Motion Activated Alarm Circuit.

Components required:-

  1. PIR Motion Sensor Module
  2. Resistor
    100 ohm
    330 ohm
  3. Transistor
    BC 547
  4. Buzzer (3v-12v)
  5. LED (5mm or 3mm)
  6. Power Supply / Battery.



This is a simple motion-activated alarm circuit based around PIR Motion sensor module.
This circuit will simply raise an alarm when motion is detected by the PIR sensor.

Before trying to understand the working of this Motion-activated alarm circuit, let us first see, What is this PIR sensor module??
If you hold this PIR sensor module in your hand you will see that few components such as PCB mount chips, resistors, presets e.t.c are mounted on a small double-sided Printed Circuit Board, on one of the sides of board we can see a white dome type structure, inside this dome a PIR( Passive Infra-Red) sensor is mounted on the PCB, you can remove this white dome and look at it.

This white igloo looking structure is actually a Fresnel Lens which directs the light(Infrared) coming from a different position in surrounding towards the PIR sensor, or we can say that this Fresnel lens mounted over the PIR sensor increases the viewing angle of PIR sensor.

The viewing angle of this sensor module can vary according to its manufacturer, but most of them have a viewing angle between 100 degrees to 120 degrees.

The detection range of sensor module is around 7 meters maximum and it is adjustable.
There are two presets(variable resistor) mounted on the module, one of which is used to adjust the detection range. This preset can be rotated in a clockwise or anticlockwise direction to increase or decrease the detection range. A small screwdriver can be used to rotate these preset.
The detection range can be adjusted between 3 meters to 7 meters.

In the sensor module we can see three pins marked as Vcc, Out and Gnd . Let’s see what are these pins used for:-
Vcc and GND pins are used to provide power i.e. voltage to the module; Vcc pin is connected to the +Ve of power supply and GND pin is connected to the -Ve of the power supply.
When any kind of motion is detected by the sensor then the logic at output pin changes from 0 to 1.
In ideal condition voltage at the output pin will be around 0v to 1v, and on detection of motion by the sensor this voltage will jump to 3.3V.

Now we will see working of this PIR Motion activated alarm circuit,,..


When motion is detected by the sensor, the voltage at output pin rises up to 3.3v (approx.) which will provide base current to Transistor T1, This will turn ON the transistor i.e. transistor will allow flow of current from E(emitter) to C(collector), which will cause Buzzer and LED to turn ON.
Resistor R1 is used to limit the base current of transistor and Resistor R2 which is connected in series with LED is used to limit current across the LED.

Let’s see something more about PIR Motion Sensor Module

This PIR sensor can only detect motion done by any warm body such as Human or animals,
It will not detect the swinging curtain in your room. This sensor actually detects a thermal change in the surrounding; for example, If you move inside your room, the sensor will register it as a thermal change, as a warm body had changed its position.

There are two presets(Variable resistor) on this module, one of which is used to adjust sensors detection range( we have already discussed about it) and another is used to adjust the delay-time.
Ok, what is this Delay time??
When motion is detected by PIR sensor module, the logic at the output pin goes high and remains high for some time and we are referring this time as delay time.
This delay time can be adjusted between 5 seconds to 300 seconds.
Now for example, If the delay-time is adjusted for 300 seconds then on detecting any motion buzzer will be continuously ON for the next 300 seconds.

Power Supply

The circuit can be powered using any voltage source ranging between 4.5V DC to 12V DC.
” Why this voltage range ?? ,, because most of PIR sensor module available on market are specified to work on this voltage range but It can be different and can vary depending on the manufacturer, so it will be good to check specification of the module before using it.


Most of buzzer available on market will work but choosing it according to circuit’s Power Supply is important. suppose If we are using 12V power supply for circuit and connected a 5V buzzer then this 5V buzzer will burn out. So, If we are using 12V power supply then we should use 12V buzzer. similarly, If we are using 5V power supply a 5V buzzer is to be used.

Question:- What If we are using 3 AA battery for this circuit, it will give total 4.5V and there’s no 4.5V buzzer available??
Short answer:- We can use a 5V buzzer.
Most of the buzzer also works on a little lower voltage than it’s rated voltage.

There are some buzzers available on market which are designed to works on a wide voltage range such as “3v-12v buzzer” or “3v-24v buzzer”. so this will be a better option.



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