HomeDesign complete (stages 1–6), not built

AlarmEcho

Hears the alarms you already have chirp, sound or get tested, and tells the owner over LTE

A battery disc next to an existing smoke or CO alarm reports when it sounds, chirps for a low battery or is tested, through an LTE-M Link the owner controls, with no rewiring and no tenant Wi-Fi.

Who buys it: Small landlords and property managers of long-term rentals (10–500 doors); families of an older parent whose home has no Wi-Fi.
Why it pays: A chirp-driven after-hours call costs about $75–150 (est., unverified). Catching the chirp the day it starts moves the battery swap to a planned visit. At an assumed 0.3–0.5 calls per door per year that is about $22–75 per door per year (est.).
Target price: $39 disc + $99 Link + $3/door/month; family pack $149 + $5/month (targets)
First-minute wow: Press the alarm's test button and the dashboard shows 'Test heard · Unit 4B hallway' over LTE within seconds, with no Wi-Fi password anywhere.

See the CAD design Concept page

AlarmEcho concept: the idea and where each part goes — The concept. The CAD design is below, under Printable CAD.

What the research found

And how this design answers it

Big players already listen for alarms (Ring listener $34.99 + base kit, Alexa Emergency Assist $5.99/mo, Minut $10/unit/mo) → all need the occupant's Wi-Fi; AlarmEcho ships its own LTE-M Link and sells to landlords, not consumers
Life-safety liability → positioned as a notifier and maintenance log: no 911, no 'protects', and every alert names the listed alarm as the safety device
Battery life of always-on audio → TDK T5838 acoustic-activity wake (~20 µA) keeps a 2x AA disc near 1.8 mAh/day, about 2+ years (target)
ICS-43434 on last-time-buy (Digi-Key, 15 Jun 2026) → production mic is the T5838 on an ESP32-S3, which has hardware PDM-to-PCM
False alarms from TV and oven timers → install-time learning of the alarm's own tone, strict T3/T4 grammar over 2 cycles, plus a small CNN second opinion
Silent failure → daily health beacons; a disc not heard for 26 h opens a ticket

Use case

Chirp in unit 4B on a Sunday

Property managerTenantMaintenance tech

01Trigger: the hallway alarm's battery runs low and it chirps about once a minute
02Detection: after 3 chirps at a steady period the disc logs 'chirp started'
03Alert: the Link sends it over LTE-M; the dashboard opens a ticket and the tenant gets an optional 'battery on its way' text
04Action: the tech swaps the battery at Tuesday's planned visit

Outcome: No 2 am call and no pulled battery; about $75–150 of after-hours visit avoided (est.)

How it works

Architecture

01
Sense: T5838 MEMS mic in acoustic-activity mode wakes an ESP32-S3 only on loud sound
02
Decide: tone match to the learned alarm, T3/T4/chirp timing grammar, int8 CNN second opinion; no audio stored or sent
03
Send: AES-CCM BLE coded-PHY frame to the Echo Link, then MQTT/TLS over LTE-M (SIM7080G) or Wi-Fi
04
Tell: push, SMS and voice escalation until acknowledged; landlord tickets and webhooks; offline-disc alerts

Hardware

Real parts, buyable today

Bill of materials: about $22 per disc and $65 per Link (incl. 10-year IoT SIM) at qty 10 (est.).

Block	Part	Why this part
Disc MCU + radio	ESP32-S3-MINI-1-N8	Hardware PDM-to-PCM and BLE 5 coded PHY; $4.51 at LCSC
Microphone	TDK T5838 (MMICT5838-00-012)	Acoustic-activity wake at ~20 µA; replaces the last-time-buy ICS-43434
Disc power	2x Energizer L91 + TI TPS63900	75 nA buck-boost down to 1.8 V; 2+ year target
Link radio	ESP32-C6-MINI-1 + SIMCom SIM7080G	BLE scanner plus LTE-M the owner controls; $22.62 modem at qty 1
Link backup	TI BQ24074 + 18650 cell	Keeps reporting through a power cut, at least 72 h

Mechanical

Printable CAD

8 parts, generated from code and checked as closed solids. Pick a part to inspect it, or download the STL files to print.

See the CAD parts render

AlarmEcho printable parts, rendered from the CAD — Render of the printable parts from the actual CAD. Not yet printed or photographed.

Loading the CAD…

Real CAD · drag to rotate · scroll to zoomReal CAD · swipe sideways

Open risks we are working on

Liability: even as a notifier, a missed alarm in a fire can draw a claim; product-liability insurance terms are unknown
Willingness to pay: landlords may not pay $3/door/month for chirp and vacant-unit alerts alone; test with 5 Austin property managers
T5838 acoustic-activity wake is under-documented (app note AN-000298 unpublished); wake behaviour must be proven on the bench
LTE-M carrier certification for the Link adds cost and months (unverified)

Design stage only: nothing on this page is for sale. Specifications and prices are targets.