Utility Distribution Transformer Load Monitor

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This reference application is intended to provide inspiration and help you get started quickly. It uses specific hardware choices that may not match your own implementation. Focus on the sections most relevant to your use case. If you'd like to discuss your project and whether it's a good fit for Blues, feel free to reach out.

Building with an AI assistant? Point it at this accelerator's firmware — firmware/transformer_load_monitor/ — so it can learn how the device works and help you adapt it to your hardware.

This project is an energy monitoring solution that gives utilities real-time load visibility at the pole — where transformer failures actually happen. A Notecarrier CX with three split-core current transformers on the secondary terminals and an I²C temperature sensor tracks per-phase loading, detects dangerous current imbalances, and correlates load with enclosure temperature as a thermal-stress proxy, all over cellular, with no site infrastructure required.

1. Project Overview

The problem. A distribution transformer is the last powered device between the high-voltage grid and a customer's service entrance. Utilities typically instrument their substations carefully (real-time SCADA, demand data, fault recorders), but the transformer hanging on the pole outside a neighborhood has none of that. The utility knows what's flowing into the substation; it doesn't know what's flowing through any individual transformer until something fails.

That ignorance is expensive. Distribution transformers age and fail for identifiable, preventable reasons: sustained overloading (insulation degrades with each degree of excess temperature), phase imbalance (uneven single-phase loads on a three-phase transformer create circulating currents and localized hot spots), and thermal stress from the combination of high ambient temperature and high loading. None of these conditions are sudden. They develop over hours to days, and a transformer that is running hot on a Tuesday will fail during Friday's peak demand, taking down a neighborhood and requiring an emergency truck roll that costs many times what proactive replacement would have.

This project puts an end to that ignorance. A handful of sensors clamped to the transformer secondary, a Notecard for the uplink, and a threshold-checking firmware loop turn an opaque pole-top asset into a continuously-monitored piece of equipment that tells the operations center what's happening in real time and pages the right people before the neighborhood goes dark.

Why Notecard. Utility poles are among the least connected pieces of infrastructure there is. They have no Ethernet, no building WiFi, and often nothing except the transformer's own secondary power. Cellular is the only practical backhaul. The Notecard Cell+WiFi variant (MBGLW) carries a prepaid SIM with 500 MB of data and 10 years of service: no per-site activation, no carrier contracts to negotiate, no IT ticket to file for network access. A utility deploying these monitors across its service territory can use a single SKU on every pole in every neighborhood, from dense urban blocks to rural feeders. The Notecard's automatic carrier selection handles varied network conditions without firmware changes.

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The power picture tells the same story. Cellular removes the site infrastructure dependency entirely. The Notecard Cell+WiFi idles at ~18 µA @ 5V between samples while the host MCU sleeps in STOP2 at a few µA, and the 5-minute sample / hourly transmit cadence means the radio wakes for roughly 20–30 seconds per hour, well within what a small AC/DC supply tapped from the transformer's own secondary can comfortably sustain.

Deployment scenario. A NEMA 4X weatherproof enclosure zip-tied or U-bolted to the transformer pole, powered from a small AC/DC supply connected to the 120VAC secondary, with three split-core CT leads exiting through weatherproof cable glands and clamped directly onto the secondary conductors.

CT installation is non-invasive: the split-core sensors clamp directly onto energized secondary conductors without breaking the circuit. The power supply connection is a separate matter — tapping the 120VAC secondary is live utility electrical work; depending on utility policy and local jurisdiction, this step may require a scheduled outage, a qualified hot-work procedure, or written utility approval before the tap is energized (see §11 Limitations). No invasive tap into the transformer case and no on-site network provisioning are required. A two-person crew, including an electrician qualified for live secondary work, can install and commission one unit in under an hour once all necessary work authorizations are in hand.

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Power supply scope. This reference design assumes an accessible 120VAC secondary tap — the standard case for North American single-phase distribution transformers installed by a qualified crew. Three-phase padmount transformers, sealed primary-side enclosures, or installations where utility rules prohibit tapping the secondary require an alternate power arrangement. See §11 Limitations for the inductive harvesting alternative.

2. System Architecture

Device-side responsibilities. The host on the pole has a simple, repeating job. Every 5 minutes (configurable) the Notecarrier CX's STM32L433 host wakes, reads the configured CT channels — two for the common split-phase install, three when phase_count=3 is set for three-phase — reads the enclosure-internal I²C temperature sensor, runs three threshold checks in firmware, and queues the results as Notes via the Notecard over I²C. The moment that's done it goes back to sleep. Between wakes the host sits in STM32 STOP2 (a few µA) until the Notecard's ATTN pin wakes it at the end of a card.attn sleep. STOP2 retains RAM, so all the state that has to survive that gap (summary accumulators, alert cooldown counters, elapsed time) simply stays in memory — nothing is serialized to the Notecard.

Notecard responsibilities. The Notecard takes everything the host just queued and decides when it goes over the air. Notes sit in its on-device queue and flush on the hub.set outbound cadence (default 60 minutes), but any Note flagged sync:true (every alert) bypasses that timer and ships immediately. The Notecard also handles environment variable distribution from the Blues Notehub cloud service, so an operations engineer can retune thresholds across the fleet without re-flashing firmware; the device picks them up on the next inbound sync. (GNSS is available on supported hardware if a future revision needs one-time asset location, but it isn't used in this design — enabling it has antenna, power, and cost implications not covered here.)

Notehub responsibilities. Once a Note leaves the pole, the Notecard's embedded global SIM carries it over supported carriers worldwide and delivers it to Notehub, where events are ingested, stored, and routed downstream. Two Notefiles carry everything the device says: xfmr_summary.qo for hourly templated rollups suitable for long-term trending, and xfmr_alert.qo for event-triggered, immediate-delivery Notes. Organizing devices into Fleets by service territory or transformer rating lets engineers set threshold environment variables that apply across an entire group (all 25-kVA residential units on one feeder share one threshold profile) while still allowing per-device overrides for unusual transformers. Smart Fleet rules can automatically assign a Notecard to the correct fleet based on its reported data.

Routing to the cloud (high level only). Notehub supports HTTP, MQTT, AWS IoT Core, Azure IoT Hub, GCP Pub/Sub, Snowflake, and other destinations; route setup is project-specific. See the Notehub routing docs; this project ships no specific downstream endpoint. Alert and summary Notefiles are deliberately separate so each can be routed independently: alerts to an outage-management or on-call system with low-latency delivery, summaries to a historian or analytics store with higher-volume, batched ingest.

3. Technical Summary

What you need:

• Notecarrier CX + Notecard Cell+WiFi (MBGLW)
• 3× YHDC SCT-013-000 CTs + 3.5 mm jack breakouts
• Per-CT burden circuit: 22 Ω, 2× 10 kΩ, 10 µF cap (see §5)
• MCP9808 I²C temperature sensor
• 5V/2A AC/DC supply (e.g., MeanWell IRM-10-5)
• Computer with Arduino IDE or arduino-cli

Bench validation (no live transformer needed):

1. Assemble the three CT bias circuits on A0, A1, A2 (see §5 wiring diagram), and fit the ATTN→D5 jumper for host sleep.
2. Connect MCP9808 to I²C (SDA/SCL).
3. Flash firmware: set PRODUCT_UID in transformer_load_monitor.ino to your Notehub project ID, then upload with the board options STMicroelectronics:stm32:Blues:pnum=CYGNET,usb=none (see §7.1).
4. Open a serial monitor at 115200 baud on the ST-LINK virtual COM port from the Notecarrier CX debug jack. You should see [sample] i_a=... lines every 5 minutes.
5. Create a project in Notehub if you haven't — the Notecard auto-associates on first cellular connect.
6. After 60 minutes you'll see [summary] queued on serial. Wait for the device's next outbound sync (default hourly) and check Notehub Devices → Events for an xfmr_summary.qo entry.
7. Clamp a CT on a known 120VAC load (a lamp works well) and verify the current value in the summary matches the expected load (within ~5%).

Here is a sample Note this device emits:

{
  "file": "xfmr_alert.qo",
  "body": {
    "alert":    "phase_imbalance",
    "i_a_rms":  91.4,
    "i_b_rms":  34.2,
    "i_c_rms":  88.7,
    "temp_c":   44.1,
    "extra":    62.6
  }
}

4. Hardware Requirements

All Blues hardware ships with an active SIM including 500 MB of data and 10 years of service — no activation fees, no monthly commitment.

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CT current range Note. The SCT-013-000 rated at 100A suits transformers up to approximately 20 kVA at 240V secondary (~83A full-load, with ~17% headroom to the CT's 100A rated output). A common 25 kVA unit draws ~104A at full load — 4% above the CT's rating; use a higher-range current-output CT for transformers at or above 25 kVA. Larger transformers (50–167 kVA) draw 200–400A and require a higher-range current-output CT such as the YHDC SCT-036-200 (200A/50mA); update CT_TURNS_RATIO and CT_BURDEN_OHMS in firmware accordingly. Voltage-output CTs (such as the YHDC SCT-019-200, which produces 0.333V full-scale from an internal burden) are not drop-in replacements — they do not interface with the external bias circuit above and require a different analog front end. See §11 Limitations for the production path.

warning

POC protection scope. The BOM above provides primary overcurrent protection (the slow-blow fuse) and nothing else on the 120VAC mains entry. For a bench prototype or controlled field trial this is adequate starting hardware, but it is not a complete protection story for a permanent outdoor pole-mount installation. A production design must add: a surge protective device (SPD) rated for the installation's overvoltage category (Category C / lightning-level, per IEC 61643-11 or UL 1449) on the Line/Neutral entry; enclosure grounding and bonding to the pole ground system per applicable NEC sections and utility rules; verified creepage and clearance in all mains-voltage wiring and connectors; UV and temperature qualification of all external cabling and the enclosure; and written utility approval before the secondary tap is permanently energized. Component selection for all of the above is installation-specific and outside this POC's scope. See §5 and §11 for further discussion.

5. Wiring and Assembly

All host I/O lands on the Notecarrier CX dual 16-pin header. The Notecard Cell+WiFi (MBGLW) seats into the carrier's M.2 slot. The Mojo, if used for bench validation, sits inline between the 5V supply and the Notecarrier's +VBAT pad; connect its Qwiic cable to any available Qwiic port on the Notecarrier so it can log charge to the Notecard. The transformer firmware does not poll the Mojo — its coulomb-count readings appear as separate Notecard events in Notehub and are a bench commissioning tool only; the Mojo is not installed in the deployed unit.

Bias circuit (build once per CT channel, identical for A, B, and C)

Each CT channel requires a small passive network that converts the CT's AC current to a voltage centered at Vcc/2 so it sits squarely within the host ADC's 0–3.3V input range.

(per channel — repeat for A0, A1, A2)

 +3V3
   │
 R1 10 kΩ          ← top half of bias divider
   │
   ├─────────────────── ADC pin (A0 / A1 / A2)
   │                         │
 R2 10 kΩ          ←  C 10 µF (+ plate toward ADC pin)
   │                         │
  GND                   CT+ (TRRS tip)
                             │
                         Rb 22 Ω   ← burden resistor
                             │
                    CT– (TRRS sleeve) ──── GND

How it works:

• Burden (Rb, 22 Ω): The SCT-013-000 is a current-output CT; Rb converts its 50 mA RMS rated output to approximately 1.1 V RMS (≈1.56 V peak swing) at 100 A RMS primary current. Rb is wired directly across the CT output terminals (tip to sleeve).
• Bias divider (R1 + R2, 10 kΩ each): Splits +3.3V to establish a DC bias of ≈1.65V (Vcc/2) at the ADC pin. This centers the AC signal within the ADC's single-supply input range.
• Coupling capacitor (C, 10 µF electrolytic): AC-couples CT+ to the ADC pin. The positive plate faces the ADC pin (higher DC potential, ≈1.65 V); the negative plate faces CT+ (≈0 V DC). C passes the AC signal to the ADC pin while blocking DC from disrupting the bias point.
• Each CT channel needs its own R1, R2, C, and Rb — do not share bias nodes or burden resistors across channels.

Pin-by-pin connections

• +3V3 → R1 top leg of each bias circuit; MCP9808 VDD.
• GND → R2 bottom leg of each bias circuit; CT– (TRRS sleeve) of each jack; MCP9808 GND.
• A0 → bias-divider mid-point (junction of R1 and R2) and positive plate of C for the Phase-A channel. The negative plate of C connects to CT+ (TRRS tip). CT+ also connects to one leg of Rb (22 Ω); the other leg of Rb connects to GND / CT–.
• A1 → same circuit, Phase-B CT.
• A2 → same circuit, Phase-C CT. For split-phase installations (L1/L2 on A0/A1), leave A2 unconnected — the firmware defaults to phase_count=2, so no env var change is needed and the unconnected channel is never sampled. For three-phase installations, wire this bias circuit and set phase_count=3 in Notehub before first power-on (see §6); without that setting, A2 is never read and imbalance calculations are incomplete.
• SDA → MCP9808 SDA (Notecarrier CX has on-board 4.7 kΩ pull-ups on the I²C bus).
• SCL → MCP9808 SCL.
• ATTN → D5 jumper (host sleep). Fit a short jumper between the ATTN and D5 pins on the Notecarrier CX header — both are on the same 16-pin header (ATTN at position 3, D5 at position 14). The Notecard pulls ATTN low while the host sleeps and raises it when the next sample is due; the rising edge on D5 wakes the host from STOP2. Leave the Notecarrier CX EN pin unconnected: on the CX it enables the shared 3.3 V VIO rail (host, Qwiic, 3V3_OUT, and Notecard I/O), so driving it from ATTN would brown out the whole board rather than sleep the host. See Putting a Host to Sleep Between Sensor Readings for the full pattern.
• +VBAT → Mojo LOAD output (bench only); Mojo BAT input → 5V from the IRM-10-5 supply.
• IRM-10-5 LINE/NEUTRAL → 120VAC tapped from the transformer secondary. The Line conductor must pass through a 2A 250V slow-blow fuse (in a rated fuse holder) before reaching the supply.

Cellular antenna

Connect the cellular antenna to the Notecard Cell+WiFi (MBGLW) u.FL port on the front edge of the Notecard. The Blues development kit includes a compatible stub antenna; for permanent pole-mount installations, a dedicated LTE antenna rated for outdoor use is preferred.

Placement guidelines:

• Polycarbonate or fiberglass NEMA 4X enclosure: the antenna can mount inside the enclosure on a standoff or against the lid. RF penetrates these materials with negligible loss.
• Steel or aluminum enclosure: run a u.FL-to-SMA pigtail to a bulkhead-mount SMA antenna on the outside wall of the enclosure. Steel fully blocks the cellular signal from the inside.
• Orient the antenna element vertically (parallel to the pole) for the best cellular radiation pattern.
• Route the antenna cable away from the transformer core, primary conductors, and the 120VAC supply wiring to avoid coupling noise into the RF path.

3.5mm TRRS jack wiring

The SCT-013-000 uses a 3.5mm plug with tip = CT+ and sleeve = CT–. Wire the breakout board:

• Tip → one end of the 22Ω burden (Rb) and to the AC coupling capacitor top plate
• Sleeve → GND (and the other end of Rb)

CT installation

Clamp each CT around a single secondary conductor, never around two conductors simultaneously, as the fields would cancel. For a single-phase (120/240V center-tap) transformer, clamp the Phase-A CT on the X1 secondary conductor (L1 hot leg) and the Phase-B CT on the X3 secondary conductor (L2 hot leg). X2 is the center-tap neutral — do not monitor X2 unless neutral current is an intentional additional metric; clamping one CT on X1 and the other on X2 would monitor one hot leg and the neutral, not the L1/L2 pair. For a three-phase transformer, clamp each CT on one secondary phase conductor. The third CT slot (A2/Phase-C) can be left unconnected for split-phase installations; the firmware defaults to phase_count=2, so no env var change is required. For three-phase installations, clamp the Phase-C CT on the third secondary conductor and set phase_count=3 in Notehub before first power-on — without that setting, A2 is never sampled and any C-phase overload or imbalance contribution is invisible to the firmware.

warning

CT open-circuit hazard. The SCT-013-000 is a current-output CT. Never disconnect the 22Ω burden resistor or unplug a CT jack while the CT is clamped on an energized conductor. An open-circuited current-output CT can develop dangerous high voltages across its terminals. Always verify the burden resistor is in-circuit before clamping the CT onto a live conductor; always remove the CT from the conductor before disturbing the burden wiring or disconnecting the 3.5mm jack.

Safety. Distribution transformer secondary conductors carry lethal voltages. Installation must be performed by qualified electrical workers with appropriate PPE, following utility safety procedures and applicable codes. This firmware is read-only — it never commands any switching of the transformer or its connected loads. The CTs are non-invasive and clamp without breaking the circuit, but proximity to energized conductors remains hazardous.

Surge and transient protection (POC omission). The 2A slow-blow fuse protects the IRM-10-5 supply against a sustained short but does not suppress lightning-induced transients or switching surges, which are routinely present on distribution transformer secondaries. For bench bring-up this is acceptable; for any unit intended for permanent pole-mount deployment, install a surge protective device (SPD) rated for the site's overvoltage category (Category C / lightning-level, per IEC 61643-11 or UL 1449) across Line/Neutral ahead of the fuse holder. Bond the enclosure to the pole grounding system and verify conductor routing, creepage/clearance distances, and any utility-specific installation approval requirements before energizing the tap. See §11 for the full list.

6. Notehub Setup

1. Create a project. Sign up at notehub.io and create a project. Copy the ProductUID (format: com.your-company.your-name:xfmr-monitor) and set it as PRODUCT_UID in the firmware before flashing.

2. Claim the device. Power the assembled unit. On first cellular connect, the Notecard associates with your project automatically — no manual claim required. The device appears in the Devices tab within a minute or two.

3. Create a Fleet per service territory. Fleets group devices for shared configuration. The natural unit is one fleet per transformer class or service territory — all 25-kVA residential transformers in a feeder get the same rated-amps threshold, while 50-kVA commercial units get a different one. Fleet-level environment variables propagate to every device in the fleet without re-flashing.

4. Set environment variables. In Notehub: Fleet → Environment (or Device → Environment for a per-unit override). All variables are optional for split-phase (L1/L2) installations — the firmware defaults are set for that common case. Three-phase installations must set phase_count=3 and force an inbound sync before live monitoring begins; with the default phase_count=2 on a three-phase transformer, the A2/Phase-C channel is never read, and any C-phase overload or imbalance contribution is invisible to the firmware. The device picks up all other env var changes on its next inbound sync (default every 2 hours) with no truck roll.

   Variable	Default	Purpose
   sample_interval_sec	300	Seconds between sensor readings (minimum 30).
   summary_interval_min	60	Minutes between summary Notes. Changing this also re-applies hub.set to match the Notecard's outbound cadence.
   rated_amps	100.0	Transformer secondary rated amps per phase. The default matches the 100A SCT-013-000 CT range. Set to the nameplate-derived secondary current (kVA rating ÷ secondary voltage), e.g., 83 for a 20-kVA / 240V unit or 62 for a 15-kVA / 240V unit. A 25-kVA / 240V transformer draws ~104A at full load, which exceeds the default CT's 100A rating; swap in a higher-range current-output CT and set this value to the transformer's actual nameplate current before deploying on transformers at or above 25 kVA.
   overload_pct	95.0	Any single phase exceeding this percentage of rated_amps triggers an overload alert.
   imbalance_pct_thresh	20.0	(max_phase − min_phase) / max_phase above this percentage triggers a phase_imbalance alert. Only evaluated when at least one phase is carrying >2A.
   temp_alert_c	70.0	Enclosure temperature (°C) above which a high_temp alert fires. Typical enclosure internal temperature limit is 50–65°C; 70°C is a conservative field alert.
   alert_cooldown_sec	1800	Minimum seconds between successive alerts of the same type (default 30 minutes). Prevents alert storms on a slow-drifting condition.
   phase_count	2	Number of active CT channels (1–3). The default 2 matches the common split-phase (L1/L2 on A0/A1) installation — no env var change needed out of the box. Set to 3 for three-phase transformers (A0/A1/A2 all wired) before first power-on; without this, A2 is never read. Set to 1 for single-leg monitoring. Imbalance is only meaningful across two or more phases; with phase_count=1, imbalance_pct is always reported as 0.0 in both real-time alerts and summary Notes. With phase_count=2, L1 and L2 are compared without the unconnected C channel (always 0A) inflating the imbalance reading.

5. Configure routes. Add one route for xfmr_alert.qo (to an outage-management system, on-call paging, or CMMS endpoint) and a second for xfmr_summary.qo (to a long-term store or analytics platform). Keeping the two Notefiles separate means each can be fanned out to a different destination at a different urgency without filter logic in the route.

What you should see in Notehub

Note: Template fields use numeric type hints to encode type and size: 14.1 = 4-byte IEEE-754 float; 12 = 2-byte signed integer. This compact binary format is decoded automatically by Notehub.

• _session.qo — Notecard session housekeeping on each cellular connect. If this is showing up, the radio link is healthy.
• xfmr_summary.qo — one per summary_interval_min. Annotated example payload (field comments are explanatory, actual JSON contains no comments):
  COPY

  {
    "i_a_rms":       47.3,   // phase-A mean RMS over loaded sample intervals only (A) — see §11
    "i_b_rms":       44.8,   // phase-B mean RMS over loaded sample intervals (A)
    "i_c_rms":       49.1,   // phase-C mean RMS over loaded sample intervals (A)
    "i_total":      141.2,   // sum of per-phase loaded-interval means (A)
    "loading_pct":   39.2,   // i_total / (rated_amps × phase_count) × 100; excludes idle intervals
    "imbalance_pct":  9.0,   // (max_phase − min_phase) / max_phase × 100; from loaded-interval means
    "temp_c":        38.5,   // mean enclosure temperature over valid temperature readings (°C)
    "overloads":       0,    // sample intervals during which any phase exceeded the overload threshold
    "samples":        12,    // loaded sample intervals (at least one phase above the noise floor)
    "total_wakes":   12      // all host wakes in the window (loaded + idle)
  }

  overloads counts sample intervals during which any phase exceeded the overload threshold, not distinct overload events. A single sustained overload lasting the full summary window at the default 5-minute sample rate would report overloads: 12. samples counts only intervals in which at least one phase was above the 0.5 A noise floor; total_wakes counts every host wake regardless of load. For time-weighted utilization analytics, multiply loading_pct by samples / total_wakes to account for idle intervals (see §11).
• xfmr_alert.qo — emitted only on a threshold trip, transmitted immediately. The alert field is one of overload, phase_imbalance, or high_temp.

7. Firmware Design

The firmware lives in the sketch directory firmware/transformer_load_monitor/ and is split across four files that must be compiled together:

The Arduino IDE and arduino-cli automatically compile every .ino, .h, and .cpp file in the same sketch folder together — always open or target the folder (firmware/transformer_load_monitor/), not an individual source file.

7.1 Installing and flashing

Dependencies:

• Arduino core for STM32 — stm32duino/Arduino_Core_STM32. Add the board index URL https://github.com/stm32duino/BoardManagerFiles/raw/main/package_stmicroelectronics_index.json under File → Preferences → Additional Boards Manager URLs, then install "STM32 MCU based boards." The Notecarrier CX's host uses the Cygnet board definition (see build settings below).
• Blues Wireless Notecard library (note-arduino) — Install via the Arduino Library Manager (search "Blues Wireless Notecard") or arduino-cli lib install "Blues Wireless Notecard".
• Adafruit MCP9808 Library — install via the Arduino Library Manager (search "MCP9808"). Requires the Adafruit BusIO dependency, which the Library Manager installs automatically.
• STM32duino Low Power (and its dependency STM32duino RTC) — stm32duino/STM32LowPower. Install via the Library Manager or arduino-cli lib install "STM32duino Low Power" "STM32duino RTC". Provides the STOP2 entry and ATTN-pin wake used in cx_sleep.h.

Build settings: the Notecarrier CX's host is the same STM32L433 as the Blues Cygnet, so the board is Blues boards → Cygnet (pnum=CYGNET), with USB support (if available) set to None (usb=none). With the USB CDC stack enabled and no USB host attached (or the Notecarrier CX USB-C switch in NC), the STM32's USB wakeup interrupt fires continuously and the host never stays in STOP2. Debug output goes to the LPUART on the Notecarrier CX debug jack instead — an ST-LINK V3 exposes it as a virtual COM port; see Serial Logging With STLINK.

Flashing — Arduino IDE: use File → Open… and select the folder firmware/transformer_load_monitor/ (or double-click transformer_load_monitor.ino, the IDE loads all four files from the folder automatically). Apply the build settings above and click Upload. With the Notecarrier CX's USB-C switch in HST, the host's DFU bootloader is reachable over the USB-C port (hold BOOT, tap RST); alternatively upload over SWD with an ST-LINK on the debug jack.

Flashing — arduino-cli:

# Install the STM32 core and libraries first if not already done
arduino-cli core install STMicroelectronics:stm32
arduino-cli lib install "Blues Wireless Notecard" "Adafruit MCP9808 Library" "STM32duino Low Power" "STM32duino RTC"

# Compile and upload — target the sketch folder so all four source files
# are included (/dev/cu.usbmodem* for Mac, /dev/ttyACM* for Linux).
# usb=none disables the USB CDC stack, which must be off for the host to sleep.
arduino-cli compile -b STMicroelectronics:stm32:Blues:pnum=CYGNET,usb=none firmware/transformer_load_monitor
arduino-cli upload  -b STMicroelectronics:stm32:Blues:pnum=CYGNET,usb=none -p /dev/cu.usbmodem* firmware/transformer_load_monitor

firmware/transformer_load_monitor/sketch.yaml sets the same board options, so running arduino-cli compile in that directory without -b picks them up too.

After flashing, open a serial monitor at 115200 baud on the ST-LINK virtual COM port. On first boot you should see [sample] i_a=... lines every sample_interval_sec seconds (default 5 minutes). After summary_interval_min minutes (default 60) you will see [summary] queued (0 loaded / 12 total wakes) if no CT is connected — the summary Note is queued to the Notecard's local on-device store at that point; it reaches Notehub on the next outbound cellular sync (default 60 minutes later). The summary is always emitted as a liveness heartbeat even with no load. When CTs are clamped on live conductors the sample lines show non-zero current values and the summary line reports the number of loaded intervals versus total wakes in that window.

7.2 Firmware constants and environment variables

The firmware defines several key tuning constants in transformer_load_monitor_helpers.h:

7.3 Modules

7.4 Sensor reading strategy

CTs. Each SCT-013-000 produces an AC current signal proportional to the line current flowing through its core. The external 22Ω burden resistor converts this to an AC voltage of approximately 1.1 V RMS (≈1.56 V peak) at 100 A RMS primary current, centered on the ADC's DC bias point (~1.65V = Vcc/2). The firmware uses a two-pass algorithm: 256 samples establish the actual DC offset (which may drift slightly from Vcc/2 due to component tolerance), then 1480 samples compute the RMS of the centered signal. Both passes pace samples to a fixed 225 microseconds period (≈4.44 kHz) using micros() and delayMicroseconds(), so the 1480-sample RMS window spans a deterministic ~333 milliseconds: approximately 20 mains cycles at 60 Hz, or ~16.7 cycles at 50 Hz. Without that pacing, the host ADC's native single-digit-µs throughput would finish the burst in well under one full mains cycle and produce RMS values that drift with whichever waveform fragment was captured. Three channels are read sequentially, so total active measurement time is approximately 1 second. The scale factor CT_SCALE = (3.3/4096) × (2000/22) converts ADC RMS counts directly to primary amps. Readings below 0.5 A are zeroed to suppress noise-floor pickup.

Temperature. The Adafruit MCP9808 is initialized on every wake (a cheap, idempotent begin() that also doubles as a presence check), configured to 0.0625°C resolution, read once, and put to shutdown mode before the host sleeps. An absent or unresponsive sensor returns –999.0°C. The firmware tracks a separate valid_temp_samples counter that increments only when the reading passes the –40°C to +125°C sanity range; the summary average divides sum_temp_c by that counter rather than by total wake cycles. A window with zero valid temperature readings emits –999.0°C in the summary rather than a biased-low average. The Notecarrier CX's on-board I²C pull-ups are shared by the MCP9808 and the Notecard on the same bus — no external resistors are needed.

7.5 Event payload design

One template-backed Notefile for summaries (xfmr_summary.qo), one untemplated Notefile for alerts (xfmr_alert.qo). Templates trade flexibility for efficiency: the Notecard stores fixed-length binary records internally and the wire payload is ~3–5× smaller than free-form JSON, which matters across a fleet that may transmit daily summaries for a decade.

Sample alert (phase imbalance event):

{
  "file": "xfmr_alert.qo",
  "body": {
    "alert":    "phase_imbalance",
    "i_a_rms":  91.4,
    "i_b_rms":  34.2,
    "i_c_rms":  88.7,
    "temp_c":   44.1,
    "extra":    62.6
  }
}

The extra field carries alert-type-specific context: for overload it is the loading percentage of the worst phase; for phase_imbalance it is the computed imbalance percentage (max−min)/max; for high_temp it is the total current across all phases at the time of the alert.

7.6 Low-power strategy

The device is grid-tied (120VAC tap on the transformer secondary), so energy budget is not life-critical, but the sleep pattern matters anyway. Keeping the host in STOP2 between samples eliminates a continuous 50–80mA baseline, reduces enclosure heat buildup, and makes the power profile immediately legible on a Mojo trace during bench validation.

After each sample cycle loop() calls cxSleepUntilAttn() (in cx_sleep.h), which sends a card.attn sleep request for sample_interval_sec seconds, waits for the Notecard to pull ATTN low, and puts the STM32 host into STOP2, where it draws a few microamps with its RAM intact. When the interval elapses the Notecard raises ATTN; the rising edge on D5 wakes the host and execution resumes on the next line of loop(). Because the host resumes in place rather than cold-booting, the AppState struct — accumulators, cooldown counters, pending alerts, cached config — simply lives in RAM, nothing is serialized to the Notecard, and setup() runs only once per power-up. From the firmware author's perspective, the sleep call is a single function call; the Notecard keeps the schedule. If the sleep request fails or ATTN never goes low (jumper missing), loop() waits out the interval awake and tries again on the next cycle, so the sample cadence is preserved either way.

The Notecard Cell+WiFi itself idles at ~18 µA @ 5V between cellular sessions. Alert Notes with sync:true bypass the outbound queue and wake the radio immediately; summary Notes accumulate in the on-device queue and are flushed together once per hour.

7.7 Retry and error handling

• hub_configured and template_configured are set only when the Notecard confirms the power-up hub.set and note.template requests, and loop() retries anything still unconfirmed on every wake.
• If cxSleepUntilAttn() returns false — the Notecard did not accept the card.attn request, or ATTN never went low because the ATTN→D5 jumper is missing — loop() waits out sample_interval_sec awake with delay() and tries again on the next cycle, so sampling continues at the normal cadence (at a much higher power cost).
• Individual env.get calls return the default value if the Notecard returns NULL or an empty text field. Config values are then clamped to sane ranges (e.g., sample_interval_sec floored at 30 seconds) to guard against a misconfigured env var causing erratic behavior.
• CT readings below the 0.5 A noise floor are zeroed; sample intervals where no phase exceeds the noise floor are excluded from current accumulation via the valid_samples counter, making the per-phase RMS averages loaded-interval averages rather than true time-weighted window averages. The total_wakes field in the summary payload records all host wakes (loaded + idle), so dividing samples by total_wakes gives the fraction of the window that carried detectable load — useful for utilization weighting (see §11). A summary window with zero valid CT samples still emits a xfmr_summary.qo Note with zero current fields and samples: 0, confirming the device is alive even when no load is detectable on the CTs.
• Alert cooldown counters per alert type (default 6 cycles = 30 minutes at 5-min sample rate) prevent a sustained overload condition from paging the operations center every 5 minutes. Each alert type arms and disarms independently.
• fetchEnvOverrides updates the hub.set outbound period if summary_interval_min has changed since the last successful fetch, keeping the Notecard's transmit timer synchronized with the firmware's summary window.

7.8 Key code snippet 1: Note template definition

Template fields use numeric type hints: 14.1 means 4-byte IEEE-754 float; 12 means 2-byte signed integer.

J *req = notecard.newRequest("note.template");
JAddStringToObject(req, "file", "xfmr_summary.qo");
JAddNumberToObject(req, "port", 50);
J *body = JAddObjectToObject(req, "body");
JAddNumberToObject(body, "i_a_rms",       14.1);
JAddNumberToObject(body, "i_b_rms",       14.1);
JAddNumberToObject(body, "i_c_rms",       14.1);
JAddNumberToObject(body, "i_total",       14.1);
JAddNumberToObject(body, "loading_pct",   14.1);
JAddNumberToObject(body, "imbalance_pct", 14.1);
JAddNumberToObject(body, "temp_c",        14.1);
JAddNumberToObject(body, "overloads",     12);
JAddNumberToObject(body, "samples",       12);  // loaded intervals
JAddNumberToObject(body, "total_wakes",   12);  // all wakes (loaded + idle)
notecard.sendRequest(req);

7.9 Key code snippet 2: CT RMS two-pass algorithm

Pass 1 determines the actual DC bias offset (should be ~2048 counts for a well-built circuit; the measured value accounts for component tolerance). Pass 2 computes RMS of the centered AC signal using float accumulation to avoid 32-bit integer overflow at full-scale input. Both passes pace samples to a fixed 225 microseconds period so the RMS burst spans a deterministic ~333 milliseconds (≈20 mains cycles at 60 Hz) regardless of the host ADC's native throughput. See §7.4 for why this matters.

float readCtRms(uint8_t pin) {
    // Pass 1: establish DC offset (paced sampling — see §7.3).
    long sum = 0;
    uint32_t next_us = micros();
    for (int i = 0; i < CT_DC_SAMPLES; i++) {
        sum += analogRead(pin);
        next_us += CT_SAMPLE_PERIOD_US;
        int32_t wait = (int32_t)(next_us - micros());
        if (wait > 0 && wait < (int32_t)CT_SAMPLE_PERIOD_US) {
            delayMicroseconds((uint32_t)wait);
        }
    }
    int offset = (int)(sum / CT_DC_SAMPLES);

    // Pass 2: AC RMS over ~333 ms (≈20 mains cycles at 60 Hz).
    float sum_sq = 0.0f;
    next_us = micros();
    for (int i = 0; i < CT_RMS_SAMPLES; i++) {
        float s = (float)(analogRead(pin) - offset);
        sum_sq += s * s;
        next_us += CT_SAMPLE_PERIOD_US;
        int32_t wait = (int32_t)(next_us - micros());
        if (wait > 0 && wait < (int32_t)CT_SAMPLE_PERIOD_US) {
            delayMicroseconds((uint32_t)wait);
        }
    }
    float i_rms = sqrtf(sum_sq / CT_RMS_SAMPLES) * CT_SCALE;
    return (i_rms < CT_NOISE_FLOOR_A) ? 0.0f : i_rms;  // zero noise floor
}

7.10 Key code snippet 3: immediate-sync alert

sync:true tells the Notecard to skip the outbound queue timer and open a cellular session immediately. On cellular the alert typically reaches Notehub within 15–60 seconds of the threshold trip.

J *req = notecard.newRequest("note.add");
JAddStringToObject(req, "file", "xfmr_alert.qo");
JAddBoolToObject(req,   "sync", true);
J *body = JAddObjectToObject(req, "body");
JAddStringToObject(body, "alert",    "phase_imbalance");
JAddNumberToObject(body, "i_a_rms",  i_a);
JAddNumberToObject(body, "i_b_rms",  i_b);
JAddNumberToObject(body, "i_c_rms",  i_c);
JAddNumberToObject(body, "temp_c",   temp_c);
JAddNumberToObject(body, "extra",    imbalance_pct);
notecard.sendRequest(req);

7.11 Key code snippet 4: sleeping until the Notecard raises ATTN

After sending any queued Notes, loop() asks the Notecard to hold ATTN low for sample_interval_sec seconds, confirms the pin has gone low, and puts the STM32 into STOP2. The host stays alive, so the card.attn call is a request whose result is checked rather than a fire-and-forget command. When the Notecard raises ATTN, execution resumes on the next line and loop() runs again with state intact.

// In setup() — arm the ATTN wake once
cxSleepBegin();

// At the end of every loop() pass — sleep until ATTN rises
if (!cxSleepUntilAttn(notecard, cfg.sample_interval_sec, NULL, &debugSerial)) {
    // The Notecard didn't take the sleep request, or ATTN never went low
    // (check the ATTN -> D5 jumper).  Keep the sample cadence and retry.
    debugSerial.println("[sleep] ATTN sleep failed — waiting out the interval awake");
    delay(cfg.sample_interval_sec * 1000UL);
}

8. Data Flow

Every 5 minutes (sample_interval_sec), the host wakes, reads the configured CT channels (A0 always; A1 when phase_count≥2; A2 when phase_count=3) and the temperature sensor, and evaluates three threshold rules against the freshly-read values and accumulated history.

Collected. Per-phase RMS current (A) on phases A, B, and C; enclosure temperature (°C); total CT current (sum of phases); derived loading percentage; derived phase-imbalance percentage; count of overloaded sample intervals in the current summary window.

Transmitted:

• xfmr_summary.qo — one record per summary_interval_min (default 24 records/day), containing per-phase means over loaded sample intervals (intervals in which at least one phase exceeded the 0.5 A noise floor), mean enclosure temperature, count of overloaded sample intervals, number of loaded samples (samples), and total host wakes in the window (total_wakes). Template-encoded; queued and flushed by the Notecard's hourly outbound sync. Multiply loading_pct by samples / total_wakes to recover the true time-weighted window average (see §11).
• xfmr_alert.qo — emitted immediately on threshold trip, with sync:true to bypass the outbound interval. Each alert type has a 30-minute cooldown to prevent alert storms.

Triggers. Three independent rules:

• overload — any single phase exceeds rated_amps × (overload_pct / 100). The extra field carries the peak phase's loading percentage. A transformer persistently above 95% of nameplate rating faces insulation aging that shortens its service life measurably.
• phase_imbalance — (max_phase − min_phase) / max_phase exceeds imbalance_pct_thresh while at least one phase is above 2A. The imbalance check is skipped entirely when phase_count=1 (single-leg mode); imbalance_pct is always 0.0 in both the real-time alert path and the summary average. With phase_count=2, only L1 and L2 are compared — the unconnected C channel (always 0A) is excluded from the min/max calculation. The extra field carries the computed imbalance percentage. On three-phase transformers, phase imbalance generates negative-sequence currents that heat the core, waste capacity, and can push a lightly-loaded phase into overvoltage.
• high_temp — enclosure temperature exceeds temp_alert_c. The extra field carries the total current across all phases at the time of the alert. Temperature combined with load is the primary mechanism of accelerated transformer aging; the Arrhenius rule of thumb says every 10°C above design temperature halves insulation life.

Routed. Both Notefiles flow through Notehub. From there, operators configure routes to send xfmr_alert.qo to an outage-management system or on-call paging tool and xfmr_summary.qo to a long-term historian for trending and transformer-life analytics.

9. Validation and Testing

Expected steady-state behavior. On a healthy, lightly-loaded transformer with well-balanced phases, the Events tab in Notehub should show one xfmr_summary.qo per hour and zero xfmr_alert.qo events. The serial monitor should print [sample] lines every 5 minutes and [summary] queued once per hour (the Note reaches Notehub on the next outbound sync).

Bench bring-up without a live transformer. Connect the CTs to a known-current AC circuit (a 120VAC lamp load on one phase works well) and verify the i_a_rms field in the summary matches the expected current (within ~5%). With no load on a CT channel, the reading should be 0.0A — if it's non-zero, the bias network has a wiring error or the ADC offset is drifting outside the noise floor.

Triggering alerts for validation. Drop overload_pct to 50.0 in the Fleet environment (the minimum the firmware permits. See §7.6 for the full clamp range). The next inbound sync (≤2 hours by default) will pull the new value, and the next sample will fire an overload alert on any load above half of rated_amps. Reset overload_pct to your intended value afterward.

Power validation with Mojo. The Notecard Cell+WiFi (MBGLW) idle current is ~18 µA @ 5V. Cellular transmit sessions draw ~250 mA average with brief peaks up to ~2 A. The host MCU, fully gated off by card.attn, contributes essentially nothing between samples.

Expected power profile at the defaults (5-min sample, hourly transmit):

A useful Mojo trace to look for:

• Healthy: a sustained low baseline at the board's quiescent current level (establish the exact value with Mojo during commissioning, it reflects Notecarrier and regulator draws with the host in STOP2), brief ~1 second blips every 5 minutes (host ADC reads across three channels), one 10–30 second burst at ~250 mA per hour (cellular sync).
• Host not sleeping: a sustained elevated baseline significantly above the quiescent level you measured with the host asleep. The host is not entering STOP2 — check the ATTN→D5 jumper (the serial log shows [sleep] ATTN sleep failed when ATTN never reaches D5) and confirm the firmware was built with USB support set to None (usb=none); with USB CDC enabled the host leaves STOP2 within a millisecond.
• Weak signal: correctly-spaced hourly bursts but each burst is >60 seconds at peak. Route the cellular antenna away from the transformer casing.

Mojo is a bench-validation and regression tool — it is not required in production. Once a firmware revision passes the trace check, deployed units don't need it.

10. Troubleshooting

11. Limitations and Next Steps

A pole-mount monitor that drives operational decisions about live distribution equipment has a high bar to meet in production: surge protection, signed utility approvals, billing-grade accuracy where it matters, and survivable comms when the transformer itself fails. This POC keeps the implementation narrow on purpose, with three signals (per-phase load, imbalance, enclosure thermal stress) over one cellular path, so the alerting and trending pieces stay readable. The items below mark both where that scope was drawn and the natural production additions.

Simplified for this POC

Each of these is a deliberate scope boundary that keeps the alerting and trending pieces readable, paired with what a production build would change.

The CT current range is bounded by the chosen sensor. The SCT-013-000 (100A) suits transformers up to approximately 20 kVA at 240V (~83A full-load). Transformers at or above 25 kVA at 240V draw ~104A, beyond this CT's rating, and require a higher-range current-output CT (200A or 400A). Voltage-output CTs are not drop-in replacements; see §4. The firmware CT_TURNS_RATIO and CT_BURDEN_OHMS constants need to be updated to match the chosen CT; all downstream calculations scale automatically.

RMS accuracy is monitor-grade, not billing-grade. The two-pass ADC algorithm covers approximately 20 mains cycles per channel (~330 milliseconds per channel, ~1 second total for three channels) at the host ADC's throughput rate. This gives adequate accuracy (~5–10%) for a load-threshold monitor, but is not suitable for billing-grade energy metering. Extending the sample count to cover ~200 mains cycles per channel would improve RMS accuracy at the cost of proportionally longer host-active time.

Summary averages reflect loaded intervals only. i_a_rms, i_b_rms, i_c_rms, i_total, loading_pct, and imbalance_pct in xfmr_summary.qo are computed only over the sample intervals (samples) where at least one phase exceeded the 0.5 A noise floor. Sample intervals where no load is detectable are excluded from the denominator. This means a lightly-loaded or intermittently-loaded window reports average current during the intervals when load was present, not the true time-weighted average across the full window. For fault detection and threshold alerting this is the appropriate behavior — alert thresholds should be evaluated against actual load conditions, not diluted by idle time. For utilization reporting or transformer-life analytics, multiply loading_pct by samples / total_wakes (both fields are in the summary payload) to recover the time-weighted window average.

Power factor is not measured. The firmware reports apparent current (A RMS), not real power (watts) or reactive power (VAR). A transformer nameplate rating is in kVA (apparent power), so loading percentage is correct as stated. Measuring true power would require voltage sensing in addition to current sensing — a meaningful production enhancement for a billing or power-quality application.

Temperature is single-point, an enclosure proxy and not a true ambient probe. This implementation uses an MCP9808 mounted inside the enclosure. It measures internal enclosure temperature, not true outdoor ambient air temperature and not transformer core or winding temperature. Enclosure temperature tracks thermal stress directionally but can diverge significantly from outdoor ambient, especially on a sun-exposed pole, and lags winding temperature by minutes to hours under transient load changes. A true ambient probe (external housing, shielded from direct sun) or a winding-contact sensor (PT100 RTD or thermocouple on the transformer case) would provide more actionable data; see Production Next Steps below.

This design uses conventional split-core CTs, not Rogowski coils. This implementation uses conventional current-output split-core CTs (YHDC SCT-013-000) rather than flexible Rogowski-type coils. True Rogowski coils produce a signal proportional to di/dt, requiring an external integrator board before the ADC. They offer no inherent accuracy advantage for power-frequency monitoring over calibrated current-output CTs and add meaningful cost and complexity. Flexible Rogowski coils (such as the Dent Instruments PowerScout or Magnelab RCT series) are the right choice for physically constrained installations where a rigid split-core jaw cannot open wide enough to encircle a large busbar or conductor bundle. That constraint does not apply to standard residential or commercial distribution transformer secondary leads, so this design uses the simpler current-output CT.

There is no inductive power harvest. The project description mentions optional inductive energy harvesting from the transformer. It is omitted from this POC because the complexity and installation constraints of a validated inductive harvester are substantial relative to the simplicity of a direct 120VAC tap on the transformer secondary. For installations where tapping the secondary is impractical (sealed enclosures, third-party owned equipment), inductive harvesting is the right production next step, but it would require a separate, validated power-stage design and is out of scope here.

There is no backup power. If the transformer fails (the scenario we're trying to detect early), the monitoring device also loses power and cannot report the outage itself. Adding a Blues Scoop with a small LiPo battery would allow the device to survive a brief outage and transmit a "power lost" event before the battery dies, a meaningful production addition for "last gasp" outage detection.

Alert cooldown is cycle-based. The cooldown counter tracks sample cycles, not wall-clock time. If sample_interval_sec is changed via env var after an alert fires, the cooldown duration shifts proportionally. For most deployments this is acceptable; for strict SLA compliance, store the alert timestamp in persistent state instead.

There is no surge or transient protection. The 2A slow-blow fuse on the 120VAC tap protects against a sustained overcurrent fault but provides no suppression of lightning-induced transients, switching surges, or capacitor-bank switching events — all of which are common on distribution transformer secondaries. A permanent pole-mount deployment requires: a surge protective device (SPD) rated for the site's overvoltage category (Category C / lightning-level, per IEC 61643-11 or UL 1449) installed on the Line/Neutral entry ahead of the fuse; enclosure grounding and bonding to the pole ground system per applicable NEC sections and utility rules; verified creepage and clearance in all mains-voltage wiring and connectors; UV and temperature qualification of all external cabling and the enclosure; and written utility approval before the secondary tap is permanently energized. This POC omits the SPD and grounding circuit as out of scope for a bench or controlled field-trial context; do not omit them in any design intended for permanent field deployment.

Production Next Steps

These additions take the monitor from a controlled trial toward a permanently energized, fleet-scale pole-mount deployment.

Surge and grounding protection is mandatory before permanent field deployment: a surge protective device (SPD) on the AC mains entry (Category C rated, per IEC 61643-11 or UL 1449), enclosure grounding/bonding to the pole ground system per applicable NEC sections and utility rules, and written utility approval.

Higher-range CTs (200A, 400A) cover 50–167 kVA distribution transformers; update CT_TURNS_RATIO and CT_BURDEN_OHMS accordingly.

A contact temperature probe on the transformer case enables direct winding-temperature monitoring — a PT100 RTD via MAX31865 (SPI, requires three additional GPIO pins for SCLK/MISO/CS) or a thermocouple with a MAX31856 (also SPI). An I²C RTD option such as the Texas Instruments TMP117 (±0.1°C, I²C) avoids the extra SPI pins but is limited to –40°C to +125°C, which covers typical transformer enclosure ranges.

Voltage measurement on the secondary (a resistor divider plus a precision analog input or an external ADC) would let the device compute real power, power factor, and true apparent kVA loading.

Inductive energy harvesting from a current-carrying secondary conductor is an alternative to the 120VAC tap — valuable for sealed enclosures where the tap is not accessible.

A Blues Scoop with LiPo enables last-gasp outage reporting when the primary supply fails.

Over-the-air host firmware updates across the deployed fleet are critical when physical re-flash of hundreds of pole-mount units is impractical. Notecard Outboard Firmware Update (ODFU) provides this capability; verify the specific wiring and bootloader support for the Notecarrier CX's STM32L433 host against Blues documentation before committing to a production design.

Per-transformer commissioning would add a one-time rated_amps calibration via env var at install time to account for field-measurement of the actual secondary current at known load.

12. Summary

The pole-top transformer that used to be invisible until Friday's peak demand took down the neighborhood now reports for itself every hour. Three split-core CTs clamped on the secondary leads plus one I²C temperature sensor give the operations center the three signals that matter most for transformer health (per-phase loading, phase imbalance, and thermal stress) with hourly summaries for capacity planning and immediate xfmr_alert.qo Notes when a phase climbs past 95% of rated current or the enclosure runs hot. The same SKU, the same SIM, and the same firmware image work on a dense urban feeder and a rural single-wire run, which is the difference between a pilot that works on paper and a fleet program that actually scales. The dispatcher gets the heads-up before the streetlights go out; the planner gets a year of loading history without a single truck roll to read it.