How to Build a Motion Activated Alarm Using Basic Electronic Components

How to Build a Motion Activated Alarm Using Basic Electronic Components


Building a motion activated alarm is a fun electronics project that can be completed using just a few basic components. This type of alarm can be useful for home security or for pranks. The alarm will sound when it detects motion in its range.

In this article, I will walk through the full process of building a motion activated alarm from start to finish. I will cover:

By the end, you will have the knowledge to build your own motion activated alarm using common electronic parts. Let's get started!

Required Components

To build the motion activated alarm, you will need the following components:

That covers the key components needed. Now let's look at how the circuit works.

How the Motion Activated Alarm Circuit Works

The alarm uses the PIR sensor to detect motion. When motion is detected, the PIR sensor outputs a high signal voltage. This signal is sent to the base of the transistor, turning it on. When the transistor is on, it allows current to flow through the buzzer, creating the alarm sound.

The 10kΩ resistor between the PIR sensor and transistor protects the transistor by limiting current. The 1kΩ resistor controls the buzzer volume.

Here is a diagram showing how the components are connected:

When motion triggers the PIR sensor, the buzzer turns on. When motion stops, the PIR sensor stops signaling the transistor, turning the buzzer off again. This creates an audible alarm that sounds only when motion is detected.

Step-by-Step Assembly Instructions

Follow these instructions to build the motion activated alarm circuit:

1. Insert the PIR sensor, transistor, and resistors into the breadboard

Place the PIR motion sensor, transistor, 10kΩ resistor, and 1kΩ resistor into the breadboard. Leave space between the components. Refer to the circuit diagram for proper placement.

2. Connect the PIR sensor to the transistor

Use a jumper wire to connect the output pin of the PIR sensor to the base pin of the NPN transistor. Place the 10kΩ resistor along this wire between the sensor and transistor.

3. Connect the transistor and buzzer

Use a jumper wire to connect the collector pin of the transistor to one pin of the buzzer. Place the 1kΩ resistor along this wire.

4. Connect power between the PIR sensor and buzzer

Use jumper wires to connect the positive voltage rail on the breadboard to the power pins on the PIR sensor and buzzer. This will provide power when the battery is connected.

5. Connect the ground rail

Connect the negative/ground rail on the breadboard to the ground pins on the PIR sensor, transistor, and buzzer.

6. Connect battery power

Connect the 9V battery to the power rails at the end of the breadboard. Red wire to positive, black wire to negative.

The circuit is now assembled! When powered, the buzzer will sound when the PIR sensor detects motion.

Tips for Customizing and Improving the Alarm

There are several ways to customize this project:

Some troubleshooting tips - ensure all connections are secure, use fresh battery, and adjust component placement on breadboard.

With just a few low-cost components, you now have your own motion activated alarm for security or practical jokes! Adjust and enhance the circuit to fit your specific needs.


Building a motion activated alarm is an easy and enjoyable electronics project. With just a PIR sensor, transistor, buzzer, resistors, breadboard, and battery, you can construct an alarm circuit that sounds when motion is detected. Follow the assembly instructions to properly connect the components. Customize the alarm using the tips provided for your specific application. With the knowledge gained, you can now build your own motion sensing alarms using basic electronic components!