Motion Detector Online: Camera and Phone Alarm
Turn a phone or laptop into a simple motion detector. The camera watches for movement and logs each event with a time, or the phone sounds an alarm if anyone picks it up.
- Frame changed
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- Events
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- Last event
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Prop the phone up so it faces the area you want to watch.
Two ways to detect motion
Camera mode watches what the camera sees and compares each frame with the one before. When enough of the picture changes, it counts a motion event, highlights the moving area in green, writes the time in the log and, if you leave the alarm on, beeps. Use it to see whether a pet leaves its bed, whether someone comes into a room, or how often a door opens.
Phone moved mode uses the phone's accelerometer rather than the camera. Put the phone down, press start, and after three seconds it is armed. If anyone picks it up, slides it or knocks it, the alarm sounds. It is a quick anti-theft alarm for a phone left on a café table or a bag left on a train seat.
Setting it up
- Lean the phone against something or use a stand, so the camera does not move. A camera that moves sees the whole room change at once.
- Point it at the doorway or area you care about, with as little window as possible: passing clouds and car headlights change the light and can trigger the detector.
- Keep the page open and the screen on. The page asks the phone to stay awake, but browsers stop the camera if you switch apps.
- Plug the phone in for long sessions. The camera uses a lot of power.
Choosing the sensitivity
Sensitivity changes two things: how much a pixel must change to count, and how much of the frame must change to trigger an event. At 8 to 10, small movements such as a cat's tail or a hand reaching into view trigger it, but so can camera noise in a dark room. At 1 to 3, only a person walking through the picture will. Start at 6, walk through the frame, and adjust. The Frame changed reading shows how much of the picture moved, which helps you pick a level that ignores flickering lights but catches people.
What it can and cannot do
This is a simple detector that runs while the page is open. It does not record video, send alerts to another phone, or keep running in the background. The camera image is processed on your device, a few times a second, and is never saved or uploaded. For home security, a dedicated Wi-Fi camera or a PIR alarm is more reliable, keeps working with the screen off and can alert you when you are away.
Camera motion detection and lighting
Camera detectors respond to changes in brightness, not to heat like a PIR sensor. That makes them sensitive to lights switching on and off and to auto-exposure adjusting after a big change. In low light, the camera image gets grainy, which looks like small movement everywhere. A lamp left on in the room improves accuracy a lot.
Motion detector: frequently asked questions
Can I use my phone as a motion detector?
Yes. Open this page, prop the phone up facing the area, and press Start. The camera checks for movement and beeps and logs each event.
Does the motion detector record video?
No. Frames are compared on your device and thrown away. Nothing is saved, recorded or uploaded.
How does the phone moved alarm work?
It reads the phone's motion sensor. Once armed, any lift, push or knock above the sensitivity level sounds an alarm until you stop it.
Why does it trigger when nobody is there?
Changing light, a moving camera or camera noise in a dark room. Lower the sensitivity, steady the phone and keep windows and screens out of the frame.
Will it keep working with the screen off?
No. Browsers stop the camera and motion sensors when the page is hidden. Keep the page open and the phone plugged in.
Does it work on a laptop?
Camera mode works with any webcam. Phone moved mode needs a motion sensor, which most laptops do not have.