Bench Power Monitor

Every other electronics build on this bench eventually asks the same question — how much current does it actually draw, and for how long? This is the meta-tool that answers it: an ESP32 + INA219 monitor that sits in series with any project’s supply rail, reads voltage, current, and power at 10 Hz, integrates them into mAh/mWh on an OLED, and streams a CSV to a laptop for a capacity or consumption plot. It exists to verify the power budgets the other designs only assume — e.g. checking the desk-buddy’s or a plant-waterer’s deep-sleep current against its stated milliamp-hour claim instead of trusting the datasheet.

OVERVIEW & MOTIVATION

A bench power monitor is a small, permanent piece of test equipment rather than a one-off project: wire it once between a bench supply and whatever DUT (device under test) is on the bench that week, and it reports live V/mA/mW plus a running mAh/mWh total since the last reset. The INA219 does the actual current sensing (high-side, through a fixed onboard shunt) so the DUT’s own ground reference is never disturbed. An SSD1306 OLED gives a live readout without a laptop attached; a serial CSV stream gives the same data to a computer for plotting capacity curves or consumption-over-time graphs. A single button both cycles the OLED’s three pages and — held down — zeroes the integrator for a fresh run.

COMPONENTS & BOM

REF	COMPONENT	SPEC	ROLE
U1	ESP32 DevKit-C	ESP32-WROOM-32 dev board	controller — sampling, integration, OLED, logging
U2	INA219 breakout	Adafruit INA219, 0.1Ω onboard shunt, I2C addr 0x40	high-side V/I/P current-sense ADC
U3	SSD1306 OLED	128×64, I2C addr 0x3C	live / integrated / min-max readout
SW1	Pushbutton	6 mm tactile, momentary, normally open	short = cycle page, hold 1.5 s = reset integrator
—	Hookup wire	22-26 AWG, stranded	I2C bus + logic-side wiring
—	Test leads	insulated, alligator-clip or banana-to-pin	DUT power path (VIN+/VIN-, GND return)

WIRING

The maroon path is the one actually being measured: bench supply positive goes into the INA219’s VIN+, through its fixed 0.1Ω onboard shunt, out VIN-, and on to the DUT. The DUT’s ground returns straight to the bench supply’s ground (ink, top of the drawing) — it never routes through the INA219, which is what lets the chip report current without disturbing the DUT’s own ground reference. SDA/SCL (blue) are a single shared I2C bus across the INA219, ESP32, and OLED.

ESP32 PIN	NET	PERIPHERAL PIN
GPIO21	I2C SDA	INA219 SDA · SSD1306 SDA
GPIO22	I2C SCL	INA219 SCL · SSD1306 SCL
GPIO27	BUTTON, active low	PUSHBUTTON signal leg (other leg → GND)
3V3	LOGIC SUPPLY	INA219 VCC · SSD1306 VCC
GND	LOGIC GND	INA219 GND · SSD1306 GND · PUSHBUTTON GND leg
n/a — not an ESP32 pin	MEASURED PATH	BENCH SUPPLY(+) → INA219 VIN+ · INA219 VIN- → DUT(+)
n/a — not an ESP32 pin	GND RETURN	BENCH SUPPLY(-) → DUT(-) direct, bypasses INA219

FIRMWARE & MEASUREMENT MATH

The INA219 is read through the Adafruit_INA219 library (not raw register access), using its setCalibration_32V_1A() profile: ±1.3 A headroom to survive a motor-driver DUT’s inrush, with 40 µA/bit resolution — a real step up from the library’s default 2 A profile (100 µA/bit) for characterizing small idle/sleep currents, though still coarse next to a dedicated microamp meter for true deep-sleep verification.

Every 100 ms (10 Hz) the firmware reads bus voltage, current, and power, then integrates charge and energy with the trapezoidal rule instead of a rectangular sum — it averages the previous and current sample rather than holding the last value flat for the whole interval, which matters when current ramps between samples (e.g. a motor spin-up) instead of stepping:

double dtHours = dtMs / 1000.0 / 3600.0;
mAh_total += (prevCurrent_mA + current_mA) / 2.0 * dtHours;
mWh_total += (prevPower_mW   + power_mW)   / 2.0 * dtHours;

Every sample is also written to Serial as a CSV line for offline plotting:

// millis_ms,bus_V,current_mA,power_mW,mAh,mWh
Serial.print(now); Serial.print(',');
Serial.print(busVoltage_V, 3); Serial.print(',');
Serial.print(current_mA, 3);   Serial.print(',');
Serial.print(power_mW, 3);     Serial.print(',');
Serial.print(mAh_total, 5);    Serial.print(',');
Serial.println(mWh_total, 5);

The button is debounced against millis(): a short press cycles the OLED between LIVE, INTEGRATED (mAh/mWh + elapsed time), and MIN-MAX pages; holding it 1.5 s zeroes the mAh/mWh totals and the min/max tracking for a clean new run.

SIMULATION

Wokwi has no INA219 part, so diagram.json is an honest partial simulation: it wires the real ESP32 + SSD1306 + pushbutton exactly as the real build (I2C on GPIO21/22, button on GPIO27), plus two potentiometers on GPIO34/GPIO35 labeled as V stand-in / I stand-in for the missing sensor. The shipped sketch.ino talks to a real INA219 over I2C and will fail ina219.begin() in the simulator, since no such device exists on the bus. Swapping the ina219.* calls for analogRead(34)/analogRead(35) in serviceSample() — a few lines — lets the sim exercise everything except the sensor itself: OLED page cycling, the trapezoidal mAh/mWh math, the CSV log format, and the button’s short/long-press behavior. See README.md in the source folder for the exact steps.

STATUS

Design and simulation complete; not bench-verified. The firmware compiles against the Adafruit_INA219 API and the calibration/resolution figures above (±1.3 A range, 40 µA/bit) are datasheet-typical for the library’s 32V_1A profile, not numbers measured on this build. No physical INA219 has been wired to a real DUT yet, so no real V/I/mAh trace exists. Planned next: bench-wire it in series with the desk-buddy project’s supply rail, log a full sleep/wake cycle, and compare the logged mAh against that project’s assumed sleep-current budget.

USE CASES & APPLICATIONS

Battery-life validation for any of the low-power builds on this bench — logging mAh over a full duty cycle beats trusting a datasheet’s typical-current figure. Charger/supply profiling: watch voltage sag and current draw through a charge cycle. Rough solar-node sizing: log a day’s consumption profile to estimate panel/battery sizing before committing to hardware. Lab teaching: the trapezoidal-integration and high-side-sensing concepts here are the same ones behind bench multimeters’ mAh functions and commercial USB power meters.

FILES

• sketch.ino — firmware: Adafruit_INA219 driver, 10 Hz sample/integrate/log loop, trapezoidal mAh/mWh integration, three-page OLED UI, debounced short/long-press button.
• diagram.json — Wokwi diagram: ESP32 + SSD1306 + pushbutton wired as the real build, plus two potentiometers standing in for the (unsimulated) INA219 — see SIMULATION above.
• README.md — Wokwi sim steps and real-build notes: shunt placement, wire gauge, and the current-range-vs-resolution tradeoff across the INA219’s calibration profiles.