resistor

A Go library for working with fixed resistors. Decode and encode color band and SMD markings, snap values to IEC preferred series, and infer unknown properties from physical observations.

No UI dependencies. Import it as a library, or use the included CLI, TUI, and web app.

Contents

• Library
• CLI
• TUI
• Web
• Deployment
• Development
• License

---

Library

go get github.com/sss7526/resistor

Color band decode and encode

Supports 4-band, 5-band, and 6-band resistors per IEC 60062:

bands := []resistor.Color{resistor.Green, resistor.Brown, resistor.Brown, resistor.Gold}
spec, err := resistor.DecodeBands(bands)
// spec.ResistanceOhms == 510, spec.TolerancePct == 5

spec := resistor.ResistorSpec{ResistanceOhms: 510, TolerancePct: 5}
bands, err := resistor.EncodeBands(spec)
// []Color{Green, Brown, Brown, Gold}

6-band resistors include a temperature coefficient in the sixth band.

SMD markings

Supports 3-digit, 4-digit, R-notation, and EIA-96 formats:

spec, err := resistor.DecodeSMD("472")   // 4700 ohms
spec, err := resistor.DecodeSMD("4R7")   // 4.7 ohms
spec, err := resistor.DecodeSMD("01C")   // EIA-96

marking, err := resistor.EncodeSMD(4700, resistor.SMDAuto)
// "472"

E-series value selection

Snap an arbitrary resistance to the nearest IEC 60063 preferred series value (E3 through E192):

v, err := resistor.NearestStandard(487, resistor.E24, resistor.RoundNearest) // 510
v, err := resistor.NearestStandard(487, resistor.E12, resistor.RoundUp)      // 560

Standard resistor selection

Returns the snapped value, color bands, and all defaults applied:

result, err := resistor.SelectStandardResistor(resistor.SelectionRequest{
    Resistance: 487,
    Series:     resistor.E24,
})
// result.SelectedResistance == 510
// result.Bands              == [Green, Brown, Brown, Gold]
// result.Assumptions        == ["Tolerance defaulted to +/-5%", ...]

Unspecified fields default to E24, 5% tolerance, and nearest rounding.

Physical inference

Estimate unknown properties from any combination of bands, SMD marking, body color, length, and package type:

result, err := resistor.InferResistor(resistor.ObservedResistor{
    Bands:     []resistor.Color{resistor.Brown, resistor.Black, resistor.Red, resistor.Gold},
    BodyColor: resistor.Blue,
    LengthMM:  6.3,
})
// result.Spec.ResistanceOhms == 1000
// result.Spec.PowerWatts     == 0.25         (inferred from length)
// result.Spec.Type           == "metal_film" (inferred from body color)
// result.Meta.Confidence     == 0.92
// result.Meta.Assumptions    == ["Blue body assumed metal film", ...]

Decoded facts always take precedence over heuristic estimates. Confidence is a value in [0.0, 1.0].

Engineering analysis

Accepts voltage or current (or both, checked for Ohm's Law consistency):

report, err := resistor.AnalyzeResistor(resistor.AnalysisInput{
    Spec:           resistor.ResistorSpec{ResistanceOhms: 100, PowerWatts: 0.25, TolerancePct: 5},
    AppliedVoltage: 10,
})
// report.Current                == 0.1 A
// report.PowerDissipation       == 0.1 W
// report.DeratedSafePower       == 0.125 W
// report.WorstCaseResistanceMin == 95 ohms
// report.WorstCaseResistanceMax == 105 ohms
// report.Warnings               == [{Caution, "Power dissipation exceeds 50% derated threshold"}]

---

CLI

go install github.com/sss7526/resistor/cmd/resistor-cli@latest

All commands accept --json for machine-readable output.

Select a standard value:

resistor-cli select 487
resistor-cli select 487 --series E12 --tol 1 --round up

Infer from physical observations:

resistor-cli infer --bands brown,black,red,gold
resistor-cli infer --bands brown,black,red,gold --body blue --length 6.3
resistor-cli infer --smd 472 --pkg 0603

Analyze under electrical conditions:

resistor-cli analyze --r 100 --v 10 --pwr 0.25 --tol 5
resistor-cli analyze --r 100 --i 0.1

Decode and encode SMD markings:

resistor-cli smd decode 472
resistor-cli smd decode 4R7
resistor-cli smd encode 4700

---

TUI

An interactive terminal interface for all operations.

go install github.com/sss7526/resistor/cmd/resistor-tui@latest

Navigate with arrow keys, confirm with Enter, return to menu with Escape, quit with q or Ctrl+C.

---

Web

The WASM module exposes all library operations as browser-callable JavaScript functions. The included server embeds the compiled WASM and all static assets into a single binary.

Build

make build-wasm           # standard Go WASM (~3.4 MB uncompressed)
make build-server         # server + standard WASM
make build-server-tinygo  # server + TinyGo WASM (~430 KB gzip, ~1.1 MB uncompressed)

TinyGo must be installed for the TinyGo targets. See tinygo.org/getting-started/install.

Install to GOPATH/bin:

make install-server
make install-server-tinygo

Run

./bin/resistor-server               # listens on :8080
./bin/resistor-server --addr :9000  # custom port
RESISTOR_ADDR=:9000 ./bin/resistor-server

The server sets strict security headers (CSP with per-request nonce, COEP, COOP, X-Frame-Options) and is designed to sit behind a TLS-terminating reverse proxy.

JavaScript API

All functions live on the resistor global and return {ok: true, value: ...} on success or {ok: false, error: "..."} on failure. Inputs are JSON strings.

Function	Input	Returns
decodeBands	'["green","brown","brown","gold"]'	ResistorSpec
encodeBands	'{"resistanceOhms":510,"tolerancePct":5}'	Color[]
decodeSMD	"472"	ResistorSpec
encodeSMD	'{"resistance":4700,"mode":"auto"}'	marking string
nearestStandard	'{"value":487,"series":"E24","mode":"nearest"}'	number
selectStandardResistor	'{"resistance":487}'	SelectionResult
inferResistor	'{"bands":[...],"bodyColor":"blue","lengthMM":6.3}'	InferenceResult
analyzeResistor	'{"spec":{...},"appliedVoltage":10}'	AnalysisReport

---

Deployment

The server binary is self-contained and designed to run behind Caddy (or any reverse proxy). Caddy handles TLS automatically via Let's Encrypt.

Docker

# Standard Go WASM
docker build -t resistor .
docker run --rm -p 8080:8080 resistor

# TinyGo WASM (~430 KB gzip)
docker build --build-arg WASM=tinygo -t resistor .

The final image is built on scratch and runs as UID 10001.

Docker Compose + Caddy

The included docker-compose.yml pulls sss7526/resistor:latest from Docker Hub and runs it behind Caddy. Set RESISTOR_HOST to your public domain and start:

RESISTOR_HOST=resistor.example.com docker compose up -d

Caddy provisions TLS automatically. For local testing (HTTP only, no cert needed):

docker compose up

Certificate data is persisted in the caddy_data named volume. Edit Caddyfile before starting Compose to add rate limiting, authentication, or other directives.

Watchtower runs as a sidecar and polls Docker Hub every 5 minutes. When a new :latest image is pushed it pulls and restarts the resistor container automatically. Caddy is intentionally excluded from Watchtower's watch scope.

---

Development

Requires Go 1.26 or later.

Build:

make build        # CLI + TUI to bin/
make build-cli
make build-tui
make build-server

Test:

make test         # unit tests
make test-cli     # CLI integration tests
make test-all     # both
make smoke        # end-to-end smoke tests against the built binary

Fuzz:

make fuzz
make fuzz FUZZTIME=60s

Benchmark:

go test -bench=. -benchtime=1s ./...
go test -bench=BenchmarkInferResistor -benchmem ./...

Lint and vulnerability check:

make lint
make vuln

---

Contributing

See CONTRIBUTING.md.

---

License

MIT. See LICENSE.