Limit switch passive or low power sensor

Hi folks,
I'm looking for a way to monitor the status of a open/close door. I was thinking of a magnetic switch but the problem is that there is no source of power. any idea what would be the best practice to get the sensor, is there any passive sensor that I can use? or I need to design a low power wireles communication like Bluetooth or something?

I appreciate your response in advance

If there is "no source of power," then you could use a mechanical device connected to the door that raises a flag when the door is open and lowers the flag when it is closed.

A reed switch and a magnet make a fine passive sensor. What is wrong with that? And what do you mean "monitor" the door?

Paul

DaveEvans:
If there is "no source of power," then you could use a mechanical device connected to the door that raises a flag when the door is open and lowers the flag when it is closed.

Thanks Dave, the problem is how to send the status of door (open/close) to the controller (Arduino)?

Paul_KD7HB:
A reed switch and a magnet make a fine passive sensor. What is wrong with that? And what do you mean "monitor" the door?

Paul

Thanks Paul. correct, the thing is there is no possibility to hook the reed sensor with wires! I'm looking a passive solution or a low power wireless transmitter because if the door left open or close for long period so the sensor will be out of power!
Thanks for your response

Alidan:
Thanks Paul. correct, the thing is there is no possibility to hook the reed sensor with wires! I'm looking a passive solution or a low power wireless transmitter because if the door left open or close for long period so the sensor will be out of power!
Thanks for your response

Then think about how the safety sensor for a garage door operates and use that device or principal the monitor the door.

An IR light source pointed at a reflector on the door can reflect the light back to a sensor only when the door is closed. When the door is opened, the sensor can no longer see the IR light and tells you the door is not closed.

Paul

Some household intruder alert systems feature battery operated wireless door and window sensors, which you can buy as individual replacement items.

Some of those have been hacked, so you can read them with Arduino and an inexpensive receiver.