Passive rfid transponders can be made with greater range than you have
experienced. For instance the Illinois Toll Roads have implemented what
they call IPASS. This is a RFID that operates at highway distances and
speeds. With IPASS you place the rfid in the center of your car/truck
window and when you go through a toll plaza the reader induces an
electrical current in the transponder. This current activates the coded
transmitter which is read by the plaza receiving equipment. Each and
every transponder that passes the reader is interrogated. The induction
field is constant and activates the transponder as it is passed.

There is another device called SPEED PASS that is used by Mobile Oil CO.
The transponder is contained in a small plastic capsule shaped package.
I took one apart and examined the contents. The transponder is contained
inside a glass capsule and consists of a small (1/8") diameter coil that
is connected to a small pc board with an IC and some other components.
The entire thing is about 1/2" long. Range for this device is very short
since it is associated with you credit card.

I don't know how big an inductor is needed to provide enough power to
transmit over the ranges you are looking at but some simple
experimentation should answer this. Basically what the inductor is half
of a transformer. The field generator is the other half. The power
generated can be used to activate a transistor switch to provide power
to the transmitter.

For more information it would be best to do a google search on RFID and
see what else is available.

Good luck

Dave Nagel

fh03 wrote:
David, can you please elaborate on how this can be done? And how it can
be implemented so that the proper signal activates the unit.

I'm sort of a newbie in this field and would appreciate it if you could
explain how this can be done in greater detail.

Thanks again.

David G. Nagel Wrote:

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.