Infrared Sensor Based Power Saver

The simple circuit presented here will automatically switch off electrical appliances like lights or fans as you vacate a room, after a predetermined time period. It will also switch on the light when you enter the room again. This will reduce unnecessary power consumption. Fig. shows the passive infrared (PIR) motion sensor used in the circuit.

Circuit and working

The circuit diagram of the PIR sensor based power saver is shown in Fig. It is built around bridge rectifier DB107 (BR1), PIR motion sensor connected across connector CON2, timer NE555 (IC1), two 1N4007 rectifier diodes (D1 and D2) and a few other components.

The circuit uses a PIR sensor, which detects the presence of people through change in the infrared radiation from the room when people enter or leave the room. The PIR sensor outputs around 3.3V high signal whenever it detects radiation change in front of it.

IC1, resistor R3, potmeter VR1 and capacitor C3 are used as a timer here to convert small time span of PIR signal to a long delay. Output of IC1 at pin 3 drives transistor T2, which, in turn, controls relay RL1. Electrical loads like lights or fans are controlled through this relay.

230V AC mains power is connected across connector CON1. It is stepped down to 9V through transformer X1, rectified by bridge rectifier BR1 and filtered by capacitor C1. Thus we get around 9V DC at test point TP1. This 9V DC voltage is used as power supply for the circuit.

When the circuit is first switched on, capacitor C3 charges through potmeter VR1 and resistor R3. During this time, voltage at pins 2 and 6 of IC1 is less than two-thirds of its supply voltage, and so output pin 3 goes high. This energises the relay through transistor T2, and the appliance is switched on.

When capacitor C3 charges above two-thirds of the supply voltage, IC1’s output pin 3 goes low and de-energises the relay and switches off the appliance after some delay that can be adjusted through potmeter VR1.

Whenever motion is detected by the PIR, its output pin goes high (around 3.3V) for a while depending on the setting on the PIR. The high signal from the PIR is fed to the base of transistor T1, which, in turn, discharges capacitor C3 through resistor R4. When the capacitor’s charge (voltage) reaches less than two-thirds of the power supply, output pin 3 of IC1 goes high again (initial stage) and load is switched on.

When the load is switched off, LED2 glows. This indicates that the circuit is under power-save mode.

Component list

Construction and testing

An actual-size, single-side PCB for the PIR sensor based power saver is shown in Fig. 3 and its component layout in Fig. 4. Enclose the PCB in a small box in such a way that you can easily connect 230V AC input to CON1 and the light/fan to CON3 at rear end of the box. Connect the PIR using a 3-wire cable to the PCB at CON2 and install it at a suitable location in your room.

Before using the PIR in the circuit, manually check it by connecting Vcc and GND pins of PIR to a 9V (or 12V) battery. Then check for change in voltage at signal output pin with respect to ground by waving your hand in front of the sensor. Adjust sensitivity and time controls of the PIR as per requirement (else turn both presets clock-wise to have highest sensitivity and high time signal). The dome surface of the PIR should be clean for better sensing.

After manually checking the PIR, remove the battery and connect the PIR to the PCB. You can assemble the circuit on the PCB with terminal connectors for CON1 (input) and CON3 (output).

Verify that voltages at the test points are as per the table before using the circuit. Check to see if LED2 is off and relay is energized. After some time, LED2 will glow and the relay will get De-energized.

Notes

Some PIR sensors require an initial stabilization time of 10 to 60 seconds in order to function properly. During this time, any motion in its field-of-view (near 15m range) should be avoided.