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.

The PIR Sensor Motion Alarm Circuit with Transistor and Relay is simple because it uses only a few components, costs very little, and is easy to build.

The circuit works using a PIR sensor where PIR means Passive Infrared sensor and it detects human motion whenever someone moves, then the alarm activates and as a result a siren sounds.

Circuit Working:

Parts List:

Components	Values	Quantity
Resistors	1k, 4.7k 1/4 watts	1 each
Semiconductors	Transistor 2N3904 NPN	1
	LED 5mm any color	1
	Diode 1N4007	1
	PIR Sensor Module HC-SR501	1
	12V DC SPDT Relay	1
	12V Alarm Siren Speaker	1
	Power Supply 12V DC	1

First, when we turn ON the power supply, the circuit receives 12V, initially, no motion is present, so the PIR sensor output remains 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 and then it reaches transistor base and as a result transistor turns ON.

Now current flows from collector to emitter and relay coil energizes and at the same time the LED glows and this indicates motion detection.

Relay contacts change position, so siren gets power supply and therefore, alarm sounds loudly.

When motion stops then after some time the PIR output becomes LOW again and 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 1k ohm used in our circuit.

How to Build:

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

• First, take all the parts as shown in circuit diagram.
• Next, PIR Sensor V+ pin connect to +12V supply, OUT pin connect to resistor R1 and then V- pin connect to ground.
• Now transistor 2N3904 base pin connect to R1 output, collector pin connect to relay coil and anode of diode D1 and cathode of D1 connect to +12V and then emitter pin connect to ground.
• Then relay pins one coil pin connect to +12V and other coil pin connect to Q1 transistor collector.
• Also, COM terminal connect to +12V and NO terminal connect to siren positive and siren negative is connect to ground.
• Further, LED anode connect to R2 resistor one end and other end of resistor connect to +12V and cathode of LED connect to collector of transistor Q1.

Conclusion:

To conclude, 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.