection
- Output: 0-3.3V (compatible with most Eurorack gate inputs)

### Return Input
- Handles 0-10V Eurorack signals safely
- 10K/22K voltage divider scales to ~0-3.1V
- Schottky clamp diodes protect against overvoltage
- MCP6001 rail-to-rail op-amp buffers signal
- Threshold set in firmware (~1.5V default)

### I2C
- 400kHz I2C to OLED display
- No external pull-ups needed (internal to OLED module)

## Opening the Project

1. Install KiCad 8.0+
2. Open `SN-L00.kicad_pro`
3. Schematic editor opens automatically