This Home Assistant Sensor Is Off the Hook

Dillan Stock is on a mission to make his home the smartest smart home of them all. We last saw him create invisible custom recessed housings for door sensors to handle presence detection on the cheap. Now, he has come up with another interesting idea for a custom sensor: a smart hook. It looks just like those Command hooks that people stick on the wall to hang up a towel, jacket, or picture. But this one detects when something is hanging on it and integrates with Home Assistant.

The project relies on a clever reuse of the Aqara T1 Door and Window Sensor, a compact Zigbee device typically used to detect whether doors or windows are open or closed. Instead of using it in its original form, Stock dismantles the sensor and extracts its internal PCB, repurposing it as the brains of the hook. By pairing it with a simple mechanical system, he transforms a binary open/closed signal into an indicator of whether an item is hanging.

The hook itself is built around a spring-loaded sliding mechanism housed in a custom 3D-printed enclosure. In its default “idle” state, two springs push the internal assembly upward, positioning a small magnet close to the sensor’s reed switch. This keeps the circuit “closed.” When weight is added — such as a damp towel — the springs compress, pulling the magnet away and triggering the “open” state.

However, in reality, it’s not quite that easy. Reed switches don’t activate and deactivate at the same distance, meaning the magnet must travel far enough to clearly distinguish between states. To solve this, the hook’s design ensures sufficient movement range, preventing ambiguous readings. Additionally, the dual-spring setup allows for user customization: removing one spring makes the hook sensitive enough to detect lighter objects.

During assembly, the sensor PCB fits neatly into a dedicated compartment secured with a small bracket, while M3 screws hold the structure together. A separate mounting plate — attached via adhesive or screws — allows the hook to slide into place, maintaining a clean, consumer-friendly look despite its DIY origins.

What makes the smart hook truly useful is its integration with Home Assistant. Stock demonstrates a multi-stage automation designed to solve a simple but relatable problem: remembering to hang up a towel. If the hook remains empty after his morning alarm, a timer begins. If nothing changes, a notification is sent to his phone. Ignore that, and the system escalates — triggering an audible alarm through household speakers.

Beyond towels, there are countless possible applications. Parents could ensure kids hang up backpacks before screen time unlocks, or homeowners could verify keys are returned to their place each night. It’s a small device with big potential for home automation.