why does my pir activate ...when i dont want it to?

[vw754]

i bought my my 500 wat pir light around 2 years ago and i recently noticed.......it activates when theres a car going past the house (imy house is a corner house ) when cars go past ,down the side street my light comes on and it also works in the back yard (yard/garden) is around 20 feet square) but if i turn the pir any further it will drop off !

The pir on the light is turned fully to the right,yet when a car go down the stree ..which is on the left ! the light comes on.....is it beecaust its too high up (7-8 feet high) or because it cost me £7.99 new ?[/b

 

[333rocky333]

The arc of the lens, most sensors trigger more when aproached side on, sometimes you can black out the outer edge of the sensor lens to reduce it

 

[ColJack]

PIR's (as stated already) work best when something crosses in front of it, and worst when approached head on.