PIR Sensor Motion Alarm Circuit Transistor and Relay

Nowadays, security is very important that why homes and shops need protection.

So to safeguard this motion sensor circuits are useful.

PIR Sensor Motion Alarm Circuit Transistor and Relay is simple.

It uses very few components and is with low cost and is also is easy to build.

The circuit works using a PIR sensor where PIR means Passive Infrared sensor.

It detects human motion whenever someone moves.

Then the alarm activates and as a result a siren sounds.

Circuit Working:

Parts List:

At first when the power supply is ON, 12V is given to the circuit.

Initially no motion is present because PIR output is LOW.

Therefore, transistor remains OFF, along with relay is OFF and siren is silent.

Now when a person moves, PIR sensor detects infrared change.

Then PIR output becomes HIGH and this output goes through resistor R1.

Then it reaches transistor base and as a result transistor turns ON.

Now current flows from collector to emitter and relay coil energizes.

At the same time the LED glows and this indicates motion detection.

Now relay contacts change position, so siren gets power supply.

Therefore, alarm sounds loudly.

When motion stops then after some time the PIR output becomes LOW again.

By turning OFF the relay turns and siren also stops.

Formulas with Calculation:

1: Base Resistor R1 Calculation:

where,

• Supply voltage is 12V
• Base emitter voltage is 0.7V
• PIR output voltage is 3.3V (approx)
• Base current needed is 1 mA

Formula:

R1 = (Vout – Vbe) / Ib

Calculation:

R1 = (3.3 – 0.7) / 0.001
R1 = 2.6 / 0.001
R1 = 2600 ohm

Nearest standard value used in circuit is R1 with 4.7k ohm

2. LED Resistor R2 Calculation:

where,

• LED voltage is 2V
• LED current is 10 mA
• Supply is 12V

Formula:

R2 = (V – Vf) / I

Calculation:

R2 = (12 – 2) / 0.01
R2 = 10 / 0.01
R2 = 1000 ohm

So R2 is 1k ohm is used in our circuit.

How to Build:

To build a PIR Sensor Motion Alarm Circuit Transistor and Relay follow the below steps for connection:

• Take all the parts as shown in circuit diagram.
• PIR Sensor V+ pin connect to +12V supply.
• OUT pin connect to resistor R1.
• V- pin connect to ground.
• Transistor 2N3904 base pin is connect to R1 output.
• Collector pin is connect to relay coil and anode of diode D1 and cathode of D1 connect to +12V.
• Emitter pin is connected to ground.
• Relay pins one coil pin is connected to +12V.
• Other coil pin is connected to Q1 transistor collector.
• COM terminal is connected to +12V.
• NO terminal is connected to siren positive.
• Siren negative is connect to ground.
• LED anode is connected to R2 resistor one end and other end of resistor connect to +12V.
• And cathode of LED is connected to collector of transistor Q1.

Conclusion:

This PIR Sensor Motion Alarm Circuit Transistor and Relay is easy to understand.

The circuit uses basic components and it works very effectively.

Therefore, it is best for home security projects and for learning electronics.

References:

PIR sensor – Relay automatically turning lights on, heeeeeelp..!!