PIR sensor:

fig.(a) experimental circuit diagram

fig.(b) power supply

fig.(c) relay module

fig.(d) pir sensor




fig.(e)
over all connection here one phase and neutral remains 

 

1. AC to DC Power Supply Section

(Left side of the circuit)fig(a)

This part converts 230V AC mains to 5V DC, which is needed to power the PIR sensor and the relay section.

1.1 Capacitive Dropper (C1 + R1 + R5)

  • C1 (105J) drops the AC voltage without heating (reactive dropper).
  • R1 (1MΩ) discharges the capacitor when power is off (safety bleed resistor).
  • R5 (100Ω, 1W) limits inrush current and protects the diodes.

This replaces a transformer and produces a low-current AC voltage.

1.2 Bridge Rectifier (D1, D2, D3, D4)

These four 1N4007 diodes convert AC to DC.

1.3 Filtering Capacitor (C2)

  • Smooths the pulsating DC.
  • Stores charge so PIR sensor gets stable power.

1.4 Zener Regulation (D6 – 5V Zener)

  • Keeps the voltage at 5V.
  • Provides regulated 5V for the PIR module.

1.5 LED Indicator (D5 + R2)

  • Shows power status.
  • Helps confirm DC supply is present.

2. PIR Sensor Module Section

(Middle block with PIR1)fig.(a)

The PIR sensor has 3 pins:
Vcc (5V), OUT, GND

How it works:

  • When human motion is detected, the PIR output pin becomes HIGH (3.3V or 5V).
  • When no motion, PIR output stays LOW.
  • power the PIR sensor from the regulated 5V produced earlier.

3. Relay Driver Section

(Right block: Q1, D7, R4, RL1)

The PIR signal is weak, so a transistor is needed to drive the relay.

3.1 Transistor (Q1 – BC547)

  • Works as a switch.
  • When PIR output is HIGH, base of Q1 gets current through R4 (330Ω).
  • Q1 turns ON → Relay energizes.

3.2 Relay (RL1 – 5V Coil)

  • Controls the 220V AC load (bulb).
  • Relay coil gets 5V from power supply when Q1 activates.

3.3 Diode D7

  • Protects the transistor from back-EMF generated when relay coil is turned OFF.

4. Load Section (AC Bulb or Lamp)

Final right side

Once the relay switches:

  • Load (L1 – 12W or any AC bulb) turns ON.
  • When no motion, PIR goes LOW → Q1 OFF → Relay OFF → Bulb OFF.

5. Step-by-Step Working Summary

(1) AC mains enters the circuit

Voltage is reduced by capacitor C1.

(2) Bridge rectifier converts AC to DC

Using four 1N4007 diodes.

(3) Capacitor C2 smooths DC

Provides stable supply.

(4) Zener diode regulates to 5V

Supplies PIR and transistor circuit.

(5) PIR detects motion

  • Output becomes HIGH.

(6) Q1 transistor turns ON

Relay activates.

(7) Relay connects AC supply to bulb

Bulb lights up automatically.

(8) No motion detected

  • PIR output LOW → Q1 OFF → Relay OFF → Light turns OFF.

6. Why This Circuit Is Useful

  • Fully automatic
  • Saves electricity
  • No transformer (cheap)
  • PIR sensor works reliably on regulated 5V
  • Relay allows control of high-voltage AC safely

 

 

 

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