LED Feedback Power Budget

LED Feedback Power Budget

Ada re-derives this chapter’s own numbers step by step, at full precision

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Ada ADA · CALCULATION AUDIT

LED Feedback Power Budget

The chapter budgets visual feedback from real parts: a single indicator LED at 2.1 V and 20 mA, and a 30-pixel WS2812B strip whose pixels each draw about 60 mA at full white, all sized with a 20% supply margin. It is tempting to treat an alert colour as a free software choice. This audit re-derives the LED, PWM, and addressable-strip figures and asks whether the device can actually show its alert without browning out the supply or draining the battery.

Companion to the chapter Visual and Audio Actuators — every number here comes from that chapter.

See the relationship before changing it

The figure reads from left to right. The blue card is full-white pixels. The middle card applies this page's rule. The green card is strip current. Walk the arrows once: set the input, apply the rule, then read the result with its unit.

The retained audit below checks several chapter fixtures. This model keeps those stated values fixed and changes only full-white pixels, so the numeric fixture does not switch without explanation.

Full-white pixels changes strip current An input card leads through the rule current = pixels x 60 mA / 1,000 to the strip current result. INPUT PAGE INPUT APPLY THE RULE predict calculate check units OUTPUT RESULT
Walk the arrows. A software colour becomes a supply load when many addressable pixels light together.

Derive the baseline in four named moves

  1. 1

    Name the input. The chapter baseline is 30 pixels.

  2. 2

    Name the relationship. current = pixels x 60 mA / 1,000

  3. 3

    Substitute with units. 30 x 60 mA / 1,000 = 1.80 A

  4. 4

    Read the result. Keep the unit beside the value. Use it only inside the technical boundary on this page.

Predict, then change full-white pixels

Try Predict the direction of current = pixels x 60 mA / 1,000. Test another full-white pixels, then compare strip current.

30 pixels
Chapter baseline
Strip current

Observe A software colour becomes a supply load when many addressable pixels light together. Reset full-white pixels to 30 and compare strip current.

Explain A software colour becomes a supply load when many addressable pixels light together.

Check yourself

What should you do before trusting a moved-control result?
Answer: Predict its direction, apply the shown relationship, keep the units, and reset to the worked baseline.
What does this small model leave out?
Answer: Only full-white pixels moves here. Field effects named in the technical boundary stay fixed.

Honesty note. This teaching model changes only full-white pixels. It is not a field prediction.

Ada: Visual feedback is not just a color choice. The LED physics, PWM timing, and addressable-strip current budget decide whether the device can show the alert without browning out the supply or wasting battery. Let me carry every value in the chapter’s own table to full precision.

  • Single LED at full brightness. P = V_f x I_f = 2.1 x 0.020 = 0.042 W – about 42 mW for one full-bright indicator.
  • 50% PWM current and power. Average current is 0.50 x 20 mA = 10 mA, so P = 2.1 x 0.010 = 0.021 W – about 21 mW, half the full-brightness draw, because PWM scales average current linearly rather than lowering the LED’s forward voltage.
  • 24-hour PWM saving. (0.042 - 0.021) x 24 = 0.504 Wh, rounding at the end to the chapter’s “about 0.5 Wh” saved per LED-day.
  • 5 kHz PWM timing. One period is 1 / 5000 = 0.0002 s = 0.2 ms. At 50% duty that splits into 0.2 x 0.50 = 0.1 ms on and 0.1 ms off – about 100 microseconds each, far faster than the eye’s flicker-fusion threshold.
  • 8-bit brightness step. One PWM code is 1 / 255 = 0.0039216 of full scale, or 0.392%. On the chapter’s 5 V driver that step is 5 x 0.0039216 = 0.0196 V, about 19.6 mV of ideal average – the finest brightness change the 8-bit dimmer can resolve.
  • 30-pixel strip at full white. 30 x 60 mA = 1800 mA = 1.8 A, so the strip alone draws 5 x 1.8 = 9 W from the 5 V rail. Adding the chapter’s 20% safety margin gives 1.8 x 1.20 = 2.16 A as the supply floor.
  • The code’s actual ceiling. strip.setBrightness(50) caps output at 50 / 255 = 0.1961 of full scale, so the worst full-white frame the firmware can actually produce draws 1.8 x 0.1961 = 0.353 A, or 5 x 0.353 = 1.76 W – comfortably under the 2.16 A supply, but only while that brightness cap stays in the code.

Every one of these figures reproduces the chapter’s own table to the digit, so both design rules stand: PWM genuinely halves LED power at 50% duty (0.5 Wh per LED-day is real, not marketing), and the 30-pixel full-white current – not the data pin – is what sizes the 5 V supply, ground return, and fuse.

Every number above is taken from the chapter’s own material and re-derived step by step.

TryRecalculate LED power, 50% PWM saving, timing, code resolution, and strip supply current.
ObserveA 30-pixel full-white strip needs 1.8 A; brightness 50/255 caps the ideal draw near 0.353 A.
ExplainAlert colour is not free: firmware brightness and supply headroom determine whether it can be shown.
Technical boundaries. The audit assumes linear PWM brightness and fixed per-pixel full-white current. It does not model LED efficacy, gamma perception, colour-channel imbalance, animation patterns, regulator efficiency, wiring drop, USB limits, temperature, or simultaneous controller loads.
Audit result

The 20% full-white supply floor is 2.16 A; the lower firmware cap is safe only while that limit remains enforced.