PIR Power Saving System

The simple circuit presented here will be automatically switch ON the light / fan on entry into the room and switch OFF the load with preset delay on leaving the room. This will reduce unnecessary power consumption.

This circuit is useful in drawing room, study room, dinning hall, TV room, work place, cubicle, visitors room, washroom lighting, exhaust fan, corridor lighting, staircase lighting, lift etc. Using LED2, where much lighting is not required, saves more power than using the relay RL1.

Typical Image of PIR Sensor

Fig. 1: Typical Image of PIR Sensor

PIR ( Pyroelectric / Passive Infra Red ) sensor , which outputs signal (3.3VDC max. is marked as HIGH signal) on detection of presence of humans by their motion, i.e., change in IR radiation emitted by the body. The high signal time can be adjusted from 2.5 seconds to 250 seconds (approx.). The delay can be further increased by using the circuit. The extra delay is directly proportional to the values of C3 and VR1.

Initially, on connecting to power supply, capacitor C3 does not contain any charge. So, P-Channel MOSFET, T2 conducts and switches ON the LED2 and Relay RL1 as per selection of jumper(s) J1 and J2 respectively.

As the charging voltage of C3 increases through R4 and VR1, the LED2 gets dimmer slowly and switches OFF. Mean while the Relay RL1 is also deactivates and switches OFF the 230VAC load. The delay can be adjusted by combination of increasing the VR1 and C3 (1000uF max.).

If, any human motion is observed by the PIR, it sends HIGH signal for the specified time delay set on the PIR. The signal, in-turn switches ON the transistor T1 and discharges the capacitor C3 through R3, until PIR stops sending signal. So, the MOSFET T2 again starts conducting and switches ON the LED2 and RL1. Once, the signal of the PIR stops ( or 0V ), the transistor T1 switches OFF and C3 gets charging through R4 and VR1, which matches to initial condition.

So, to switch OFF the relay, the total delay is addition of Signal delay on PIR and delay due to charging of capacitor C3 i.e., signal time HIGH set on PIR + delay set by VR1 for charging C3. The delay for charging C3 (100uF) through VR1 set of 1M (max.) is about 3 to 4 minutes.

A 6VDC power supply is shown in the circuit comprising Transformer T1, rectifiers D1,D2 and filter capacitor C1. The status of power supply is indicated using LED1 and R1.

The dome surface of PIR should be clean for better sensing. The sensitivity and signal HIGH time can be adjusted for the PIR. Turn the trimmer(s) of the PIR clockwise for mode delay and more sensitive to motion detection.

The total circuit can be assembled on a small PCB with terminals/connectors for 230VAC input and output, except transformer TR1.

Test Points:

TP0 = Ground or 0V
TP1 w.r.t. Gnd. = power supply voltage ( 7V approx. )
TP2 w.r.t. Gnd. = change in voltage on motion detection ( 0V or 3.3V approx.)
TP3 w.r.t. Gnd. = charging level of T2 (0V to TP1)

Caution/notes:

1. Care should be taken while connecting to mains supply after switch board.

2. The circuit may switch ON by the movement of pets also.

Components:

R1 & R3 = 4.7K ¼ watt
R2 = 10K ¼ watt
R4 = 100K ¼ watt
R5 & R6 = 33ohm 1watt
VR1 = 1M preset
C1 = 1000uF 16V
C2 = 0.1uF
C3 = 100uF 16V
D1 to D4 = 1N4007 (4Nos.)
LED1 = 3mm Red
LED2 = 0.5W White
T1 = NPN transistor BC547
T2 = MOSFET (P) – IRF9540
TR1 = 6-0-6V 500mA transformer
PIR1 = HC-SR501, PIR Motion sensor
RL1 = 6V relay
PCB, connectors, jumpers, wires etc.