Searching http://home.howstuffworks.com/question238.htm
Knowing that "some grocery stores have automatic door openers that use a very simple form of radar to detect when someone passes near the door. The box above the door sends out a burst of microwave radio energy and waits for the reflected energy to bounce back. When a person moves into the field of microwave energy, it changes the amount of reflected energy or the time it takes for the reflection to arrive, and the box opens the door."

I would like to build such simple radar, for intruder alarm.
I know there is other method like IR, PIR, ultasonic, but I like to try this radar technology for learning/fun.

Anybody can give me some hint on the TX/RX radio circuit?
What freq range is using?
Thank you.

 

You send out a continuous radio wave of a certain frequency and then you read what the return frequency is. If the some of the incoming signals have a differenf requency (due to doppler shift of something moving away or towards the sensor) then that means there is movement in the field...a lot easier to do then to measure the time of flight of a pulsed RF signal.

It is not reliable to measure the returning field strength.

 

Paul Obrien

The best intruder detector I've used had a combination of microwave and IR detection in the same box, virtually eliminated flase triggering.

 

Why shouldn't u use modulated IR burst alone.?

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Paul Obrien

IR burst may OK for a beam but for room coverage PIR is easiest.

 

What freq range you suggest?

 

Paul Obrien

A really cheap ready made radar is the Mattel Hotwheels Radar gun, around $30