Key transponder

[Biff383]

What do I need to measure and build a transponder chip?

[AG6QR]

What kind of transponder chip?

A mode C transponder for flight radar?  A transponder for a telecommunications satellite?  These devices are quite different from one another, despite having the same name.

But regardless, in order to build any kind of chip, you probably need access to a silicon fab facility, and you'll probably need the right computer technology to produce the chip design in a format that the fab facility can handle.  You also need a bit of specialized knowledge in how to design the circuitry, and how to implement the design in silicon.  This is not a trivial task.

Have I completely misunderstood the question?

[Biff383]

  Actually I was thinking along the lines of a car key. I haven't been able to find much information on this.

[codeboy2k]

Remote car door openers?  Those are just rolling codes and they have been hacked already.

Physical car keys with embedded chips in them I have no idea. Probably they are just 1 wire serial code chips like those from maxim or maybe something more sophisticated.

[Biff383]

Yup there dosent seem to be a lot of information floating around about them, it's a PATs system. I was just hopeing to save the $200 that it's going to cost for a new key.    Oh well I guess I just gotta bite the bullet this time. I just hate to have to let someone else do it.

[Galenbo]

I was just hopeing to save the $200 that it's going to cost for a new key.   
... I just hate to have to let someone else do it.

If it's a passive anti-theft device integrated in the key, like mine, a new one has to be programmed in the ECU, so you will pay anyway.