The problem

In long apartment hallways, one chirp sounds exactly like another no matter where you're standing. Without a system, finding the dead battery means walking 200 feet of corridor and pressing your ear to every unit door — sometimes multiple passes, sometimes needing the tenant home. A 30-minute service call for a $3 battery.

How it works

A coordinator ESP32 broadcasts a radio timing pulse via ESP-NOW every second. Two sensor nodes — mounted at opposite ends of the hallway — each run an INMP441 digital microphone through a tight bandpass filter centered on 3,300 Hz (the NFPA smoke detector chirp frequency). When a chirp arrives, each node records the exact microsecond offset from the nearest coordinator pulse. The coordinator compares those two timestamps: the time difference, combined with the known node spacing, resolves which unit the chirp came from — logged live with unit position, timestamp, and confidence to a dashboard the technician can check from anywhere. No walking required.

Self-ranging — no manual measuring

The two nodes measure their own separation acoustically instead of requiring a tape measure at install time: the coordinator fires a buzzer pulse from one node, times how long its partner takes to hear it, then reverses the shot. The round-trip pair resolves both distance and clock offset. A boot-time and periodic (6-hour) auto-recalibration keeps the system accurate even if a node gets bumped or repositioned, with an outlier-rejecting median-of-several-shots average smoothing out single-measurement noise.

Why the filter matters

HVAC noise, footsteps, voices, and slamming doors all compete with a 50 ms chirp. A biquad bandpass filter with Q=4 tuned to 3,150–3,500 Hz passes the chirp and rejects nearly everything else, making reliable detection possible in a working building.

Superhearing mode

When one node picks up a candidate chirp, it immediately broadcasts a prime signal to its partner via ESP-NOW. That node drops into high-sensitivity mode — tightened filter, lower detection threshold — ready for the next event. Low-battery chirps repeat on a fixed interval (typically every 30–60 seconds) and never stop, so the system learns exactly when to expect the next one, and confidence compounds with every confirmed pass.

Field results

Deployed at real hallway distance (10–14 m node spacing, self-measured via the ranging system above) with 81 confirmed alarms fired live during testing and development. An initial bench test (37 chirp events, controlled conditions) measured ±2.5 ft positioning accuracy; ongoing field work is refining that further — including ruling out a firmware sensitivity regression found via git archaeology, and a from-scratch waveform cross-correlation mode (in development) that resolves timing directly from the recorded chirp shape instead of a simple threshold crossing, to cut out amplitude-dependent jitter in the final few milliseconds of precision.