Works in Complete Darkness
LiDAR uses infrared, not visible light. The sensor fires invisible beams and measures distance. No camera image, no ambient light required. Monitors the room without emitting any visible light.
Alarmist: LiDAR Motion Detector
Point your iPhone at a doorway, arm the app, and get a loud alarm if anyone enters. LiDAR uses infrared, so it works in complete darkness.
Requires iPhone 12 Pro or newer with LiDAR
Alarmist is a convenience tool, not a security system. It cannot guarantee detection of all motion or protect you from harm. Test thoroughly and never rely on it as your sole means of protection.
How Alarmist Works
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.
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.
What You Get
Measures Actual Depth
A person walking through a doorway creates a large, sustained depth change. This is fundamentally different from pixel-based motion detection and far more reliable in low-light conditions.
Loud Alarm That Ignores Silent Mode
The alarm plays at full volume regardless of your phone settings. You will wake up. The alarm triggers immediately when motion is confirmed, not after a delay.
Configurable for Any Environment
Adjust sensitivity thresholds, set distance bands to ignore objects too close or too far, and tune temporal smoothing to require motion across multiple frames before triggering.
Evidence Capture
When motion triggers the alarm, Alarmist can turn on the flashlight and capture a photo automatically. Evidence is saved to your camera roll.
One Extra Layer of Security
Alarmist is not a replacement for proper security systems. It is one more tool for sleeping in hotels, Airbnbs, dorms, and anywhere you want a little more awareness.
Why I Built Alarmist
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.
See It in Action
Important Limitations
Alarmist is not a replacement for proper security systems. It is one extra layer of awareness, not a complete solution.
- Device must stay plugged in. LiDAR and screen-on drain battery quickly. Plug in your iPhone or iPad for extended monitoring.
- App must remain in foreground. Screen lock or switching apps stops monitoring. Keep Alarmist on screen.
- Requires LiDAR hardware. iPhone 12 Pro or newer, or iPad Pro 2020 or newer. Standard iPhones and iPads do not have LiDAR.
- Test before relying on it. Every environment is different. Adjust sensitivity and distance bands for your room before arming.
I built this for my own travel use. It works for me. Test it in your environment before depending on it.
Built by Peter. Bootstrapper in Beaverton.