diff --git a/Sources/SstUI/Instruments.swift b/Sources/SstUI/Instruments.swift
new file mode 100644
index 0000000..d5e2913
--- /dev/null
+++ b/Sources/SstUI/Instruments.swift
@@ -0,0 +1,110 @@
+import CoreGraphics
+import Foundation
+
+/// dBFS → VU → needle position and angle, all from the meter tokens.
+public enum VUScale {
+    public static func vu(fromDbfs dbfs: Double) -> Double {
+        dbfs - MeterTokens.vuReferenceDbfs
+    }
+
+    /// 0 = bottom of the scale, 1 = top. The needle may swing slightly past the end (up to 1.03).
+    public static func fraction(forVU vu: Double) -> Double {
+        min(1.03, max(0, (vu - MeterTokens.vuMin) / (MeterTokens.vuMax - MeterTokens.vuMin)))
+    }
+
+    public static func fraction(forDbfs dbfs: Double) -> Double {
+        fraction(forVU: vu(fromDbfs: dbfs))
+    }
+
+    /// Radians from vertical; negative is left of vertical.
+    public static func angle(forFraction fraction: Double) -> Double {
+        (-MeterTokens.vuArcDegrees / 2 + MeterTokens.vuArcDegrees * fraction) * .pi / 180
+    }
+}
+
+/// Second-order needle: reaches ~99 % in `response` seconds, slight overshoot for damping < 1.
+public struct NeedleSpring: Equatable {
+    public private(set) var position: Double
+    public private(set) var velocity: Double = 0
+    private let omega: Double
+    private let zeta: Double
+
+    public init(response: Double = Motion.needleResponse, damping: Double = Motion.needleDamping, position: Double = 0) {
+        self.zeta = damping
+        self.omega = 4.6 / (damping * response)
+        self.position = position
+    }
+
+    /// Advances the simulation by `dt` seconds in small fixed sub-steps (stable for any frame time).
+    @discardableResult
+    public mutating func step(target: Double, dt: Double) -> Double {
+        var remaining = max(0, dt)
+        let h = 1.0 / 240.0
+        while remaining > 1e-9 {
+            let s = min(h, remaining)
+            let acceleration = omega * omega * (target - position) - 2 * zeta * omega * velocity
+            velocity += acceleration * s
+            position += velocity * s
+            remaining -= s
+        }
+        return position
+    }
+}
+
+public enum SegmentID: CaseIterable, Hashable {
+    case a, b, c, d, e, f, g
+}
+
+/// Which segments of a seven-segment digit are lit, and where each segment sits (from the segment tokens).
+public enum SegmentGlyph {
+    static let map: [Character: Set<SegmentID>] = [
+        "0": [.a, .b, .c, .d, .e, .f], "1": [.b, .c], "2": [.a, .b, .d, .e, .g], "3": [.a, .b, .c, .d, .g],
+        "4": [.b, .c, .f, .g], "5": [.a, .c, .d, .f, .g], "6": [.a, .c, .d, .e, .f, .g], "7": [.a, .b, .c],
+        "8": Set(SegmentID.allCases), "9": [.a, .b, .c, .d, .f, .g],
+    ]
+
+    public static func lit(for character: Character) -> Set<SegmentID> {
+        map[character] ?? []
+    }
+
+    /// Hexagonal bar for one segment, inside a `Segment.digitWidth` × `Segment.digitHeight` box.
+    public static func polygon(_ id: SegmentID) -> [CGPoint] {
+        let width = Segment.digitWidth, height = Segment.digitHeight
+        let h = Segment.thickness / 2, gap = Segment.gap
+        let left = h, right = width - h, mid = height / 2
+
+        func horizontal(_ x0: CGFloat, _ x1: CGFloat, _ cy: CGFloat) -> [CGPoint] {
+            [CGPoint(x: x0, y: cy), CGPoint(x: x0 + h, y: cy - h), CGPoint(x: x1 - h, y: cy - h),
+             CGPoint(x: x1, y: cy), CGPoint(x: x1 - h, y: cy + h), CGPoint(x: x0 + h, y: cy + h)]
+        }
+        func vertical(_ cx: CGFloat, _ y0: CGFloat, _ y1: CGFloat) -> [CGPoint] {
+            [CGPoint(x: cx, y: y0), CGPoint(x: cx + h, y: y0 + h), CGPoint(x: cx + h, y: y1 - h),
+             CGPoint(x: cx, y: y1), CGPoint(x: cx - h, y: y1 - h), CGPoint(x: cx - h, y: y0 + h)]
+        }
+        let x0 = left + h + gap, x1 = right - h - gap
+        switch id {
+        case .a: return horizontal(x0, x1, h)
+        case .g: return horizontal(x0, x1, mid)
+        case .d: return horizontal(x0, x1, height - h)
+        case .f: return vertical(left, h + h + gap, mid - h - gap)
+        case .b: return vertical(right, h + h + gap, mid - h - gap)
+        case .e: return vertical(left, mid + h + gap, height - h - h - gap)
+        case .c: return vertical(right, mid + h + gap, height - h - h - gap)
+        }
+    }
+}
+
+public enum LEDBar {
+    public enum Tone { case green, amber, red }
+
+    /// The last segment is red, the three before it amber, the rest green.
+    public static func tone(forSegment index: Int, segments: Int = MeterTokens.ledSegments) -> Tone {
+        if index >= segments - 1 { return .red }
+        if index >= segments - 4 { return .amber }
+        return .green
+    }
+
+    public static func litCount(forFraction fraction: Double, segments: Int = MeterTokens.ledSegments) -> Int {
+        Int((min(1, max(0, fraction)) * Double(segments)).rounded())
+    }
+}
diff --git a/Tests/SstUITests/InstrumentsTests.swift b/Tests/SstUITests/InstrumentsTests.swift
new file mode 100644
index 0000000..145eb0e
--- /dev/null
+++ b/Tests/SstUITests/InstrumentsTests.swift
@@ -0,0 +1,108 @@
+import CoreGraphics
+import Testing
+@testable import SstUI
+
+struct VUScaleTests {
+    @Test func zeroVUIsMinusTwentyDbfs() {
+        #expect(VUScale.vu(fromDbfs: -20) == 0)
+        #expect(VUScale.vu(fromDbfs: -40) == -20)
+    }
+
+    @Test func fractionSpansTheScale() {
+        #expect(VUScale.fraction(forVU: -20) == 0)
+        #expect(abs(VUScale.fraction(forVU: 3) - 1) < 1e-9)
+        #expect(abs(VUScale.fraction(forVU: 0) - 20.0 / 23.0) < 1e-9)
+    }
+
+    @Test func fractionIsClampedAtBothEnds() {
+        #expect(VUScale.fraction(forVU: -60) == 0)
+        #expect(VUScale.fraction(forVU: 30) == 1.03)
+        #expect(VUScale.fraction(forDbfs: -120) == 0)
+    }
+
+    @Test func angleIsSymmetricAroundVertical() {
+        #expect(abs(VUScale.angle(forFraction: 0) + VUScale.angle(forFraction: 1)) < 1e-9)
+        #expect(abs(VUScale.angle(forFraction: 0.5)) < 1e-9)
+        #expect(abs(VUScale.angle(forFraction: 1) - 45 * .pi / 180) < 1e-9)
+    }
+}
+
+struct NeedleSpringTests {
+    func simulate(_ spring: NeedleSpring, seconds: Double, target: Double = 1) -> (final: Double, peak: Double) {
+        var spring = spring
+        var peak = 0.0
+        let dt = 1.0 / 240
+        for _ in 0..<Int(seconds / dt) { peak = max(peak, spring.step(target: target, dt: dt)) }
+        return (spring.position, peak)
+    }
+
+    @Test func reachesTheTargetWithinTheResponseTime() {
+        let result = simulate(NeedleSpring(), seconds: Motion.needleResponse)
+        #expect(abs(result.final - 1) < 0.02)
+    }
+
+    @Test func overshootsSlightlyWithTheTokenDamping() {
+        let result = simulate(NeedleSpring(), seconds: 1)
+        #expect(result.peak > 1.003)
+        #expect(result.peak < 1.03)
+        #expect(abs(result.final - 1) < 0.001)
+    }
+
+    @Test func criticalDampingNeverOvershoots() {
+        let result = simulate(NeedleSpring(damping: 1), seconds: 1)
+        #expect(result.peak <= 1.000001)
+    }
+
+    @Test func largeFrameStepsStayStable() {
+        var spring = NeedleSpring()
+        for _ in 0..<40 { spring.step(target: 1, dt: 0.05) }
+        #expect(abs(spring.position - 1) < 0.001)
+    }
+
+    @Test func restsAtTheStartWithoutATarget() {
+        var spring = NeedleSpring()
+        for _ in 0..<60 { spring.step(target: 0, dt: 1.0 / 60) }
+        #expect(spring.position == 0)
+    }
+}
+
+struct SegmentGlyphTests {
+    @Test func everyDigitHasASegmentPattern() {
+        for digit in "0123456789" { #expect(!SegmentGlyph.lit(for: digit).isEmpty) }
+        #expect(SegmentGlyph.lit(for: "8").count == 7)
+        #expect(SegmentGlyph.lit(for: "1") == [.b, .c])
+        #expect(SegmentGlyph.lit(for: ":").isEmpty)
+    }
+
+    @Test func polygonsStayInsideTheDigitBoxAndDiffer() {
+        var seen: [[CGPoint]] = []
+        for id in SegmentID.allCases {
+            let polygon = SegmentGlyph.polygon(id)
+            #expect(polygon.count == 6)
+            for point in polygon {
+                #expect(point.x >= 0 && point.x <= Segment.digitWidth)
+                #expect(point.y >= 0 && point.y <= Segment.digitHeight)
+            }
+            #expect(!seen.contains(polygon))
+            seen.append(polygon)
+        }
+        #expect(seen.count == 7)
+    }
+}
+
+struct LEDBarTests {
+    @Test func tonesFollowTheEightThreeOneSplit() {
+        let tones = (0..<12).map { LEDBar.tone(forSegment: $0) }
+        #expect(tones.prefix(8).allSatisfy { $0 == .green })
+        #expect(tones[8...10].allSatisfy { $0 == .amber })
+        #expect(tones[11] == .red)
+    }
+
+    @Test func litCountScalesAndClamps() {
+        #expect(LEDBar.litCount(forFraction: 0) == 0)
+        #expect(LEDBar.litCount(forFraction: 0.5) == 6)
+        #expect(LEDBar.litCount(forFraction: 1) == 12)
+        #expect(LEDBar.litCount(forFraction: 1.03) == 12)
+        #expect(LEDBar.litCount(forFraction: -1) == 0)
+    }
+}