Here is an ultra simple Arduino Night Alarm project wired around a handful of inexpensive and easily available components. One advantage of using Arduino as a security system is the flexibility!

The code is an Arduino project for the Digispark Attiny85 to turn on a light source when the PIR sensor detects something. Note that after powering the PIR sensor module, it needs about 10 to 60 ...

Present consists of just a PIR sensor and Arduino in a 3D printed enclosure to snap onto your monitor.

With just a Wemos D1 Mini, a standard PIR sensor, and some open source code, you can create a practical self-contained motion sensor module that can be placed anywhere you want to keep an eye on.