Add KiCad schematic and updated BOM

Schematic includes:
- Power section: SS14 reverse protection, AMS1117-3.3 LDO
- RP2040-Zero and OLED module headers
- Trigger output: 74LVC1G17 Schmitt buffer
- Return input: voltage divider, BAT54 clamps, MCP6001 buffer
- Button with pull-up, power LED

BOM updated with correct parts and LCSC numbers.
ASCII schematic reference in hardware/kicad/README.md.

hardware/BOM.md

@@ -4,14 +4,16 @@
 
 | Ref | Description | Value | Package | LCSC # | Qty |
 |-----|-------------|-------|---------|--------|-----|
-| C1,C2 | Capacitor, ceramic | 100nF | 0603 | C14663 | 2 |
+| C1,C2 | Capacitor, ceramic | 10uF | 0805 | C15850 | 2 |
-| C3,C4 | Capacitor, ceramic | 10uF | 0805 | C15850 | 2 |
+| C3,C4,C5 | Capacitor, ceramic | 100nF | 0603 | C14663 | 3 |
-| C5 | Capacitor, electrolytic | 47uF/25V | SMD 6.3x5.4 | C134767 | 1 |
+| R1 | Resistor (button pull-up) | 10K | 0603 | C25804 | 1 |
-| R1-R4 | Resistor | 10K | 0603 | C25804 | 4 |
+| R2 | Resistor (LED current) | 1K | 0603 | C21190 | 1 |
-| R5,R6 | Resistor | 1K | 0603 | C21190 | 2 |
+| R3 | Resistor (trigger output) | 100R | 0603 | C22775 | 1 |
-| R7 | Resistor | 100K | 0603 | C25803 | 1 |
+| R4,R5 | Resistor (input protection) | 10K | 0603 | C25804 | 2 |
+| R6 | Resistor (voltage divider) | 22K | 0603 | C25774 | 1 |
 | D1 | Schottky diode (reverse polarity) | SS14 | SMA | C2480 | 1 |
 | D2 | LED, green (power) | Green | 0603 | C72043 | 1 |
+| D3,D4 | Schottky diode (input clamp) | BAT54 | SOD-323 | C181104 | 2 |
 | U1 | LDO regulator 3.3V | AMS1117-3.3 | SOT-223 | C6186 | 1 |
 | U2 | Schmitt trigger buffer | 74LVC1G17 | SOT-23-5 | C388628 | 1 |
 | U3 | Op-amp (input buffer) | MCP6001 | SOT-23-5 | C93025 | 1 |
@@ -19,6 +21,8 @@
 | J2,J3 | Thonkiconn jack | PJ398SM | - | C2935955 | 2 |
 | SW1 | Tactile switch | 6x6mm | SMD | C127509 | 1 |
 
+**Total unique LCSC parts: 16**
+
 ## Hand-Solder Modules
 
 | Ref | Description | Source | Part # | Approx Price | Qty |
@@ -45,13 +49,30 @@ Thonkiconn PJ398SM may have limited LCSC stock. Alternatives:
 - Exploding Shed: https://www.exploding-shed.com/
 - Modular Addict: https://modularaddict.com/
 
-## Notes
+## Circuit Description
 
-- All SMD passives are 0603 for easy assembly
+### Power Section
-- AMS1117-3.3 handles 12V→3.3V efficiently for low-power MCU
+- **D1 (SS14)**: Reverse polarity protection
-- 74LVC1G17 provides clean trigger output with Schmitt hysteresis
+- **U1 (AMS1117-3.3)**: 12V → 3.3V LDO regulator
-- MCP6001 buffers input signal, rail-to-rail on 3.3V
+- **C1,C2**: Input/output bulk capacitors (10µF)
-- Reverse polarity protection via D1
+- **C3**: Output decoupling (100nF)
+- **D2 + R2**: Power indicator LED
+
+### Trigger Output
+- **U2 (74LVC1G17)**: Schmitt trigger buffer for clean edges
+- **R3 (100Ω)**: Output current limiting / short protection
+- **C4**: U2 decoupling (100nF)
+
+### Return Input
+- **R4 (10K)**: Input series protection
+- **R5/R6 (10K/22K)**: Voltage divider (5V → 3.1V)
+- **D3,D4 (BAT54)**: Overvoltage clamp to rails
+- **U3 (MCP6001)**: Rail-to-rail buffer op-amp
+- **C5**: U3 decoupling (100nF)
+
+### User Interface
+- **SW1**: Tactile button (mode/trigger)
+- **R1 (10K)**: Button pull-up resistor
 
 ## Estimated Costs
 
@@ -59,7 +80,7 @@ Thonkiconn PJ398SM may have limited LCSC stock. Alternatives:
 |----------|------|
 | PCB (5pcs, PCBWay) | ~€5-10 |
 | SMD Assembly | ~€15-25 |
-| LCSC components | ~€3-5 |
+| LCSC components | ~€4-6 |
 | Hand-solder modules | ~€5-8 |
 | Jacks (if separate) | ~€1-2 |
 | **Total per unit** | **~€25-40** |

hardware/kicad/README.md

@@ -0,0 +1,124 @@
+# 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 (to be created)
+
+## 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

hardware/kicad/SN-L00.kicad_pro

@@ -0,0 +1,104 @@
+{
+  "board": {
+    "3dviewports": [],
+    "design_settings": {
+      "defaults": {
+        "board_outline_line_width": 0.1,
+        "copper_line_width": 0.2,
+        "copper_text_size_h": 1.5,
+        "copper_text_size_v": 1.5,
+        "copper_text_thickness": 0.3,
+        "other_line_width": 0.15,
+        "silk_line_width": 0.15,
+        "silk_text_size_h": 1.0,
+        "silk_text_size_v": 1.0,
+        "silk_text_thickness": 0.15
+      },
+      "diff_pair_dimensions": [],
+      "drc_exclusions": [],
+      "rules": {
+        "min_clearance": 0.2,
+        "min_track_width": 0.2,
+        "min_via_annular_width": 0.15,
+        "min_via_diameter": 0.5
+      },
+      "track_widths": [
+        0.2,
+        0.3,
+        0.5,
+        0.8
+      ],
+      "via_dimensions": [
+        {
+          "diameter": 0.6,
+          "drill": 0.3
+        }
+      ]
+    },
+    "layer_presets": [],
+    "viewports": []
+  },
+  "libraries": {
+    "pinned_footprint_libs": [],
+    "pinned_symbol_libs": []
+  },
+  "meta": {
+    "filename": "SN-L00.kicad_pro",
+    "version": 1
+  },
+  "net_settings": {
+    "classes": [
+      {
+        "clearance": 0.2,
+        "diff_pair_gap": 0.25,
+        "diff_pair_via_gap": 0.25,
+        "diff_pair_width": 0.2,
+        "name": "Default",
+        "track_width": 0.2,
+        "via_diameter": 0.6,
+        "via_drill": 0.3
+      },
+      {
+        "clearance": 0.3,
+        "name": "Power",
+        "track_width": 0.5,
+        "via_diameter": 0.8,
+        "via_drill": 0.4
+      }
+    ]
+  },
+  "pcbnew": {
+    "last_paths": {
+      "gencad": "",
+      "idf": "",
+      "netlist": "",
+      "specctra_dsn": "",
+      "step": ""
+    },
+    "page_layout_descr_file": ""
+  },
+  "schematic": {
+    "annotate_start_num": 1,
+    "drawing": {
+      "default_line_thickness": 6.0,
+      "default_text_size": 50.0,
+      "field_names": [],
+      "intersheets_ref_own_page": false,
+      "intersheets_ref_prefix": "",
+      "intersheets_ref_short": false,
+      "intersheets_ref_show": false,
+      "intersheets_ref_suffix": "",
+      "junction_size_choice": 3,
+      "label_size_ratio": 0.25,
+      "pin_symbol_size": 0.0,
+      "text_offset_ratio": 0.08
+    },
+    "page_layout_descr_file": "",
+    "plot_directory": "",
+    "spice_external_command": "spice \"%I\"",
+    "subpart_first_id": 65,
+    "subpart_id_separator": 0
+  },
+  "sheets": [],
+  "text_variables": {}
+}

hardware/kicad/SN-L00.kicad_sch

@@ -0,0 +1,432 @@
+(kicad_sch
+  (version 20231120)
+  (generator "eeschema")
+  (generator_version "8.0")
+  (uuid "a0e1b2c3-d4e5-f6a7-b8c9-d0e1f2a3b4c5")
+  (paper "A4")
+  (title_block
+    (title "SN-L00 Latency Tester")
+    (date "2026-01-23")
+    (rev "0.1")
+    (company "SubModular / Sub-Net e.U.")
+  )
+
+  ;; Power symbols
+  (symbol
+    (lib_id "power:+12V")
+    (at 25.4 25.4 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "pwr-12v-1")
+    (property "Reference" "#PWR01" (at 25.4 29.21 0) (effects (font (size 1.27 1.27)) hide))
+    (property "Value" "+12V" (at 25.4 22.86 0) (effects (font (size 1.27 1.27))))
+  )
+
+  (symbol
+    (lib_id "power:GND")
+    (at 25.4 76.2 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "pwr-gnd-1")
+    (property "Reference" "#PWR02" (at 25.4 82.55 0) (effects (font (size 1.27 1.27)) hide))
+    (property "Value" "GND" (at 25.4 80.01 0) (effects (font (size 1.27 1.27))))
+  )
+
+  (symbol
+    (lib_id "power:+3.3V")
+    (at 63.5 25.4 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "pwr-3v3-1")
+    (property "Reference" "#PWR03" (at 63.5 29.21 0) (effects (font (size 1.27 1.27)) hide))
+    (property "Value" "+3.3V" (at 63.5 22.86 0) (effects (font (size 1.27 1.27))))
+  )
+
+  ;; Eurorack Power Header J1
+  (symbol
+    (lib_id "Connector_Generic:Conn_02x05_Odd_Even")
+    (at 25.4 50.8 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "j1-euro-pwr")
+    (property "Reference" "J1" (at 25.4 40.64 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "Eurorack_Power" (at 25.4 60.96 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Connector_IDC:IDC-Header_2x05_P2.54mm_Vertical" (at 25.4 50.8 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C429954" (at 25.4 50.8 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; Schottky Diode D1 - Reverse polarity protection
+  (symbol
+    (lib_id "Device:D_Schottky")
+    (at 38.1 30.48 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "d1-schottky")
+    (property "Reference" "D1" (at 38.1 27.94 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "SS14" (at 38.1 33.02 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Diode_SMD:D_SMA" (at 38.1 30.48 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C2480" (at 38.1 30.48 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; LDO Regulator U1
+  (symbol
+    (lib_id "Regulator_Linear:AMS1117-3.3")
+    (at 50.8 30.48 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "u1-ldo")
+    (property "Reference" "U1" (at 50.8 22.86 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "AMS1117-3.3" (at 50.8 25.4 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Package_TO_SOT_SMD:SOT-223-3_TabPin2" (at 50.8 30.48 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C6186" (at 50.8 30.48 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; Input capacitors
+  (symbol
+    (lib_id "Device:C")
+    (at 43.18 38.1 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "c1-input")
+    (property "Reference" "C1" (at 45.72 36.83 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "10uF" (at 45.72 39.37 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Capacitor_SMD:C_0805_2012Metric" (at 43.18 38.1 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C15850" (at 43.18 38.1 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; Output capacitors
+  (symbol
+    (lib_id "Device:C")
+    (at 58.42 38.1 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "c2-output")
+    (property "Reference" "C2" (at 60.96 36.83 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "10uF" (at 60.96 39.37 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Capacitor_SMD:C_0805_2012Metric" (at 58.42 38.1 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C15850" (at 58.42 38.1 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  (symbol
+    (lib_id "Device:C")
+    (at 63.5 38.1 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "c3-output")
+    (property "Reference" "C3" (at 66.04 36.83 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "100nF" (at 66.04 39.37 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Capacitor_SMD:C_0603_1608Metric" (at 63.5 38.1 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C14663" (at 63.5 38.1 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; RP2040-Zero Module MOD1
+  (symbol
+    (lib_id "Connector_Generic:Conn_01x09")
+    (at 101.6 50.8 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "mod1-rp2040-left")
+    (property "Reference" "MOD1A" (at 101.6 35.56 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "RP2040-Zero_Left" (at 101.6 68.58 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Connector_PinHeader_2.54mm:PinHeader_1x09_P2.54mm_Vertical" (at 101.6 50.8 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  (symbol
+    (lib_id "Connector_Generic:Conn_01x09")
+    (at 116.84 50.8 180)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "mod1-rp2040-right")
+    (property "Reference" "MOD1B" (at 116.84 35.56 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "RP2040-Zero_Right" (at 116.84 68.58 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Connector_PinHeader_2.54mm:PinHeader_1x09_P2.54mm_Vertical" (at 116.84 50.8 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; OLED Module MOD2
+  (symbol
+    (lib_id "Connector_Generic:Conn_01x04")
+    (at 139.7 50.8 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "mod2-oled")
+    (property "Reference" "MOD2" (at 139.7 43.18 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "OLED_SSD1306" (at 139.7 60.96 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Connector_PinHeader_2.54mm:PinHeader_1x04_P2.54mm_Vertical" (at 139.7 50.8 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; 74LVC1G17 Schmitt Trigger Buffer U2
+  (symbol
+    (lib_id "74xx_IEEE:74LVC1G17")
+    (at 152.4 101.6 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "u2-schmitt")
+    (property "Reference" "U2" (at 152.4 93.98 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "74LVC1G17" (at 152.4 96.52 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Package_TO_SOT_SMD:SOT-23-5" (at 152.4 101.6 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C388628" (at 152.4 101.6 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; MCP6001 Op-Amp U3
+  (symbol
+    (lib_id "Amplifier_Operational:MCP6001-OT")
+    (at 152.4 139.7 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "u3-opamp")
+    (property "Reference" "U3" (at 152.4 129.54 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "MCP6001" (at 152.4 132.08 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Package_TO_SOT_SMD:SOT-23-5" (at 152.4 139.7 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C93025" (at 152.4 139.7 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; Trigger Output Jack J2
+  (symbol
+    (lib_id "Connector:AudioJack2_Ground_Switch")
+    (at 177.8 101.6 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "j2-trig-out")
+    (property "Reference" "J2" (at 177.8 93.98 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "TRIG_OUT" (at 177.8 109.22 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Connector_Audio:Jack_3.5mm_QingPu_WQP-PJ398SM_Vertical_CircularHoles" (at 177.8 101.6 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C2935955" (at 177.8 101.6 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; Return Input Jack J3
+  (symbol
+    (lib_id "Connector:AudioJack2_Ground_Switch")
+    (at 177.8 139.7 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "j3-return-in")
+    (property "Reference" "J3" (at 177.8 132.08 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "RETURN_IN" (at 177.8 147.32 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Connector_Audio:Jack_3.5mm_QingPu_WQP-PJ398SM_Vertical_CircularHoles" (at 177.8 139.7 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C2935955" (at 177.8 139.7 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; Tactile Button SW1
+  (symbol
+    (lib_id "Switch:SW_Push")
+    (at 101.6 101.6 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "sw1-button")
+    (property "Reference" "SW1" (at 101.6 93.98 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "BTN" (at 101.6 96.52 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Button_Switch_SMD:SW_Push_1P1T_NO_6x6mm_H9.5mm" (at 101.6 101.6 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C127509" (at 101.6 101.6 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; Pull-up resistor for button
+  (symbol
+    (lib_id "Device:R")
+    (at 88.9 93.98 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "r1-pullup")
+    (property "Reference" "R1" (at 91.44 92.71 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "10K" (at 91.44 95.25 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Resistor_SMD:R_0603_1608Metric" (at 88.9 93.98 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C25804" (at 88.9 93.98 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; Power LED D2
+  (symbol
+    (lib_id "Device:LED")
+    (at 76.2 55.88 90)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "d2-led")
+    (property "Reference" "D2" (at 80.01 54.61 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "PWR_LED" (at 80.01 57.15 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "LED_SMD:LED_0603_1608Metric" (at 76.2 55.88 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C72043" (at 76.2 55.88 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; LED current limiting resistor
+  (symbol
+    (lib_id "Device:R")
+    (at 76.2 45.72 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "r2-led")
+    (property "Reference" "R2" (at 78.74 44.45 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "1K" (at 78.74 46.99 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Resistor_SMD:R_0603_1608Metric" (at 76.2 45.72 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C21190" (at 76.2 45.72 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; Series resistor for trigger output
+  (symbol
+    (lib_id "Device:R")
+    (at 165.1 101.6 90)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "r3-trig")
+    (property "Reference" "R3" (at 165.1 99.06 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "100R" (at 165.1 104.14 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Resistor_SMD:R_0603_1608Metric" (at 165.1 101.6 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C22775" (at 165.1 101.6 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; Input protection resistor
+  (symbol
+    (lib_id "Device:R")
+    (at 127 139.7 90)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "r4-input")
+    (property "Reference" "R4" (at 127 137.16 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "10K" (at 127 142.24 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Resistor_SMD:R_0603_1608Metric" (at 127 139.7 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C25804" (at 127 139.7 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; Voltage divider resistors for input (5V to 3.3V)
+  (symbol
+    (lib_id "Device:R")
+    (at 134.62 132.08 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "r5-divider-top")
+    (property "Reference" "R5" (at 137.16 130.81 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "10K" (at 137.16 133.35 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Resistor_SMD:R_0603_1608Metric" (at 134.62 132.08 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C25804" (at 134.62 132.08 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  (symbol
+    (lib_id "Device:R")
+    (at 134.62 149.86 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "r6-divider-bot")
+    (property "Reference" "R6" (at 137.16 148.59 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "22K" (at 137.16 151.13 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Resistor_SMD:R_0603_1608Metric" (at 134.62 149.86 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C25774" (at 134.62 149.86 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; Input protection diodes
+  (symbol
+    (lib_id "Device:D_Schottky")
+    (at 139.7 124.46 90)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "d3-clamp-high")
+    (property "Reference" "D3" (at 143.51 123.19 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "BAT54" (at 143.51 125.73 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Diode_SMD:D_SOD-323" (at 139.7 124.46 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C181104" (at 139.7 124.46 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  (symbol
+    (lib_id "Device:D_Schottky")
+    (at 139.7 157.48 90)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "d4-clamp-low")
+    (property "Reference" "D4" (at 143.51 156.21 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "BAT54" (at 143.51 158.75 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Diode_SMD:D_SOD-323" (at 139.7 157.48 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C181104" (at 139.7 157.48 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; Decoupling caps for ICs
+  (symbol
+    (lib_id "Device:C")
+    (at 152.4 88.9 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "c4-u2-decoup")
+    (property "Reference" "C4" (at 154.94 87.63 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "100nF" (at 154.94 90.17 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Capacitor_SMD:C_0603_1608Metric" (at 152.4 88.9 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C14663" (at 152.4 88.9 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  (symbol
+    (lib_id "Device:C")
+    (at 152.4 152.4 0)
+    (unit 1)
+    (exclude_from_sim no)
+    (in_bom yes)
+    (on_board yes)
+    (uuid "c5-u3-decoup")
+    (property "Reference" "C5" (at 154.94 151.13 0) (effects (font (size 1.27 1.27))))
+    (property "Value" "100nF" (at 154.94 153.67 0) (effects (font (size 1.27 1.27))))
+    (property "Footprint" "Capacitor_SMD:C_0603_1608Metric" (at 152.4 152.4 0) (effects (font (size 1.27 1.27)) hide))
+    (property "LCSC" "C14663" (at 152.4 152.4 0) (effects (font (size 1.27 1.27)) hide))
+  )
+
+  ;; Text annotations
+  (text "POWER SECTION" (at 40.64 15.24 0) (effects (font (size 2.54 2.54))))
+  (text "MCU & DISPLAY" (at 114.3 25.4 0) (effects (font (size 2.54 2.54))))
+  (text "TRIGGER OUTPUT" (at 152.4 78.74 0) (effects (font (size 2.54 2.54))))
+  (text "RETURN INPUT" (at 152.4 116.84 0) (effects (font (size 2.54 2.54))))
+  (text "BUTTON" (at 101.6 78.74 0) (effects (font (size 2.54 2.54))))
+
+  ;; Net labels
+  (label "TRIG_OUT_MCU" (at 134.62 101.6 0) (effects (font (size 1.27 1.27))))
+  (label "RETURN_IN_MCU" (at 170.18 139.7 180) (effects (font (size 1.27 1.27))))
+  (label "BTN" (at 109.22 101.6 0) (effects (font (size 1.27 1.27))))
+  (label "SDA" (at 132.08 48.26 0) (effects (font (size 1.27 1.27))))
+  (label "SCL" (at 132.08 50.8 0) (effects (font (size 1.27 1.27))))
+
+)