Use the passive receiver to power a transistor switch to activate the
transmitter. You should be able to induce enough power in the unit to
switch some kind of transistor switch connecting the battery to the
second stage of the unit. You would have to be careful that only the
proper signal activates the unit though.


Dave WD9BDZ



fh03 wrote:

thanks for your input guys. I havent been able to come online due to
power outages caused by hurricane wilma.

anyway. the reed switch mentioned would work, but the distance is the
issue. I currently have an rfid reader and passive tag, but they too
work only within a few inches. What I need is for the device to be
activated within 5 feet of the receiver and from my understanding a
reed switch cant do that.

I know electronic toll payment systems seem to conserve battery, and
thats what i need. Bascially I want to have a battery in the unit, but
I dont want the battery to be connected until we're in that range. The
reason passive devices dont work is because even though the antenna can
provide an induced current, without a battery the tag doesnt have enough
power to be able to transmit over say a few inches, let alone five feet.
So we need battery in the transmitter. The user only carries a
transmitter, so it doenst know when it is within range. I have a
microcontroller that will transmit an ID as soon as it is powered up,
but I dont want it to be on at all times, and I dont want a switch.
I'll look into the "Hall Effect" now and see what its about.

Thanks again guys, keep it coming.