Hypothesis

Use a PDM MEMS microphone for audio capture. PDM preferred over I2S for simplicity and power efficiency. Audio processing offloaded entirely to Beacon Module MCU.

Alternatives considered

Vesper VM1010 and Knowles SPH0641LU4H are leading candidates due to wake-on-sound (WoS) hardware capability enabling near-zero standby current — well aligned with ultra-low-power design goal.

Open questions

• Always-on ambient monitoring, or event-triggered capture?
• SNR floor requirement?
• Mono sufficient, or stereo?

Next step

Settle use-case intent (ambient vs event-triggered) before selecting part. WoS capability is a strong differentiator if event-driven.

Hypothesis

Use a temperature and humidity sensor for environmental baseline monitoring. Pressure measurement is out of scope — not relevant to the remote surveillance use case.

Use case value: freeze alerts, fire/heat anomaly detection, humidity anomalies (e.g. water ingress on a boat, cabin door opened in cold weather).

BME680 VOC sensing remains conditionally relevant for fuel leak detection on boats — flagged as a variant, not the baseline.

Alternatives considered

SHT4x is the preferred baseline — lowest power, best T/RH accuracy, no unused pressure channel. BME680 as a variant for deployments where VOC/fuel leak detection is needed.

Open questions

• Is VOC/fuel leak detection a required variant (boat deployment)?
• Temperature anomaly threshold for alerting?

Next step

Confirm whether VOC variant is needed before finalising BOM. Baseline = SHT4x.

Hypothesis

GPS is not a standard fit for the Accordion Sentry Model. The device is placed at a known fixed location — GPS adds cost, board area, and is the highest-power sensor on the board (~10–30 mA active). It earns its place only when asset mobility is a use case.

Conditional inclusion: GPS is justified for boat deployments where drift detection or theft tracking is required. For cabin and woodland fixed installations, GPS is redundant.

Alternatives considered

If included, u-blox M10 platform remains the preferred choice for low-power GNSS. A small backup supply is needed to retain almanac between duty cycles and reduce TTFF.

Open questions

• Is GPS a standard feature or a hardware variant (DNP for fixed installations)?
• Does Beacon Module provide a backup power rail for GNSS almanac retention?

Next step

Decide: standard fit vs DNP variant. Recommend treating GPS as optional/variant to keep baseline power budget lean.

Hypothesis

Use a photoacoustic NDIR sensor for true CO2 measurement. Photoacoustic technology offers the best balance of accuracy and low power. Traditional NDIR (~40 mA active) requires heavy duty-cycling; photoacoustic variants achieve low average current natively.

Note: MOX-based eCO2 sensors (CCS811, ENS160) are NOT true CO2 — they estimate from VOC readings. If actual CO2 ppm is required, NDIR or photoacoustic is mandatory.

Alternatives considered

SCD41 in single-shot mode is the leading candidate — trigger measurement, wait ~5 s, sleep. Infineon XENSIV PAS CO2 is a strong contender if power budget is tight, but carries more supply chain and maturity risk.

Open questions

• True CO2 ppm required, or is eCO2 estimate acceptable?
• Measurement interval? (drives average current significantly)
• Concentration range requirements beyond standard indoor 400–5000 ppm?

Next step

Confirm true CO2 vs eCO2 requirement and measurement interval before selecting part.

Hypothesis

Use an accelerometer (not a full 6-DoF IMU) for tamper detection and asset movement monitoring. Gyroscope is out of scope — angular rate is not needed for the surveillance use case and draws ~5 mA active. Magnetometer remains conditional on heading requirement.

Use case value: detect if a boat is drifting or moved, a cabin door slammed, or the Sentry device itself is tampered with. Hardware wake-on-motion interrupt keeps the Beacon MCU asleep until movement is detected.

Alternatives considered

BMA400 or LIS2DH12 are preferred — purpose-built accelerometers with no unused gyro die drawing power. BMI270 remains viable if future firmware needs gesture classification.

Open questions

• Is absolute heading (magnetometer) needed for any deployment scenario?
• Any specific vibration or shock threshold for tamper alerting?

Next step

Select between BMA400 (lowest power) and LIS2DH12 (simplest, most supported). Confirm magnetometer requirement.

Hypothesis

Add a PIR (Passive Infrared) sensor as a second presence detection tripwire alongside audio. PIR detects body heat movement without requiring visual line of sight through walls — it works within a space, at a doorway, or outdoors within range. Complements the MEMS microphone well: audio detects sound events, PIR detects thermal movement independently.

Use case fit: cabin entry detection, perimeter trigger in the woods, boat deck presence. Very low standby power; output is a simple digital interrupt — no bus traffic until triggered.

Alternatives considered

EKMB series (Panasonic) is the leading candidate — ~6 µA standby with digital interrupt output, no processing required on the Sentry side.

Open questions

• Indoor-only, outdoor-only, or both? (affects lens/Fresnel selection)
• Detection range requirement?
• Is PIR a standard fit or deployment-specific variant?

Next step

Confirm deployment environments and range requirement. PIR is low cost and near-zero power — lean towards including as standard.