Quite often you forget to switch off the light or fan when going out of the room. The simple infrared sensor based power saver 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. 1 shows the pyroelectric/passive infrared (PIR) motion sensor used in the circuit.
Infrared sensor based power saver circuit
The circuit diagram of the infrared sensor based power saver is shown in Fig. 2. 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.