As i am going deeper into this issue, it seems very odd to me .
If you go over the most popular magnetometers around, you will see that most of them does not have an interrupt on change pin . only interrupt for data ready .
That means, if connected to MCU , like Arduino , the MCU has to always sample the result from the sensor to detect a movement .
For example sampling each second, will wake up the Atmega for some time, and then back to sleep .
This approach seems to make it impossible for battery operated devices because current consumption.
What am i missing here ? is there no way to detect movement with a very low current? (accelerometers and gyros are not good for a slow movement) .
Mobile phone chipsets (the target market for most of these types of sensors) typically have single low power core like an ARM 6 which is active all the time, handling sensor input.
One more thing, to wake the MCU every once in a while , do you need external device for interrupt ? i know that to wake from sleep you need INT0 or INT1 to change (?)
Magnetometers are not intended to detect movement. It is possible to move without significant change in magnetic fields. Usually they are used for orientation.
Yes, a measurement can in principle be made in a millisecond.
jremington:
Magnetometers are not intended to detect movement. It is possible to move without significant change in magnetic fields. Usually they are used for orientation.
I was thinking the same thing.
BenStlr:
accelerometers and gyros are not good for a slow movement
What sort of movement are you trying to detect? Have you tried using an accelerometer to detect these movements?
What is your power limitation? What sort of batteries and how long do you want the batteries to last?
What do you want the microcontroller to do when a movement is detected?
I think you are very wrong about the assumption that magnetometer are not good .
There are academic articles about it, that says, exactly what we found- accelerometer will not detect the slowly opening of a door, no matter what algorithms you would implement. As its name , he detects acceleration, and if you open a door slowly there is no acceleration .
A compass-magnetometer- is quite simply detect that, because you change his position relative to earth, that means 20 degrees opening of a door, would make the change , no matter what the speed is .
(put your iPhone on a door with the compass on, and see it )
Our pcb has to awake from sleep when a door was opened, but it turned out that accelerometer is not good for that (ADXL345) in any configuration .
Do you have any other ideas- on how to detect a door opening, or a window opened, with only one part on the object ?
@Groove Ridiculous you are . I will not even start reacting to this. Read some academic articles about it and you will quickly find how wrong you are . Constant speed will not create acceleration . thats basic physics.
This guy , made his Thesis (oh yea) on what called ecompass- means to detect a slow movement of a door by accelerometer and compass . i dont think he just "missed" your genius idea to use a sensitive accelerometer
Hi,
But from the latch or just moved from any open position to another open position.
I would like to know that the door was opened in any way, slowly,fast,just a little bit, etc .
To me , opened, means from closed, that is off a latch, door frame. So any movement from the door frame can cause a normally open mag/reed assy to close, sound an alarm, no current flow until door opened.
@Groove, so if someone carry a magnet, that means the magnetometer was changed and i know that its being hacked . Oh yea, as simple as that. its the change you are looking for.
Dont be so arrogant ,if you read just a little bit more about it, you will find quickly enough that this is what is being used today in most of these systems . There is an option to just give up your ego and admire you had a mistake.
Unless you have another idea other then saying "use a more sensitive device" . The device we use has 4mg sensitivity . Less then that, is more like noises .
@Tom , thanks , well again, we only need one part , not 2, i am aware of all other crapy bad user experience ideas such as reed sensors that requires other strange objects being installed .
