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Automatic Light using PIR

2017-11-07 12:27:25 1 By: Fayaz Hassan

The simple circuit presented here will automatically switch ON an 12V LED light, readily available in the market as strip or array, on entering into the room and switch OFF the LED light 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, corridor lighting, staircase lighting, lighting inside a lift etc.  Using a relay in place of LED strip/array,  it can control any electrical equipment like fan, tube light etc.




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 increasing the value of C (47uF) and VR (1Mega ohm trim pot). The extra delay and dimming of LED strip/array is directly proportional to the values of C and VR.

Initially, on connecting 12VDC power supply to the circuit and finding any human interference/ movement, PIR emits 3.3V signal to the OP-AMPs.  The output of the OP-AMP charges the capacitor C  through 4.7K resistor and switches ON the LED strip/array quickly through MOSFET (IRFZ44).

Once, there is no change in IR input to PIR (means no human movement is observed), the PIR output signal drops to 0V (low) level after specified time delay set on the PIR and OP-AMP stops charging the capacitor C.  Then, the charge in the capacitor discharges through VR and 470K resistance, which further adds the delay.  As the charging voltage of C decreases through 470K and VR, the LED strip/array gets dimmer slowly and then switches OFF. 



If, any human motion is observed by the PIR again, it sends HIGH signal to OP-AMP, charges capacitor C and switches ON the LED strip/array through MOSFET.

So, the total delay is addition of high signal delay on PIR and delay due to discharging of capacitor C  i.e., signal time HIGH set on PIR + delay set by VR and 470K for discharging C.

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 more delay and more sensitive to motion detection.

The total circuit can be assembled on a small PCB and connect to 12VDC adaptor or 12V battery.  A 12V relay can be used instead of (or parallel to)  LED strip/array in case any 230VAC equipment to be controlled.

Caution/notes:
1. The circuit may switch ON by the movement of pets also.
2. HC-SR501, PIR Motion sensor is used in the circuit.




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