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!

Present consists of just a PIR sensor and Arduino in a 3D printed enclosure to snap onto your monitor. When the PIR sensor no longer detects someone in range, it sends a notification over serial ...

Back in the old days hardware manufacturers ... the same one found on the Arduino Uno. The entry-level $99 kit includes a WiFi and motion hub, a button, a buzzer, and an LED.