circuit-design

name	circuit-design
description	Use when designing an electronic circuit from requirements, choosing topology or regulator type, selecting component values, planning protection circuits, or deciding what goes on a board before schematic capture begins. Also use when the user asks "what components do I need" or "how should I power this."

NEVER use the Read, Write, or Edit tools on KiCad files (.kicad_sch, .kicad_pcb, .kicad_sym, .kicad_mod, .kicad_pro, .kicad_prl). ALL KiCad file manipulation MUST go through the kicad MCP tools. NEVER run kicad-cli commands via Bash. If an MCP tool returns an error, try different parameters — do NOT fall back to manual file editing.

EVERY KiCad operation has a corresponding MCP tool. Do NOT claim a tool does not exist without first listing all available tools. Key tools that MUST be used instead of file writes:

• add_symbol — create custom symbol definitions in .kicad_sym files
• create_symbol_library — create new .kicad_sym library files
• create_schematic — create new .kicad_sch files
• create_project — create new .kicad_pro project files If you find yourself thinking "there's no MCP tool for this," you are wrong. Check the tool list again.

Circuit Design

Design the circuit completely — every component, every value, every rating — before opening the schematic editor. This skill produces a component list and block diagram that feeds directly into the schematic-design skill.

Response Format

When this skill activates, print exactly:

Using circuit-design to design the circuit before schematic capture.

Then, if the user has not already described what they want, ask exactly:

What circuit are we designing? I need:

1. Function — what does it do?
2. Input power — voltage range, AC/DC
3. Output requirements — voltages, currents, signals
4. Environment — indoor/outdoor, temperature, enclosure
5. Specific parts — any components you already want to use?

Or if you have a BOM already, I can run validation-only mode.

Do not list what you found in the project. Do not suggest example circuits or subsystems. Wait for the user's answer before proceeding to the Process checklist.

If the user already described the circuit (in the same message that invoked this skill, or in prior conversation), skip the question and proceed directly to step 1 of the Process.

Process

1. Clarify requirements — voltage, current, environment, cost, form factor. Ask the user if anything is ambiguous.
2. Gather constraints — before topology selection:
   • Board size constraints (if known)
   • Mechanical constraints (connector positions, mounting holes)
   • Thermal constraints (ambient temp, enclosure, airflow)
   • EMI/EMC requirements
   • Target manufacturer capabilities (trace width, layer count)
   • IPC design class (1 = consumer, 2 = industrial, 3 = high-rel). Default to Class 2 if unknown. This determines trace width, spacing, and annular ring minimums downstream. Record in BOM artifact header. Use defaults for unknowns.
3. Block diagram — decompose into functional stages. Name each stage and define the interface between them (voltage, signal type).
4. Topology selection — choose a circuit topology for each block.
5. Component selection — pick every part with value, rating, and package. Verify against KiCad symbol libraries.
6. Protection and filtering — add input protection, decoupling, and EMI filtering. These are not optional.
7. Bill of materials — produce the final BOM table.
8. Hand off to schematic-design for placement and wiring.

Topology Quick Reference

Need	First choice	When to upgrade
Fixed step-down, <500mA	LDO (e.g., AMS1117, LM1117)	Dropout < 1V, low noise needed
Fixed step-down, 0.5–3A	Buck converter (e.g., LM2596, MP1584)	Efficiency matters, dropout > 1V
Step-down, >3A	Synchronous buck (e.g., TPS54360)	High current, tight regulation
Step-up	Boost converter (e.g., MT3608, TPS61040)	Vout > Vin required
Negative voltage	Inverting buck-boost or charge pump	Analog circuits needing -V
Isolation	Flyback with optocoupler	Safety, galvanic isolation
Relay/solenoid drive	N-ch MOSFET low-side switch	Inductive load, >50mA
Motor (brushed DC)	H-bridge driver IC	Bidirectional control needed
LED constant current	Linear CC driver or buck CC	String voltage determines choice
Signal level shift	BSS138 MOSFET level shifter	Bidirectional, I2C/SPI
ADC input	Voltage divider + buffer opamp	High-impedance source

These are starting points. Always verify against the specific requirements — an LDO is wrong if the dropout is too high even if current is <500mA.

Protection Patterns

Every board needs protection. Select from these standard patterns:

Input protection (power entry):

• Fuse → TVS diode → bulk capacitor
• Fuse: rated 1.5–2x normal operating current
• TVS: clamp voltage below max Vin of downstream regulator
• Bulk cap: 10–100uF electrolytic/ceramic at input

Reverse polarity protection:

• P-ch MOSFET (preferred, low loss) — gate to ground, source to Vin
• Series Schottky diode (simple, 0.3–0.5V drop)

ESD protection (connectors):

• TVS array on data lines (USB, UART, SPI exposed externally)
• Place physically close to the connector

Inductive load protection:

• Flyback diode (1N4148 for small relays, 1N4007 for larger)
• Diode cathode to supply, anode to drain

Component Selection Rules

Voltage rating: Derate to 60–80% of rated voltage. A 16V cap on a 12V rail, not a 16V cap on a 15V rail.

Current rating: Derate to 70–80%. A 2A fuse for 1.2A continuous.

Capacitor values from datasheets: For regulators, the datasheet specifies input and output capacitor values and ESR requirements. Use those values, not arbitrary ones. If the datasheet says 22uF low-ESR on the output, use 22uF low-ESR.

Resistor values from datasheets: Feedback dividers, current-sense resistors, and timing resistors are calculated from datasheet formulas. Show the formula and calculation for each.

Preferred values: Use E12 or E24 series values (1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2). Do not specify values like 2.35k — round to nearest preferred value (2.2k or 2.4k) and recalculate to verify the result is within tolerance.

Packages: Default to 0805 for resistors and capacitors unless space-constrained (then 0603). Use through-hole for prototyping if the user requests it.

Verifying Symbols AND Footprints (Mandatory)

Before finalizing the BOM, you MUST confirm each component has BOTH a symbol and a footprint in KiCad's libraries. This is blocking — do not proceed until every row is verified.

For each component:

1. Call list_lib_symbols on the expected library — confirm the exact symbol name exists
2. Call list_lib_footprints on the expected footprint library — confirm the footprint exists
3. If either is not found, resolve immediately — pick a generic symbol/footprint or create a custom one via add_symbol
4. Record the verified lib_id and footprint in the BOM

Common symbol libraries:

• Regulator_Linear.kicad_sym — LDOs, linear regulators
• Regulator_Switching.kicad_sym — buck, boost, flyback controllers
• Device.kicad_sym — R, C, L, D, LED, fuse, ferrite bead
• Transistor_FET.kicad_sym — MOSFETs
• Transistor_BJT.kicad_sym — BJTs
• Connector_Generic.kicad_sym — pin headers, screw terminals
• Relay.kicad_sym — relays
• Diode.kicad_sym — diodes, TVS, Zener, Schottky
• power.kicad_sym — VCC, GND, +3V3, +5V, etc.

If an exact part is not in the library:

• Generic symbol: Use a generic symbol with the correct pin count and set the value property to the actual part number.
• Custom symbol via add_symbol: For ICs and complex parts, use the add_symbol MCP tool to create a custom symbol in a project-local .kicad_sym library. NEVER write .kicad_sym files with the Write or Edit tools — always use add_symbol.

MCP Tools for This Skill

These are the kicad MCP tools you should be using during circuit design:

Library browsing (verify parts exist):

• list_lib_symbols — list symbols in a .kicad_sym library
• get_symbol_info — get pin names, types, and properties for a symbol

Custom symbol creation (when parts aren't in built-in libs):

• create_symbol_library — create a new .kicad_sym library file
• add_symbol — define a custom symbol with pins, footprint, datasheet

Project setup:

• create_project — create a new KiCad project (.kicad_pro)
• create_schematic — create a new schematic sheet
• create_sym_lib_table — register symbol libraries with the project

Footprint verification:

• list_lib_footprints — list footprints in a .pretty directory
• get_footprint_info — check pad dimensions and pin mapping

Visual verification (optional):

• export_symbol_svg — export symbol library to SVG for visual review
• export_footprint_svg — export footprint library to SVG for visual review

Library maintenance:

• upgrade_symbol_lib — upgrade symbol library to current KiCad format
• upgrade_footprint_lib — upgrade footprint library to current format

Output: BOM Artifact (specs/bom.md)

Write the validated BOM to specs/bom.md in the KiCad project directory (the directory containing the .kicad_pro file).

# BOM: [Project Name]

## Constraints
- Input: [voltage range]
- Board size: [dimensions] (if known, else "TBD")
- Layers: [count] (default: 2)
- IPC class: [1/2/3] (default: 2)
- Manufacturer min trace: [width] (default: 0.15mm)
- Thermal: [ambient temp, enclosure, airflow] (if known)
- EMI/EMC: [requirements] (if any)

## Block Diagram
[stage names and interfaces between them]

## Components
| Ref | lib_id (verified) | Value | Rating | Package | Stage | Footprint (verified) | Notes |
|-----|-------------------|-------|--------|---------|-------|---------------------|-------|
| F1  | Device:Fuse       | 2A    | 32V    | 1206    | Input protection | Fuse_1206 | Slow-blow |
| ...

The lib_id and Footprint columns contain identifiers that were resolved against actual KiCad libraries via MCP tool calls, not guessed from training data.

Every component must appear in this table before the next phase begins. This includes:

• Decoupling caps for every IC
• Feedback resistors with calculated values
• Bootstrap caps if the regulator requires them
• Pull-up/pull-down resistors
• Test points (if needed)
• Mounting holes (if needed)

BOM Reviewer

After writing specs/bom.md, dispatch the BOM reviewer subagent:

Agent(
  prompt="<contents of agents/bom-reviewer.md>\n\nBOM path: <project_dir>/specs/bom.md",
  subagent_type="general-purpose"
)

Fix any issues reported by the reviewer, update specs/bom.md, and re-dispatch until the reviewer returns APPROVED.

User must explicitly approve the BOM before schematic-plan can start. "I already know the components" is not approval.

User rejection handling: If the user rejects the BOM or requests changes, fix the specific issues and re-run the BOM reviewer on the updated artifact. Do not restart from scratch unless the user asks.

Validation-Only Mode (User-Provided BOM)

When the user provides their own component list or BOM, skip topology and component selection but still validate everything:

1. Skip: Steps 3–6 (block diagram, topology selection, component selection, protection patterns) — the user has already made these decisions.
2. Do: Gather constraints (Step 2) — fill missing fields with defaults (IPC Class 2, 2-layer, 0.15mm min trace, etc.).
3. Do: For every component in the user's list, call list_lib_symbols to resolve the exact lib_id and list_lib_footprints to resolve the footprint.
4. Do: Fill in missing BOM columns (Rating, Package, Stage) with reasonable defaults or ask the user.
5. Do: Write specs/bom.md in the standard artifact format.
6. Do: Dispatch the BOM reviewer subagent.

The user's BOM is accepted, then audited. Validation-only mode skips decisions but does NOT skip verification.

Rationalization Prevention

Thought	Reality
"This symbol name is probably right"	Call list_lib_symbols. "Probably" caused the Q_PMOS_GSD failure.
"Standard 100nF decoupling is fine for this IC"	Read the datasheet. The IC specifies what it needs.
"I know this footprint exists"	Call list_lib_footprints. Verify, don't assume.
"The rating is close enough"	Show the derating math. 60-80% of rated voltage, 70-80% of rated current.

Checklist

IMPORTANT: Use TodoWrite to create todos for EACH checklist item below.

• Clarify requirements and gather constraints
• Create block diagram with named stages
• Select topology for each stage
• Select components with values and ratings
• Add protection and filtering circuits
• Verify ALL lib_ids via list_lib_symbols
• Verify ALL footprints via list_lib_footprints
• Write specs/bom.md artifact
• Dispatch BOM reviewer subagent
• Get user approval on BOM

Handoff to Schematic Plan

The BOM artifact is the input to the schematic-plan skill. Before handing off, verify:

• specs/bom.md exists and BOM reviewer returned APPROVED
• User has explicitly approved the BOM
• Every IC has decoupling capacitors listed
• Every calculated value shows its formula
• Every voltage/current rating has derating margin
• Protection components are included for every external interface
• All lib_ids verified via list_lib_symbols
• All footprints verified via list_lib_footprints
• Block diagram shows signal flow and named nets between stages