while i am usint?g a led to test the sensor,the sensor is blinking on a regular basis.What should I do to prevent it?
Is this a PIR motion
sensor? If it is, you can usually find a brand name and model number
on it, which would make it a lot
easier to answer your question. There are a lot of PIR motion
sensor models out there, and significant differences between them. If you can't find a model number, do you know if it's intended use is for turning on a light, or for use in an alarm system to detect intrusion, or some other purpose? Is it powered by line voltage (220, 240, 110, 120VAC, or whatever your local electricity is) or does it need 12VDC or something else to work?
I'm experienced with PIR motion sensors used in alarm systems in North America, but there are less complex PIR sensors used for other purposes. The PIRs motion sensors I work with are more sophisticated than most people realize because they use complex signal processing to cull out "random" IR input that would cause false alarms. If what you have is a PIR motion sensor intended for use in an alarm system, I can probably explain what is happening; but if what you have is a PIR sensor intended for use as a light switch when someone enters a room, or approaches the outside of your home, or some other kind of PIR sensor, then my explanation can be much shorter because those sensors are less complex.
Also, as davenn and CapSteel indicated, you haven't described what you're doing very well. A fuller description of what and how you're testing would be helpful.