Print — Lab 2 shared rail build sheet

Lab 2 — homeostasis by diffusion: shared voltage rail · build sheet

Eight nodes on one current-limited rail. Hard draw sags the rail; the sag is the ambient scalar every ADC feels at once. Fastest of the three to settle, most scope-legible — and the mechanism (droop control) is already grid-proven.

THE ONE RULE: the regulating scalar never becomes a packet. The rail hydraulics carry the message.

1. Bill of materials

Qty	Item	Notes
8	ESP32-S3 N16R8 (ESP32-S3-DevKitC-1 layout)	Lonely Binary / any DevKitC-1-layout S3; logic power SEPARATE from the shared rail (see warning)
1	bench supply, current-limited	e.g. 5 V @ 1–2 A limit; OR fixed supply + series R + cap bank
1	series resistor ~1 Ω / 5 W (if no CC supply)	creates the sag
1	cap bank 2200–4700 µF	smooths, slows the medium usefully
8	logic-level MOSFETs (e.g. AO3400 / IRLZ44N)	one load switch per node
8	load resistors ~22 Ω / 5 W	per-node draw on the shared rail
16	divider resistors (10 kΩ + 10 kΩ)	halve 5 V rail into 3V3-safe ADC range, one divider per node
—	bus wire / distribution board	every node ties to the SAME rail and common ground

Qty

Item

Notes

ESP32-S3 N16R8 (ESP32-S3-DevKitC-1 layout)

Lonely Binary / any DevKitC-1-layout S3; logic power SEPARATE from the shared rail (see warning)

bench supply, current-limited

e.g. 5 V @ 1–2 A limit; OR fixed supply + series R + cap bank

series resistor ~1 Ω / 5 W (if no CC supply)

creates the sag

cap bank 2200–4700 µF

smooths, slows the medium usefully

logic-level MOSFETs (e.g. AO3400 / IRLZ44N)

one load switch per node

load resistors ~22 Ω / 5 W

per-node draw on the shared rail

divider resistors (10 kΩ + 10 kΩ)

halve 5 V rail into 3V3-safe ADC range, one divider per node

—

bus wire / distribution board

every node ties to the SAME rail and common ground

Two rails, on purpose: the ESP32s run on their own USB power. Only the LOADS live on the shared 5 V rail. If logic shared the sagging rail, brownouts would reboot nodes mid-experiment and muddy the result. ADC safety: never feed the 5 V rail into an ADC pin directly — always through the divider. Common ground between USB power and the shared rail is mandatory.

2. Per-node wiring

Signal	ESP32 pin	Connects to
Rail sense	GPIO 5 (ADC1)	midpoint of 10k/10k divider across rail → GND
Load PWM	GPIO 16	MOSFET gate; drain → 22 Ω → rail; source → GND
GND	GND	common with rail ground

Signal

ESP32 pin

Connects to

Rail sense

GPIO 5 (ADC1)

midpoint of 10k/10k divider across rail → GND

Load PWM

GPIO 16

MOSFET gate; drain → 22 Ω → rail; source → GND

GND

common with rail ground

Board (ESP32-S3-DevKitC-1 layout, N16R8): sense is GPIO 5 — on the S3, ADC1 = GPIO 1–10 (GPIO 5 = ADC1_CH4) and ADC1 doesn't fight WiFi. The classic-ESP32 GPIO 34 is not an ADC pin on the S3. On octal-PSRAM N16R8 leave GPIO 33–37 (PSRAM) and GPIO 26–32 (SPI flash) alone — touching them crashes the cache; safe GPIOs are 1–18, 21, 38–42, 47.

Per-node wiring, breadboard view — bottom red rail is the SHARED 5 V rail (external supply, deliberately not fed from the board), logic stays on USB; MOSFET drawn by pin name (D·S·G), check your package's datasheet. Pin positions are correct; the USB connector shown is cosmetic (micro-USB in the drawing vs the lab boards' dual USB-C). Editable sketch: lab2_rail_wiring.fzz

3. Node loop (identical on all 8)

V_NOM = 5.0 # firm-rail voltage V_FLOOR = 4.5 # full-backoff point # duty follows a droop line between the two — same law as grid governors every 100 ms: # rail is a fast medium; loop fast v = read_rail_adc() * 2.0 # undo the divider duty = clamp((v - V_FLOOR) / (V_NOM - V_FLOOR), 0.0, 1.0) set_load_pwm(duty) # no node signals another. the sag already told everyone.

4. Read-only gauge

A multimeter or scope on the rail is the honest gauge — physically read-only by nature. For logging: one spare ESP32 with ONLY a divider tap, WiFi reporting is fine here (RF is not the medium), or wired serial for purity. Never give the gauge a load.

5. Bring-up procedure

6. Perturbation runs

Scope screenshots of dip-and-recover are the money graphs. This is the lab to show an engineer first: droop control they already trust, one level down.