Re: wireless sensors, and battery life: time synch

pixel wrote:

> rfid id cards are not powered and they are not in listen mode
> in fact rfid cards are tataly dead when they have no signal from sender
>
> i know the range of rfid is pretty short
> but maybe just the idea can be used

RFID actually takes it's power by absorbing radio energy from the (fairly
high power) transmission that's sent to it.  This gives them *just* enough
power to transmit back their ID code at VERY low power - and nothing else.

That's why the range is so short.  The tag has to get close enough to the
transmitter to pick up enough power to run it - and the signal coming
back from the tag has to be really low-powered - so it has to be close
to the reciever for the signal to make it that far.

There is also the issue of when a shopping-cart load of RFID-tagged
goods pass through the checkout, all of the tags want to talk at once.

If you imagine a typical branch of WalMart, there could easily be 50
carts and therefore maybe 1000 or more RFID tags in the checkout area.
If the range was long enough, all of those tags would clutter up the
radio frequencies to an amazing degree.

Hence, the range is deliberately kept short to minimise the number
of tags that are trying to transmit at once.

---------------------------- Steve Baker -------------------------
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