Turn Your iPhone into a LiDAR Motion Detector

Your iPhone Pro and iPad Pro have a LiDAR sensor that fires invisible infrared beams and measures the distance to every surface in front of it. Alarmist turns this into a motion detector that works in complete darkness. No visible light needed. No camera image to process. Just depth.

When you arm the app, it continuously compares depth frames. A person walking through a doorway creates a large, sustained depth change that is unmistakable. Alarmist filters out noise (curtains, pets, reflections) by requiring consistent motion across multiple consecutive frames within a configurable distance band.

The pipeline has two stages. Stage A is low-power monitoring: the LiDAR scans, Alarmist compares frames, and nothing happens until real motion is detected. Stage B is the trigger: the alarm sounds at full volume (ignoring Silent Mode), the flashlight turns on, and a photo is captured as evidence. The whole sequence happens in a fraction of a second. You can also run in silent detection mode, which suppresses the siren while still capturing photos and logging events.

Every detection event is recorded in a detection history with a timestamp and photo. Browse past events on the device, or enable iCloud sync to see your history across all your devices. Configurable retention (30, 60, or 90 days) keeps storage under control.

Unlike camera-based motion detection, LiDAR depth sensing does not need any ambient light. Point your iPhone at a doorway in a pitch-black hotel room, arm the app, and sleep. If anyone enters, you will know immediately.

When I travel, I sometimes sleep in unfamiliar places. Hotels, Airbnbs, guest rooms. I wanted one extra layer of awareness if someone entered the room while I was asleep. Not a full security system. Just something that would wake me up.

Camera-based motion detection needs visible light, and I was not going to sleep with the lights on. Then I realized the iPhone Pro's LiDAR sensor fires infrared. It builds a depth map of the room in complete darkness. A person walking through a doorway creates a depth change that is impossible to miss. So I built an app that watches the depth map and sounds an alarm when something changes.

I use Alarmist when I sleep in hotels and Airbnbs. I put my phone on the nightstand, aim it at the door, arm the app, and go to sleep. The depth heatmap shows me exactly what the sensor sees before I arm it. If anyone comes through that door, I get a loud alarm that ignores Silent Mode, and the app captures a photo with the flashlight on. Every event is logged in a detection history that syncs across my devices through iCloud, so I can review what happened from any device.

I also added a silent detection mode for situations where I do not want the siren. At home, I sometimes run Alarmist pointed at a door to log who comes and goes. Silent mode captures photos and logs events without the alarm, turning the app into a simple motion-activated camera.