PIR Light Switch.Design note 7


PIR Based Light switches are now available to use as Motion detectors. When somebody passes through its detection area, the load such as light or electric alarm turns on. The delay time can be adjusted from 30 seconds to 3 minutes. After the delay time, the load turns off. The sensitivity can also be adjusted up to 10 meters. The unit consists of a PIR sensor module and a Tric controlled circuit for AC operation. It uses a transformerless power supply to make the unit compact. Load can be any AC device like bulb, Tubelight, CFL or an AC bell. It also has an LDR based circuit for actvating the circuit only at night. It is very easy to fix the unit and only two wires are needed for connection.It costs Rs. 350 to 550 depending on the make.

How PIR Sensor works?

PIR sensor is Passive InfraRed sensor that detects IR passively. That is , it is not emitting any Infrared, instead it detects the Infrared of human body heat passively. It has two Pyroelectric sensors. One act as a reference and monitors the natural background infrared ( wall or surrounding objects). The other one is sensing the IR from human motion. In the standby state, the unit remains idle and keeps the surrounding IR as reference. When a human passes in its detection range, it causes a change in the Infrared level which triggers the unit.


Fixing the unit.

When we see the back of the circuit, two screw terminals are there. One is for connecting the Phase line ( L ) and the second one is for load. The Neutral of the load goes directly to the mains neutral. On the back side ,there is a preset for adjusting the delay time. But the sensitivity adjustment Preset is present inside. So you have to remove the front case to see this. By adjusting this preset, the range can be adjusted from few cms to 10 meters.


Adjustment of Presets.
Keep the unit upside. Turn the preset Clock wise for maximum ( See figure). If day and night operation is needed, remove the LDR in it.


You need only two wires. Connect the Phase line to the screw terminal marked as L. Then connect one wire of the Bulb to the screw terminal for Load. The other wire of load directly goes to the mains neutral. It can handle a load of 250 to 1000 Watts depending on the make.

How to fix?

The unit should be fixed in a suitable place outside the home. Best place is beneath the Sunshade directing the unit 45 degree towards ground/ gate. So a cone angle of 45 degree will be available as detection range. Direct sunlight and shadow of moving tree branches should not fall on the sensor ( In day and night operation). It may give false triggering. The unit should not be place on the wall if there is moisture . Minimum 2 meters height is needed for proper operation. If the height is low, animals such as dog or cat can trigger the circuit.

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