Anchor Alarm

An anchor alarm that works when nothing else does. Its own GPS, its own sounder, its own web interface. Free, and you build it yourself.

Install it

Plug an ESP32 board into this computer with a USB cable, then press the button. The firmware is written straight from the browser, so there is nothing to download and no software to install.

This browser cannot talk to USB devices. Use Chrome or Edge on a computer. It will not work on a phone or tablet. This page has to be served over https for the browser to allow it.

Firmware 2026-08-21-c

Installing needs a computer running Chrome or Edge. Everything after that is done from a phone.

Then set it up

With nothing configured, the device makes its own wifi network called AnchorAlarm-XXXX. Join it from a phone and the setup page opens by itself. Give it your boat's wifi, and if you have a source of NMEA data on the network, its address. Then it restarts and joins your network.

From then on you reach it at its address on your own network, from anything with a browser.

What it does

Anchor watch, with the alarm sounded by the device itself
Survives losing power and comes back armed
Push notifications to your phone when you are off the boat
Shows AIS traffic, with closest approach and time to it
Ignores anchored and moored vessels rather than crying wolf
Alarms on AIS distress beacons, including man overboard
Different thresholds for a marina, at anchor, coastal and offshore

What you need

Part	Notes
An ESP32 board	A plain ESP32 devkit works. An ESP32-S3 with PSRAM is better and costs about the same.
USB cable and power	Any phone charger. It draws very little.
A passive buzzer	On GPIO 25 and ground. Without one the alarm makes no sound at all.
GPS module	Optional so far. Without one it takes position from your boat's network instead.

This is not a substitute for a watch. It is one layer among several, built for a boat that already has others. It has no chart data and knows nothing about what is under you. Where you anchor, and whether it is safe, remains the skipper's judgement.

Why we built it

We live aboard Sabado full time, and we take anchor watch seriously. Sleeping with rocks on one side and a reef on the other means a dragging anchor is not a theoretical problem. This started as a better alarm for our own boat and grew into something worth handing to anyone else who anchors out.

It is free. There is nothing to buy from us and no account to make. It does check this site for firmware updates, and it will tell you when one is available rather than installing anything by itself. Nothing else leaves the device: your position, your track and your anchor stay aboard.

License

The firmware, the design notes and everything on this page are published under our licence. See svsabado.com/license for what you may do with it.