why not use one of the readily available systems that has the problems
already worked out? there are rfid tags that you can program with your data
to track things, and there are the smart rf activated credit card type
devices for users. both of these use a short range hf signal that is
received by the tag, converted to power the internal circuitry, then to send
out the data burst to identify the tag or card and dump their data. the
card is only active while in the activating field, otherwise it doesn't have
any power source. electronic toll payment systems usually use a more active
device since they have to work over longer distances, most of these have a
battery in the client device that powers the receiver... the receiver may
only come on for short checks every second or two to see if it is in range
and needs to send it's identification and data so battery life can be very
long.

"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