Off-Grid Livestock Water Tank Monitor

note

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/livestock_water_tank_monitor/ — so it can learn how the device works and help you adapt it to your hardware.

This project is a solar-powered remote monitoring system for off-grid livestock water tanks. It tells a rancher when a stock tank is going dry — without anyone driving the pasture roads to check. The device measures the water level, the pump's current draw, and the solar system's own battery voltage, then reports all three to a phone or dispatch system over cellular wherever a tower is reachable and over satellite via Skylo's non-terrestrial network (NTN) where it isn't. The radio stays off between an immediate alert and a 4-hour summary, so the device runs indefinitely on a modest solar panel and a single battery.

What You'll Build

After following this guide, you will have a solar-powered off-grid tank monitor that:

• Samples tank level, pump current, and battery voltage every 15 minutes via analog sensors wired to a Notecarrier CX
• Alerts the rancher immediately (via the Blues Notehub cloud service, with routes to SMS/push/webhook) when the tank drops below 20% full (alert) or 10% full (critical)
• Reports system health (battery voltage, pump current) every 4 hours to a time-series database for trend analysis
• Falls back to satellite (Skylo NTN) when cellular is unavailable, with the identical firmware handling both transports
• Runs for weeks on solar even during cloudy stretches, thanks to an adaptive sleep strategy that extends the sample interval when the battery is low

Expected data consumption: ~6 KB/month on the 500 MB included prepaid Blues data plan (cellular path; satellite is separate). First production event visible in Notehub within 4 hours of power-on.

1. Project Overview

The problem. Stock tanks in remote pastures are one of agriculture's oldest and most persistent operational problems. A rancher managing a spread across multiple pastures — each with its own poly or galvanized stock tank and a submersible pump pulling from a well — has to physically drive every road to verify that every tank is full. On a working ranch with pastures spread over thousands of acres, that inspection loop can take several hours and still miss a dry tank that empties between visits. The consequences aren't just inconvenience: cattle deprived of water for even a few hours in summer heat suffer rapid decline in health, and emergency water delivery is expensive even before factoring in animal losses.

The failure modes are simple and repeatable. Tanks go dry because a float valve sticks or fails and the tank drains down, because the pump's supply well drops below the intake (the pump keeps running but moves no water), or because the pump motor fails entirely and nothing moves even when the float calls for it. None of these require sophisticated modeling — they are observable conditions that nobody happens to be watching. The sensor suite here measures exactly the two signals a rancher or hired hand would check on a physical inspection: how high is the water, and is the pump drawing current? The third measurement — solar battery voltage — tells you whether the monitoring system itself is healthy and likely to keep reporting through a run of cloudy days.

Why Notecard. Stock tanks sit miles from the ranch house, beyond WiFi range, beyond LoRa range, and frequently beyond the reach of any terrestrial infrastructure. Notecard for Skylo addresses both halves of the connectivity problem in a single module: it uses cellular (LTE-M/NB-IoT) where a tower is reachable and falls back to the Skylo NTN satellite network where terrestrial coverage ends. Many ranch pastures sit in mixed coverage — a cellular signal may be available across most of a property, but valleys and remote corners go dark. A tank in a covered valley reports over cellular while a tank on a ridge beyond any tower still reports via satellite; the rancher sees both without understanding which network carried the data. No SIM activation, no carrier contract, no per-site configuration required. The Notecard also handles the low-power half: in periodic mode with a 15-minute sampling interval, the radio is active for tens of seconds every few hours and silent the rest of the time, making it possible to run this device indefinitely on a modest solar panel and a single battery even through overcast weeks in a northern winter.

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Learn what we do in this 3-minute video that walks you through how we simplify wireless connectivity.

Deployment scenario. A weatherproof IP65 enclosure mounted on the tank post or fence rail adjacent to the tank opening. A MaxBotix ultrasonic level sensor peers down through the lid into the tank. A clamp-on current transformer clips around one conductor of the pump's supply lead without cutting any wire. A two-resistor voltage divider reads the 12V solar battery bus. A small solar panel, charge controller, and sealed battery live in or near the enclosure. Once installed and calibrated, the system reports tank levels and pump health to the rancher's phone through Notehub — requiring no physical site visit unless something goes wrong.

2. System Architecture

Device-side responsibilities. The Notecarrier CX's onboard STM32L433 host wakes on a 15-minute interval, reads three sensors (ultrasonic level, pump RMS current, solar battery voltage), evaluates alert thresholds, accumulates readings into a rolling window for the next summary, and returns to sleep. Between wakes the host sleeps in STM32 STOP2 (a few µA) until the Notecard's ATTN pin wakes it via card.attn. All Notecard interaction happens over I²C — no AT commands, no serial buffers, no JSON hand-rolling.

Notecard responsibilities. Notecard for Skylo stores Notes in its on-device queue, selects the best available transport (cellular or Skylo satellite), manages the session on the configured hub.set outbound cadence (default 4 hours), and flushes sync:true alert Notes immediately regardless of that cadence. Transport selection is transparent to the host firmware — the same note.add JSON that queues a Note over cellular also queues it over satellite. The Notecard also distributes environment variables from Notehub on each inbound sync — alert thresholds, tank calibration values, and sampling intervals are all operator-tunable from the Notehub console without re-flashing the device.

Notehub responsibilities. Notehub receives and stores every event — whether it arrived via the cellular or satellite transport, and applies project-level routes. Periodic summaries and alerts land in separate Notefiles, so routes can fan them to different destinations: alerts to an SMS gateway or push-notification service, summaries to a long-term time-series store for trend analysis. Smart Fleets can classify devices automatically by pasture name or tank ID so that fleet-level environment variables encode site-specific calibration values without touching the firmware.

Routing to the cloud (high level only). Notehub supports HTTP, MQTT, AWS, Azure, GCP, Snowflake, and other destinations; route configuration is project-specific and outside the scope of this reference design. See the Notehub routing docs — this project ships no downstream endpoint.

3. Technical Summary

1. Assemble hardware. Order the BOM from §4, build the circuit per §5, and mount the enclosure on the tank post.
2. Flash firmware. Clone this repo, set your Notehub ProductUID in the sketch, jumper the Notecarrier CX's ATTN pin to D5 (see §5), and build and flash:
   COPY

   arduino-cli core install STMicroelectronics:stm32
   arduino-cli lib install "Blues Wireless Notecard" "STM32duino Low Power" "STM32duino RTC"
   arduino-cli compile --fqbn STMicroelectronics:stm32:Blues:pnum=CYGNET,usb=none \
     firmware/livestock_water_tank_monitor/livestock_water_tank_monitor.ino
   arduino-cli upload -p /dev/ttyACM0 --fqbn STMicroelectronics:stm32:Blues:pnum=CYGNET,usb=none \
     firmware/livestock_water_tank_monitor

   The usb=none board option disables the USB CDC stack, which must be off for the host to sleep (see §7). The board options above match firmware/livestock_water_tank_monitor/sketch.yaml, so omitting --fqbn also works when invoked from the sketch directory.
3. Set calibration. Power on and wait for the device to sync to Notehub (first cellular connection within minutes if coverage exists, or satellite within ~10 minutes if cellular is unavailable). Once a tank_status.qo Note appears in Notehub, measure your actual tank geometry and set tank_depth_mm and sensor_min_mm as environment variables in Notehub (see §6 step 4 for the path). After the next sync, level_pct will report accurate percentages.
4. Set routes. In Notehub, add two routes: one for tank_alert.qo (to SMS/push service) and one for tank_status.qo (to your time-series database or data lake). See §6 and the Notehub routing docs.

Full assembly and calibration instructions follow in later sections; this quickstart gets you to first event in under an hour.

Here is a sample Note this device emits:

{
  "file": "tank_status.qo",
  "body": {
    "_time": 1717200000,
    "level_pct": 68.4,
    "distance_mm": 542.0,
    "pump_amps": 7.2,
    "pump_on": true,
    "battery_v": 12.8,
    "alerts": 0
  }
}

4. Hardware Requirements

All Blues hardware ships with an active SIM including 500 MB of cellular data and 10 years of service — no activation fees, no monthly commitment. The NOTE-NBGLWX also includes Skylo satellite data as part of its service plan; see the Blues pricing page for current satellite data allotments and usage terms.

5. Wiring and Assembly

All host I/O lands on the Notecarrier CX dual 16-pin header. Notecard for Skylo seats into the M.2 slot. The Mojo is a bench power-measurement instrument spliced inline between the step-down regulator output and the Notecarrier's +VBAT pad during commissioning; it is not read by the deployed firmware and is removed before field installation.

warning

Safety. This installation involves the pump's supply conductors, a 12V battery, solar panel wiring, and charge-controller terminals — all of which can carry hazardous voltages or stored energy. De-energize and lock out / tag out the pump circuit before installing or modifying any wiring. Follow the pump manufacturer's and charge-controller manufacturer's installation instructions in full. Place a fuse in the positive battery lead as close to the battery terminal as practical (3–5 A for the monitor load; consult the charge-controller manual for the panel and battery leads). Electrical work should be performed by qualified personnel following applicable local electrical codes.

Solar power chain (panel, charge controller, battery):

• Solar panel + → charge controller PV + input; solar panel − → charge controller PV − input.
• Charge controller BATT + → 12V battery positive terminal; charge controller BATT − → 12V battery negative terminal. If the controller does not include a fuse, place a 3–5A automotive blade fuse in the positive wire between the battery terminal and the controller's BATT + terminal.
• Take the monitor load from the charge controller's LOAD output terminals (not directly from the battery terminals). This allows the controller's low-voltage disconnect to protect the battery from deep discharge during extended overcast periods.
• Common ground: charge controller BATT −, the LOAD − output, and the 12V battery negative terminal must share a common ground bus. A short, low-resistance wire between them eliminates ground-offset measurement errors on the A2 battery-voltage divider.

Monitor power chain (charge controller LOAD → Notecarrier): Charge controller LOAD + → Pololu D24V10F5 step-down input VIN (5V output) → Notecarrier CX +VBAT. The Notecarrier draws from +VBAT and internally generates the 3.3V rail for the host and sensors. During bench commissioning, splice the Mojo inline between the step-down output and +VBAT for power measurement, then remove it before field deployment. The battery-voltage divider top-side connects directly to the battery terminals (not the LOAD terminals) so it reads true battery voltage; see the voltage-divider circuit below for details on the PMOS high-side switch that controls when current flows through the resistor chain.

Level sensor (A0):

• MaxBotix MB7389 V+ → Notecarrier +3V3_OUT
• MaxBotix MB7389 GND → Notecarrier GND
• MaxBotix MB7389 AN (analog voltage output pin) → Notecarrier A0
• Leave the MB7389 TX (TTL serial) and PW (pulse-width) pins unconnected; only AN is used in this design.

Pump current sensor (A1): Clamp the SCT-013-030 jaws around one conductor of the pump's supply cable (either leg, clamping both will cancel and read zero). Then wire the bias circuit:

• Notecarrier +3V3_OUT → 10 kΩ resistor → bias node → 10 kΩ resistor → Notecarrier GND. These two resistors form a symmetric voltage divider; the bias node sits at approximately 1.65 V (Vcc/2).
• SCT TRRS breakout TIP → bias node (the CT's AC output voltage rides on the DC bias midpoint)
• SCT TRRS breakout SLEEVE → Notecarrier GND
• Bias node → 10 µF electrolytic capacitor positive lead; capacitor negative lead → GND (the cap stabilizes the DC midpoint during pump-start transients)
• Bias node → Notecarrier A1

Solar battery voltage divider (A2 + A3): A BSS84 P-channel MOSFET acts as a high-side switch between the 12 V battery bus and the top of the voltage-divider resistor chain. Without this switch, the divider would draw from the battery bus continuously, and A2 would remain driven whenever the host MCU is unpowered, which can back-power the host through the A2 input-protection diode. A 100 kΩ gate pullup (battery+ → gate) keeps the PMOS off by default; an MMBT3904 NPN level-shifts the 3.3 V host GPIO (A3) to override that pullup and turn the PMOS on when a battery sample is needed.

Wire the circuit as follows:

• 12 V battery positive → BSS84 source
• BSS84 drain → 47 kΩ resistor → divider node → 10 kΩ resistor → Notecarrier GND
• Divider node → Notecarrier A2
• BSS84 gate → 100 kΩ resistor → 12 V battery positive (gate pullup)
• BSS84 gate → MMBT3904 collector
• MMBT3904 emitter → Notecarrier GND
• MMBT3904 base → 10 kΩ resistor → Notecarrier A3

Operation. Firmware drives A3 HIGH before sampling: the MMBT3904 saturates, pulling the BSS84 gate to near GND (V_GS ≈ −12 V, well beyond the −0.8 to −2 V threshold). After the four ADC samples, firmware drives A3 LOW; the MMBT3904 turns off and the 100 kΩ pullup restores the gate to 12 V (V_GS = 0 V), turning the PMOS off within ≈ 100 µs. While the host sleeps in STOP2, A3 stays LOW (STOP2 retains GPIO state), and if the host is ever unpowered and A3 floats, the 100 kΩ pullup independently holds the PMOS gate at 12 V — either way A2 sits at GND through the low-side 10 kΩ. No leakage path reaches the A2 protection structures.

The voltage scaling is unchanged: battery_V = V_adc / 0.175, mapping 0–14.5 V to 0–2.54 V, within the 3.3 V ADC range. The divider connects to the battery terminals (not the LOAD terminals) so the MCU reads the true battery bus voltage.

ATTN-to-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.

Antennas: The NOTE-NBGLWX exposes two u.FL ports — MAIN (cellular AND satellite, served by a single Skylo-certified LTE-capable antenna covering both LTE and S-Band/L-Band frequencies) and GPS (passive GPS/GNSS antenna; not used by this firmware and may be left unconnected). Connect a short u.FL-to-SMA female pigtail (≥100 mm, RG316/RG178) from the MAIN u.FL port to an SMA female bulkhead in the enclosure wall, then mate the included Skylo-certified antenna to the bulkhead from outside via its SMA connector. Use the antenna provided in the kit — substituting a third-party cellular antenna will invalidate the Skylo certification and may result in network blocking by Skylo. Route the antenna to the exterior of the enclosure lid or to an elevated bracket with an unobstructed view of the equator-facing sky (southern sky in the northern hemisphere; northern sky in the southern hemisphere). An antenna inside a metal enclosure or blocked by trees, structures, or terrain on the horizon toward the equator will fail to acquire Skylo GEO satellites — even when cellular reception is sufficient, deploy with sky view in mind so the satellite fallback path remains available. Refer to the Satellite Best Practices guide for siting guidance.

Level sensor mounting: Mount the MB7389 vertically (sensing face pointing straight down) through a 1.5–2 inch hole in a tank lid or on a bracket over the open top. Keep at least 300mm of clearance between the sensor face and the maximum expected water surface — this is the MB7389's blanking zone below which readings are unreliable. Record the actual mounting height above the full waterline and enter it as sensor_min_mm in Notehub after installation.

6. Notehub Setup

1. Create a project. Sign up at notehub.io and create a project. Copy the ProductUID and paste it into firmware/livestock_water_tank_monitor/livestock_water_tank_monitor.ino as PRODUCT_UID.

2. Claim the Notecard. Power the unit; on first successful Notehub session, whether over cellular or Skylo satellite, the Notecard associates with your project automatically.

3. Create a Fleet per site. Fleets group devices for shared configuration and routing. A natural unit here is one fleet per pasture — every tank in a pasture shares the same approximate tank geometry, pump type, and battery system. Smart Fleets can classify devices automatically by a tag or environment variable so you don't have to assign them manually as your fleet grows.

4. Set environment variables. All variables are optional; firmware compile-time defaults apply if a variable is absent from Notehub. Changes sync to the device on the next inbound session — no re-flashing required. The two most important variables are the tank calibration values (tank_depth_mm and sensor_min_mm), which must be set per installation to get accurate level percentages.

   Variable	Default	Purpose
   tank_depth_mm	1200	Distance (mm) from the sensor face to the dry tank bottom. Measure with the tank empty.
   sensor_min_mm	300	Distance (mm) from the sensor face to the water surface when the tank is full. Must be ≥ 300 mm (the MB7389 blanking-zone minimum, below which the sensor cannot produce valid readings). The compile-time default equals the blanking-zone floor and is a placeholder — replace it with the measured full-tank distance during commissioning.
   level_alert_pct	20	Tank level (%) below which a level_low alert fires. The rancher has time to act.
   level_critical_pct	10	Tank level (%) below which a level_critical alert fires. Cattle may dewater within hours.
   pump_on_amps	1.0	RMS amps above which the pump is considered running. Set above the CT noise floor and below the pump's normal running current.
   battery_alert_v	11.5	Solar battery voltage (V) below which a battery_low alert fires. Indicates extended overcast weather or a charge controller fault.
   sample_interval_sec	900	Seconds between sensor samples. Default is 15 minutes. Reduce this during calibration (alongside summary_interval_min) so that each summary window contains at least one fresh sample; see §10.
   summary_interval_min	240	Minutes between periodic summary Notes. Default is 4 hours (6 Notes per day). Must be at least ⌈sample_interval_sec ÷ 60⌉ minutes — a summary window shorter than one sample wake contains no data. The firmware auto-clamps values below this floor at runtime; reduce sample_interval_sec proportionally when a short calibration window is needed (see §10). When this value changes, the firmware detects the difference on the next wake and re-issues hub.set with the updated cadence, so the Notecard's outbound sync interval stays aligned with the local summary schedule — no physical reboot required.
   alert_cooldown_sec	3600	Minimum seconds between repeated level_low and level_critical alerts. The battery_low alert is edge-triggered instead and is not subject to this cooldown — it fires once per episode and re-arms only after the voltage recovers above battery_alert_v + 0.5V.

5. Configure routes. Add at minimum one route for tank_alert.qo (to an SMS gateway, push-notification service, or on-call webhook) and a second for tank_status.qo (to a time-series store for trend analysis and historical review). The Notefile separation means you can send alerts to your phone immediately while batching summaries to a database nightly, without any filtering logic in the route.

   Example alert payload (immediate, sync:true):

   COPY

   {
     "alert_code": 1,
     "level_pct": 8.5,
     "pump_amps": 0.0,
     "battery_v": 12.4,
     "_time": 1717200000
   }

   Alert codes: 0 = tank at 20% (low alert), 1 = tank at 10% (critical), 2 = battery below 11.5V.

   Example status payload (every 4 hours):

   COPY

   {
     "level_pct": 68.4,
     "distance_mm": 542.0,
     "pump_amps": 7.2,
     "pump_on": true,
     "battery_v": 12.8,
     "alerts": 3,
     "_time": 1717200000
   }

   alerts = count of tank_alert.qo Notes sent since the last summary; pump_on is derived from window-average pump_amps vs. the pump_on_amps threshold.

7. Firmware Design

The implementation spans four files in the firmware/livestock_water_tank_monitor/ directory, with the .ino as the entry point:

Dependencies:

• Arduino core for STM32 (stm32duino/Arduino_Core_STM32).
• Blues Wireless Notecard — the note-arduino library. Install via the Arduino Library Manager or download from the note-arduino releases.
• STM32duino Low Power (and its dependency STM32duino RTC) — install via Library Manager. 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 (battery power, 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 (when TANK_MONITOR_DEBUG is defined) 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.

Modules

Sensor reading strategy

Level sensor. The MB7389 continuously outputs an analog voltage proportional to measured distance: V_out = V_cc / 5120 × distance_mm. Powered at 3.3V, the output maps to 0–3.22V across the 300–5000mm range, which sits cleanly within the host's 3.3V ADC reference. The firmware takes a 16-sample average of the 12-bit ADC counts to suppress noise, converts to millimeters, then maps to a 0–100% fill level using the calibrated tank_depth_mm (empty) and sensor_min_mm (full) values. Readings outside the MB7389's documented 300–5000mm range are flagged invalid and excluded from the accumulator. Both distance_mm and level_pct are accumulated across all valid samples in the summary window and emitted as window averages in the periodic summary Note.

Pump current. The SCT-013-030 CT generates an AC voltage centered at the bias midpoint (~1.65V). The firmware first measures the DC bias from a 256-sample mean, then collects ~1500 samples (spanning approximately 20 AC mains cycles at the STM32's ADC throughput) and computes the root-mean-square deviation from that bias. The RMS voltage converts to current at 30A per volt. Readings below a 0.15A noise floor are clamped to zero to prevent ADC noise from generating false "pump running" detections.

Battery voltage. A 4-sample average of the A2 ADC channel, scaled back through the voltage divider ratio: battery_V = (adc × 3.3 / 4095) / (10 / 57). This measures the actual 12V solar battery bus voltage, not the regulated 5V rail — giving a genuine read on the system's energy state.

Event payload design

Both Notefiles are template-backed, storing Notes as fixed-length binary records instead of free-form JSON. Compact binary encoding reduces per-Note wire size by roughly 3–5× and, critically, is required for the NOTE-NBGLWX Skylo NTN satellite path — both templates are registered with "format": "compact" and an explicit port number so either Notefile can transit via satellite or cellular using the identical note.add call. Over a multi-year deployment generating 6 summary Notes per day, that compression also meaningfully extends the useful life of the 500 MB prepaid data budget. Alert Notes carry sync:true to request an immediate radio wake; fixed-length binary encoding means pump_amps = 0.0 (pump off at alert time) is transmitted faithfully — there is no omitempty-style drop for zero-valued numeric fields in compact encoding.

tank_status.qo (periodic, every summary_interval_min; compact template-encoded, port 50):

{
  "file": "tank_status.qo",
  "body": {
    "_time": 1717200000,
    "level_pct": 68.4,
    "distance_mm": 542.0,
    "pump_amps": 7.2,
    "pump_on": true,
    "battery_v": 12.8,
    "alerts": 0
  }
}

note

pump_on is a derived field, not a per-sample boolean. Each sample contributes its RMS current reading to a running average; at summary time sendSummary receives that 4-hour window average as pumpAmps and sets pump_on = (pumpAmps >= pump_on_amps). A value of true means the average pump current over the summary window was at or above the running threshold, not that the pump ran continuously, and not that it ran for any particular fraction of the window. A pump that ran briefly at the start of a 4-hour window and then stopped can produce a non-zero pump_amps average with pump_on: false; a pump that ran continuously all window will produce pump_on: true. Use pump_amps (the numeric average) for trend analysis; treat pump_on as a coarse summary-window classification — true indicates the window-average current cleared the threshold, not that the pump ran continuously or for any particular fraction of the window.

tank_alert.qo (immediate, sync:true; compact template-encoded, port 51):

{
  "file": "tank_alert.qo",
  "body": {
    "_time": 1717200000,
    "alert_code": 0,
    "level_pct": 18.1,
    "pump_amps": 0.0,
    "battery_v": 12.6
  },
  "sync": true
}

note

alert_code is an integer type code: 0 = level_low, 1 = level_critical, 2 = battery_low. The integer field keeps the compact-template payload fixed-length binary — the same requirement as the summary template for the NOTE-NBGLWX Skylo NTN path. All five fields (including pump_amps = 0.0 when the pump is off) are always present in compact binary encoding.

Low-power strategy

The host is awake for only a few seconds per 15-minute sample cycle. After each wake, doSleep() calls cxSleepUntilAttn() (in cx_sleep.h), which sends a card.attn sleep request for the computed interval, waits for the Notecard to pull ATTN low, and puts the STM32L433 into STOP2 — a few microamps with 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 runtime state (accumulator values, alert cooldown timestamps, summary window anchor, last-applied hub cadence, and the env-var cache) simply lives in the GlobalState struct in RAM; nothing is serialized to the Notecard and setup() runs only once per power-up. 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 Notecarrier CX's +3V3_OUT rail stays up while the host sleeps, so the MB7389 level sensor and the CT bias network remain powered between samples — the MB7389 keeps ranging at a few milliamps, which sets the practical sleep floor for this design (gating its V+ from a spare GPIO is the obvious next step for a battery-constrained site). The battery-voltage measurement circuit is off between samples: A3 holds LOW through STOP2 and the BSS84 PMOS gate sits at 12 V via the 100 kΩ pullup, leaving A2 at GND through the low-side 10 kΩ and drawing zero quiescent current from the battery bus. The NOTE-NBGLWX Notecard itself idles at approximately 18 µA @ 5V in its own low-power idle state between sessions, with the Skylo satellite modem also in low-power standby when not transmitting.

Sampling and transmission are deliberately decoupled: the device samples every 15 minutes but transmits summaries only every 4 hours. Alerts bypass the transmit timer via sync:true — a critically low tank reaches the rancher's phone within a session-establishment window regardless of where the Notecard is in its outbound cycle. Over cellular, that window is typically 15–60 seconds. Over the Skylo NTN satellite link, session establishment takes longer — the Notecard must acquire the GEO satellite and complete the NTN registration before data flows, which can take a few minutes depending on satellite geometry and signal conditions. Alert Notes are still delivered via satellite when cellular is unavailable; the delivery latency is higher than over cellular, but the consequence of a missed alert on an off-grid tank with no other connectivity path is worse than a slightly delayed one. For sites with reliable cellular, the cellular path is used first and the satellite path is never exercised; satellite energy cost does not apply.

When the firmware detects battery voltage below 12.0V (moderately discharged), it doubles the sleep interval to 30 minutes. Below battery_alert_v (default 11.5V, deeply discharged), it quadruples the interval to 60 minutes and fires a battery_low alert on the first crossing below that threshold. Further battery_low alerts are suppressed while the battery remains low and re-arm only after the voltage recovers above battery_alert_v + 0.5V. This edge-triggered approach prevents the 60-minute emergency-sleep cadence from issuing a sync:true radio session on every wake — which would worsen the very depletion it signals. The extended sleep intervals give the system a multi-day power reserve before the step-down regulator drops out, trading sampling granularity for survivability in extended overcast conditions.

Retry and error handling

• The firmware tracks the last outbound value issued to hub.set in GlobalState (appliedSummaryIntervalMin). After each env.get, if summary_interval_min differs from the persisted value, reapplyHubSet() re-issues hub.set. This keeps cellular outbound cadence synchronized with the operator-configured summary interval without requiring a physical reboot.
• Both note.template registrations (summary and alert) are tracked via a single templatesInstalled flag in GlobalState. The flag is set only when both templates succeed; if either registration fails on cold boot, the flag stays false and loop() retries both registrations on every subsequent wake until both succeed. Without this, a single failed registration would leave one or both Notefiles in free-form mode, bypassing the compact binary encoding required for the satellite path.
• Level readings outside the MB7389's valid range (300–5000mm) are excluded from the window accumulator. If an entire 4-hour window yields zero valid level readings, the summary emits level_pct: -1.0 as a sentinel so downstream analytics can distinguish "sensor fault" from a real near-zero fill.
• Pump current readings below the noise floor are clamped to 0.0 rather than excluded — a zero reading means "pump is off," not "no data."
• card.time returning 0 (Notecard has not yet synced and obtained network time) causes the alert evaluator to skip threshold checks for that wake cycle, preventing spurious alerts on the first cold boot.
• Alert de-duplication: level_low and level_critical alerts are cooldown-gated (one per alert_cooldown_sec, default 1 hour) so a slowly-draining tank doesn't page the rancher continuously. battery_low is edge-triggered — it fires once on the first crossing below battery_alert_v and is suppressed until the voltage recovers above battery_alert_v + 0.5V, preventing the 60-minute emergency-sleep cadence from issuing a sync:true radio wake on every cycle while the battery is already under stress.

Key code snippet 1 — template registration

The template uses "compact" format and an explicit port — both required for the NOTE-NBGLWX Skylo NTN satellite path. Type hints are literal numeric values representing field types:

• 14.1 = 32-bit float (e.g., level_pct, pump_amps, battery_v)
• 12 = 2-byte signed integer / int16_t (e.g., alerts field, range −32,768 to +32,767)
• true = boolean field (e.g., pump_on)
• 14 = Unix timestamp field; auto-populated by the Notecard on each note.add — no change to firmware is needed

Template-backed Notes reduce per-event wire size by ~4× and are required for satellite delivery.

J *req = notecard.newRequest("note.template");
JAddStringToObject(req, "file",   NOTEFILE_SUMMARY);
JAddNumberToObject(req, "port",   50);        // required for Skylo NTN
JAddStringToObject(req, "format", "compact"); // required for Skylo NTN
J *body = JAddObjectToObject(req, "body");
JAddNumberToObject(body, "_time",       14);   // Unix timestamp (Notecard auto-fills)
JAddNumberToObject(body, "level_pct",   14.1); // float32
JAddNumberToObject(body, "distance_mm", 14.1); // float32
JAddNumberToObject(body, "pump_amps",   14.1); // float32
JAddBoolToObject(body,   "pump_on",     true); // boolean
JAddNumberToObject(body, "battery_v",   14.1); // float32
JAddNumberToObject(body, "alerts",      12);   // int16 (2-byte signed, −32 768..+32 767)
if (notecard.sendRequest(req)) {
    g.templatesInstalled = true;  // retry on subsequent wakes if this fails
}

Key code snippet 2 — immediate alert with sync

The alert Note uses the compact template registered for tank_alert.qo (port 51, "compact" format), so it travels over the NOTE-NBGLWX Skylo NTN satellite path when cellular is unavailable — the identical note.add call works over either transport. sync:true tells the Notecard to wake the radio immediately rather than waiting for the next scheduled outbound window. The alert_code integer (0 = level_low, 1 = level_critical, 2 = battery_low) keeps the body fixed-length binary as required by the compact template; all fields, including pump_amps = 0.0, are always present.

if (!g.templatesInstalled) return false;  // template required for satellite path
J *req = notecard.newRequest("note.add");
JAddStringToObject(req, "file", NOTEFILE_ALERT);
JAddBoolToObject(req,   "sync", true);
J *body = JAddObjectToObject(req, "body");
JAddNumberToObject(body, "alert_code", alertCode);  // 0=level_low, 1=level_critical, 2=battery_low
JAddNumberToObject(body, "level_pct",  levelPct);
JAddNumberToObject(body, "pump_amps",  pumpAmps);
JAddNumberToObject(body, "battery_v",  battV);
notecard.sendRequest(req);

Key code snippet 3 — battery-adaptive sleep until ATTN

Sleep duration extends when the solar battery is low, conserving energy during multi-day overcast periods without requiring any cloud-side configuration change. cxSleepUntilAttn() then asks the Notecard to hold ATTN low for that many 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.

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

// In doSleep(), called at the end of every loop() pass
uint32_t sleepSec = g_sampleIntervalSec;          // default: 900 s (15 min)
if (battV > 0.0f && battV < 12.0f) sleepSec *= 2; // 30 min: battery moderate
if (battV > 0.0f && battV < g_batteryAlertV)       // 60 min: battery critical
    sleepSec *= 2;

if (!cxSleepUntilAttn(notecard, sleepSec, NULL, log)) {
    // The Notecard didn't take the sleep request, or ATTN never went low
    // (check the ATTN -> D5 jumper). Keep the sample cadence and try again.
    delay(sleepSec * 1000UL);
}

Key code snippet 4 — dynamic hub.set cadence

When a Notehub operator changes summary_interval_min, the firmware detects the change on the next wake and re-issues hub.set so the Notecard's outbound cellular cadence stays synchronized — no physical reboot required.

// After fetchEnvOverrides() returns true and updates g_summaryIntervalMin:
if (envOk && g_summaryIntervalMin != g.appliedSummaryIntervalMin) {
    J *req = notecard.newRequest("hub.set");
    JAddStringToObject(req, "product",  PRODUCT_UID);
    JAddStringToObject(req, "mode",     "periodic");
    JAddNumberToObject(req, "outbound", (int)g_summaryIntervalMin);
    JAddNumberToObject(req, "inbound",  (int)g_summaryIntervalMin);
    if (notecard.sendRequest(req)) {
        g.appliedSummaryIntervalMin = g_summaryIntervalMin;
    }
}

8. Data Flow

Every 15 minutes the host wakes, reads three sensors, and evaluates three independent alert conditions. Valid readings accumulate across wakes into a rolling window; each 4-hour summary averages each metric over that window.

Collected each sample. Tank fill level (distance mm → level percentage), pump RMS amps, solar battery voltage (V), per-alert cooldown timestamps. (pump_on is not sampled directly, it is derived at summary time from the window-average of pump_amps; see §7.)

Transmitted.

• tank_status.qo — one compact template-encoded Note (port 50) every summary_interval_min (default 4 hours, 6 Notes per day). Every numeric field — level_pct, distance_mm, pump_amps, and battery_v — is the mean of valid samples collected over the window. A sensor that produced zero valid reads in the window emits -1.0 as a sentinel; alerts carries the count of alert Notes sent since the last summary. pump_on is a derived boolean set at summary time: true when the window-average pump_amps is at or above pump_on_amps (default 1.0 A) — it reflects the window average, not a per-sample observation.
• tank_alert.qo — compact template-encoded Note (port 51) emitted immediately on any threshold trip with sync:true. The compact template and explicit port are required for the Skylo NTN satellite path; the identical note.add delivers via cellular or satellite depending on which transport the Notecard selects. alert_code is an integer field (0 = level_low, 1 = level_critical, 2 = battery_low). level_low and level_critical are cooldown-gated (one per alert_cooldown_sec, default 1 hour) so a slowly-drifting tank doesn't generate hundreds of pages. battery_low is edge-triggered: one alert fires on the first crossing below battery_alert_v and further alerts are suppressed until the voltage recovers above battery_alert_v + 0.5V.

Alert triggers.

• level_low — tank fill percentage below level_alert_pct (default 20%). The rancher has time to act; typical time-to-empty at normal cattle consumption is several hours at this level.
• level_critical — tank fill percentage below level_critical_pct (default 10%). Urgent: cattle dehydration risk is hours away, not days.
• battery_low — solar battery voltage below battery_alert_v (default 11.5V). Indicates extended cloudy weather, a faulty solar panel, or a charge controller problem — the monitoring system itself is at risk of going offline.

Routed. Both Notefiles flow to Notehub and from there to whatever downstream the project's routes specify. The deliberate Notefile separation means tank_alert.qo can go to an SMS gateway for immediate rancher notification while tank_status.qo goes to a time-series database for historical trend analysis and predictive maintenance — without any per-Note filtering logic in the routes.

9. Validation and Testing

Expected cadence in normal operation. A healthy, full tank generates one tank_status.qo Note every 4 hours and zero tank_alert.qo Notes. After first power-on, confirm Notehub shows a summary Note within 4 hours and that level_pct is non-zero and plausible given the tank's actual fill state.

Tank calibration. distance_mm in tank_status.qo is a 4-hour window average, not a point-in-time measurement. Reading it immediately after filling or draining the tank will give a value averaged over a window that spans multiple water levels and will not match the current surface distance. Before calibrating, set both sample_interval_sec to 60 (1 minute) and summary_interval_min to 5 in Notehub, then wait for the device to pick up both changes on its next inbound sync. Summary cadence is bounded by the sample wake cadence — the firmware enforces summary_interval_min ≥ ⌈sample_interval_sec ÷ 60⌉, so reducing summary_interval_min alone without also reducing sample_interval_sec leaves the effective window length unchanged. With both set as above, the device wakes every minute and emits a summary every 5 minutes; once the tank has been held at a stable level for one full 5-minute window, the averaged distance_mm closely approximates the actual surface distance.

Fill the tank completely and keep it at its overflow point for at least one full 5-minute window. Note the distance_mm value in the summary Note that follows — that value is sensor_min_mm. With the tank completely drained to the bare bottom — no residual water — hold it at empty for another full window and Note distance_mm again; that value is tank_depth_mm. A calibration performed with the tank "nearly empty" rather than truly empty will underestimate tank_depth_mm, compressing the level-percentage range and causing all readings to read high. If you cannot safely drain the tank, record the actual residual depth in millimeters and add it to the measured distance_mm before entering it as tank_depth_mm. Enter both calibrated values as device-level environment variables in Notehub and reset both sample_interval_sec and summary_interval_min to their operational values (defaults 900 and 240 respectively). After the next inbound sync, level_pct should read ~100% when full and ~0% when empty.

Pump current verification. pump_amps in tank_status.qo is a 4-hour window average. Comparing a handheld clamp-meter reading against a summary Note produced during normal operation will give a misleading result if the pump ran intermittently during that window. To get a comparable number, first set both sample_interval_sec to 60 and summary_interval_min to 5 in Notehub (as for tank calibration above) and wait for the device to pick up the changes. Once the pump has been running continuously at a steady load for one full 5-minute window, clamp a calibrated AC clamp meter around the same conductor the CT monitors and Note the reading; it should agree with pump_amps in the next summary Note within a few percent. A discrepancy of more than a few percent typically indicates the CT jaws are not fully closed, or the CT is clamped around both conductors (cancellation). A reading near zero with the pump running suggests a wiring fault in the bias circuit midpoint. Reset both sample_interval_sec and summary_interval_min to their operational values when verification is complete.

Simulating an alert. Lower level_alert_pct to 99 in Notehub; the next inbound sync will pull the new value and the following sample cycle will fire a level_low alert visible in Notehub within the cellular session-establishment window. Reset to the desired production threshold when confirmed.

Using Mojo to validate power behavior (bench-only). The Mojo is bench instrumentation in this design — the deployed firmware does not read the Mojo's LTC2959 coulomb counter at runtime and no power telemetry appears in transmitted Notes. Use it during commissioning to verify the sleep/wake profile before field installation.

Splice the Mojo inline between the step-down regulator output and the Notecarrier's +VBAT pad. The NOTE-NBGLWX Notecard datasheet specifies approximately 18 µA @ 5V in the Notecard's own low-power idle state — that is the bare Notecard chip's quiescent draw at its VMODEM_P supply, measured in isolation. A Mojo spliced at +VBAT measures the full load on that rail: the Notecard plus the Notecarrier CX's own board quiescent consumption (regulators and any always-on carrier-board circuitry). Expect the measured idle floor to sit above the bare-Notecard datasheet figure: the STM32L433 host in STOP2 adds only ~1–2 µA, and the battery-voltage PMOS switch holds A2 at GND, but the MB7389 stays powered from +3V3_OUT and keeps ranging at a few milliamps between samples, so that sensor sets the floor you will actually see. Rather than targeting the bare-Notecard datasheet number, use the Mojo trace to validate the shape of the power profile: long flat idle intervals at the system quiescent floor, interrupted by brief active pulses every 15 minutes (host awake + sensor reads + Notecard I²C transaction), with one longer radio burst per outbound sync.

Cellular session profile: Cellular transmit sessions run approximately 250 mA average at the 5V rail for 10–60 seconds per outbound sync. Over a 4-hour summary cycle (16 sample wakes), the expected trace is 16 short pulses of tens of mA (host active plus ADC reads, ~3–5 seconds each), separated by 15-minute stretches at the system quiescent floor (Notecard ~18 µA + host in STOP2 ~1–2 µA + the MB7389's standby draw), with one ~250 mA burst when the cellular session opens. Total energy for the 4-hour window should be well under 5 mAh from the 5V rail.

Satellite session profile: When the Notecard operates on the Skylo NTN path, the power picture changes substantially. The NOTE-NBGLWX idles at the same ~18 µA @ 5V as in cellular mode, but the modem-active phase for a satellite sync spans several minutes rather than the 10–60 seconds typical of a cellular session — session duration, not per-moment current draw, is what makes per-session satellite energy meaningfully higher. The NOTE-NBGLWX datasheet (Power Information section) documents modem-active draw at approximately 250 mA @ 5V; no separate NTN-specific sub-phase current is published because the BG95-S5 NTN modem-active draw falls within the same envelope as cellular. The VMODEM_P supply must sustain 750 mA to cover peak modem activity in either mode:

Per-session energy from the published 250 mA figure: a 30-second cellular sync draws approximately 2 mAh at the 5V rail; a 3-minute satellite session (acquisition + data transfer combined) draws approximately 12.5 mAh — roughly 6× more per sync. At the default 4-hour summary interval, six cellular syncs per day total approximately 12 mAh in radio energy; six satellite syncs total approximately 75 mAh per day. For sites where satellite is the primary or frequent transport, size the solar panel and battery for the ~75 mAh/day satellite figure, not the ~12 mAh/day cellular figure.

On a satellite-primary Mojo trace you will see: an ~18 µA floor between syncs, then a multi-minute plateau at approximately 250 mA average (satellite acquisition + Skylo NTN registration + data transfer) before the radio returns to idle — qualitatively different from the short cellular burst. Both the acquisition and data-transfer sub-phases appear as a single extended block at modem-active current levels on a Mojo power trace; if acquisition stalls due to poor sky view or satellite geometry, the plateau extends beyond the 2–5 minute nominal without any step-change in current that would be visible on the trace. If the site has reliable cellular, the satellite path is never exercised and the cellular profile applies.

If the Mojo trace shows a continuous tens-of-mA draw with no idle transitions, the host MCU is not entering STOP2 — check the ATTN→D5 jumper and confirm the firmware was built with USB support set to None (usb=none).

Validating the satellite path. To confirm satellite delivery before field deployment, take the device to a location with no usable cellular coverage and trigger a tank_alert.qo Note by lowering level_alert_pct to 99 in Notehub. With a clear equator-facing sky view, the NOTE-NBGLWX will switch to the Skylo satellite path. Confirm the alert Note appears in Notehub — it will carry the same body as a cellular-delivered Note but the session metadata will reflect the satellite transport. Session establishment may take several minutes; allow up to 10 minutes before concluding satellite is not working. Common issues: antenna blocked or not facing the equator-facing direction, obstructions between the antenna and the horizon, or the device too close to a building or tree line. Refer to the Satellite Best Practices guide for a complete troubleshooting checklist.

10. Troubleshooting

11. Limitations and Next Steps

This reference design is a single-tank monitor that works on a pole next to a stock tank — not a multi-tank irrigation controller, not a herd-level analytics platform, not a polar-region satellite design. The list below names the boundaries that come with that scope so a rancher or integrator can see exactly where the design ends and where production work begins.

Simplified for this reference design

Each of the simplifications below is a deliberate scope choice — a place where a production rancher or integrator will want to add a sensor, a calibration step, or an integration once the basic single-tank monitor is proven in the field.

Satellite antenna siting is site-specific. The NOTE-NBGLWX Skylo path requires a clear, unobstructed view of the equator-facing sky (southern sky in the northern hemisphere; northern sky in the southern hemisphere). Tank sites in valleys, near tree lines, or adjacent to structures that block the horizon toward the equator may have degraded satellite performance or no satellite link at all. Survey the site for equator-facing sky exposure before selecting this design for a satellite-dependent deployment. For sites that require Iridium coverage (polar regions, sites where Skylo GEO coverage does not reach), a different satellite module is needed.

Satellite data budget deserves attention on any deployment that is frequently on the satellite path: monitor satellite data consumption in the Notehub usage dashboard, and consult the Blues pricing page for current satellite data allotments and applicable usage terms.

Manual calibration is required at commissioning. tank_depth_mm and sensor_min_mm must be measured and entered in Notehub per installation. A production design would include a commissioning sequence that auto-measures and stores these values during the first fill cycle.

Single-phase current sensing is all the CT provides, since it clamps around one conductor. Three-phase deep-well pump motors (common in high-volume agricultural wells) need three CTs and a firmware change to sum the three RMS values.

No pump fault-to-start detection. The firmware detects when the pump is running (current above threshold) but cannot determine whether the pump should be running. Adding a float-switch digital input on a host GPIO would enable a "pump failed to start" alert pattern — float says the tank needs water but no pump current is detected.

No local sample history. The Notecard queues Notes for multi-day transmission if both cellular and satellite are unavailable, but the firmware keeps only the current summary window in RAM. A device reset (lightning strike, power interruption) clears the in-progress accumulator and loses the partial window's data.

Mojo is bench-validation equipment. The firmware does not read the Mojo's LTC2959 coulomb counter over I²C at runtime; alerts is the only meta-field in the summary. Adding a mojo_mah field for fleet-level power telemetry is a straightforward extension.

Production Next Steps

Once a rancher is running the basic tank monitor, the following extensions are the natural progression — from the most immediately useful detection improvements toward the deeper fleet-management integrations.

Float-switch input on a digital GPIO adds "pump should be running but isn't" detection, closing the gap left by current-only sensing.

Over-the-air firmware updates via Notecard Outboard DFU are critical for a device that lives in a pasture for years between physical access.

Three-phase CT support brings high-volume wells into scope: A1, A4, A5 for three CTs (A3 is occupied by the battery-divider PMOS enable), with firmware summing three RMS values into a single pump_amps field.

A per-device commissioning wizard — a "first-boot setup mode" — would guide the installer through measuring and storing calibration values before entering normal operation.

env.get with the time argument fetches environment variables only when they have changed since the last sync, reducing I²C overhead on each wake.

Satellite data usage monitoring — a Notehub route or environment variable feedback loop — would alert operators when satellite data consumption approaches the plan's included allotment.

12. Summary

A Notecarrier CX and Notecard for Skylo — paired with a weatherproof ultrasonic level sensor, a clamp-on current transformer, and a handful of passive components — give a rancher continuous visibility into every stock tank on the property without driving the pasture roads. Notecard for Skylo uses cellular where a tower is reachable and falls back to the Skylo satellite network where terrestrial coverage ends, so tanks in covered valleys and tanks on remote ridges both report through the same firmware and the same Notehub project. Sampling happens locally every 15 minutes with the host MCU asleep between measurements; the radio wakes only for the 4-hour summary or an immediate alert. That duty cycle makes the system comfortable running indefinitely on a modest solar setup, with built-in power-aware sleep extension to handle extended overcast stretches in northern winters. Dry-tank alerts and solar fault alerts reach the rancher's phone before cattle suffer from dehydration; pump current is captured as observational telemetry in every summary and alert payload so operators can correlate it with tank levels after the fact. It's a simple problem — knowing when the tank is low — and the Blues stack is a proportionately simple solution: a sensor, a Notecard, and a network path that follows the asset wherever it lives.