Lets talk about transponders..

[simon.says]

Hello friends..

my very first post of many in this forum

im going straight to the matter in hand: transponder

im thinking about building a simple project that will use a transponder (RFID - 125 KHz)

i dont think its that hard to do it
i have done some study of my own about Phillip's PCF7936 and TIRIS DST 4D
its really interesting how a transponder works

so here's my ideia: build a circuit that will always send a signal and only when the RIGHT transponder responds correctly, the circuit will do something that i want, for instance, unlock a dor.

I also was thinking about using the transponder bytes to each one of them mean something different. For instance, the first byte sent means unlock the door, and the second means turn on the living room light.

The hardest thing to do will be building a antenna to work in the right frequency, i dont know much about that.

i have some knowledge about Assembly and was thinking to use to programm a microchip's microcontroller.

i also have a question: does anyone know a "starter kit" to use transponders ? like microchip's starter kit for pic, but for transponders
i found a kit that its called TED-KIT2 from NXP (produces PCF7936). has anyone ever heard of it ?

so what you guys think ?

ps: my english sucks

 

[cowboybob]

Radio Shack has an inexpensive (<$40USD) kit here.

There are, of course, many others, but this a cheap intro unit that might suit you.

 

[simon.says]

that is really cheap. but i was looking for something more 'advaced'

there are some 'crypto' features in the transponder. i wanted to work with that a little

thanks though

 

[WTP Pepper]

The transponder you are talking about are common in car keys in the UK. So you cannot simply copy the mechanical key. They have a coded signature required before the car will start. Often made by Valeo.

They use load modulation. i.e you introduce them to an electric fieild via a small coil. The device powers up from the field and switches a load across itself which is a coded signal. That on/off load creates a current change in the incidental coil (just like a transformer) and the back end equipment detects that and decodes it.

At this stage I would avoid crypto devices as some power up and actually transmit (i.e active transponders as oppose to passive). An example is the Texas Instruments TIRIS devices. Some send a coded signal via the incidental carrier signal which is analysed and returned recoded via some complex algorithm using load modulation.

The PCF device you have was originally designed by Philips and at the time (being used for car transponders) the data sheets where difficult to get hold of.

However how they work is a described above.

Making one is straight forward with a little knowledge, the decoder requires a bit of analogue signal processing experience.

I would start at a breakers yard to track a decoder down. Often part of the starter lock mechanism in the steerring column.

No I am not a car thief, I used to design similar systems.

 

[simon.says]

thanks WTP
I have read the PCF datasheet, and if im not wrong, you can use manchester coding for the PCF. There is a lot of things to do to activate the transponder, turn on the magnetic field for x uS, turn off, and on again. To write in the transponder pages, its ASK if im not wrong. Amplitude Shift Keying? You send the code by modulating the field. My guess is high amplitude means "1" and low means "0". The Texas transponders have in the crypto mode, a page that you write in but you cannot read it directly. I have read that you send a challange to the transponder and then the transponder send back a responde based on the crypto number. So if i would use texas, i would write a number that i would know and then compare the responses with the ones that i know the transponder should respond.
Its really interesting car keys. I never imagined that there was a little chip in the car key. so cool
if you used to design systems like this, did you use a tool that would modulate the field ?

 

[WTP Pepper]

You sem to have a good undersatanding of how they work. You technically send a challenge number to the transponder and await a response via the load modulation,

This command can ask for pages within the transponder, some are encrypted and require you to send the encryption key. This is how car ignition systems work (well the good ones)

A simple FET switched load was all that was needed to load modulate. You have to ensure the load isn't enough to cause a total collapse of the field as you need that for power.

 

[simon.says]

cool
thanks a lot