DIY passive RFID? [closed]

Question

I'm interested in building my own "DIY" RFID scanner and tags. For the scanner, I think I would just need:

• Transceiver
• Some kind of hardware polling mechanism to have the scanner scan, say, every 2 seconds (or whatever) --> "poller"
• Power source
• Hence, every 2 seconds the transceiver transmits a signal, which may or may not be intercepted by a tag within a certain vicinity
• Transceiver can also receive a signal from a tag (see below)

And I believe the passive tag would just consist of:

• Transceiver
• Mechanism to convert some of the energy from the scanner's signal into a temporary power source for the entire tag --> "signal-to-power converter"
• Mechanism for encoding data onto the tag (probably only done once, but perhaps overwritable) --> "data encoder"
• When it powers up, by default, it transmits a signal with the tag data encoded on it

Am I missing any major/essential components here?

Most importantly, what could be used for:

• The "hardware poller" on the scanner?
• The "signal-to-power" converter on the tag?
• The "data encoder" on the tag?

Answer 1

While you have the general concepts down correctly, assembling your own passive tags and then the reader as well is very very specialized. This field delves down into antenna design which is quite difficult in itself.

The passive tags use a very small chip, usually just a small black dot that controls the energy harvesting. The specific pattern that this chip does the energy harvesting in is also in turn how it identifies itself. Attempting to make a discrete passive tag that was truly passive is not feasible for a project, these have to be made very small and compact to have the lowest power consumption.

I like to use the analogy of a rubber band, imagine the RF is a rubber band. As the passive tag harvests from this rubber band, it stretches and pulls on it. The receiver measures the width of the rubber band and records that. Its not the greatest analogy but it helped me when I was studying RFID. I'm just a bachelors student so please comment if anybody has a better analogy.

Answer 2

The one simple fact you're missing there is:

• Passive RFIDs don't transmit data

Instead they use a technique called "backscatter":

Passive and semipassive RFID tags do not use a radio transmitter; instead, they use modulation of the reflected power from the tag antenna. Reflection of radio waves from an object has been a subject of active study since the development of radar began in the 1930s, and the use of backscattered radio for communications since Harry Stockman's work in 1949. -- EE Times