I saw a circuit that showed how simple it is to make PIR based alarm system. I build and tested Arduino and PIR based alert system but not without Arduino. So this PIR alarm circuit does not need any microcontroller like Arduino.
The circuit diagram is below. I described how the circuit works in my previous blog post Security Alarm Circuit with PIR.
I mean the circuit is so simple yet very educative. This is the most simple way demonstrate how transistors can be as a simple on/off switch. Here the PIR also acts as a intelligent switch. When it detects moving human object, it outputs HIGH signal from its output pin. This HIGH signal turns ON the transistor and current flows through the buzzer, into collector and then to the base ground.
We can imagine that we can use any sensor or detector that outputs HIGH when something happens not necessary human object.
Below is video demonstration how this PIR alarm works.
