A Hall effect device comes to mind. They're relatively inexpensive and
should be available in surface mount technology. They are used in the
automotive industry for things like engine rpm, throttle position etc. so
the technology is robust. There would be a small amount of power
consumption. The reed switch already mentioned is the simplest, draws no
power when open but they are more prone to shock damage than a solid state
device.

Mike

"fh03" wrote in message
...

Hello guys,

I am building an RF tag that will transmit some data using a
microcontroller and a 433 Mhz transmitter. But I do not want to place
any buttons on the device nor have it be on at all times. So I was
wondering is there a way to create a magnetic field at the source to
generate an induced current in this tag to perhaps flip a switch to
turn on the internal battery? And once the tag is out of the fields
range the switch will open again?

I know systems such as the Sunpass or other electronic toll payments
use something like this.

If anyone has any information on how this can be done, please help.

Thanks.


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fh03