MrChips got in before I did. Your circuit is a nice easy little package as well. Something to play around with while I try my hand at repairing key FOBS. Thanks for the replies

 

I bought one of these a couple of months ago..http://www.ebay.com/itm/Portable-50...931?pt=LH_DefaultDomain_0&hash=item3a9d2b4d5b

It only operates in close proximity of the transmitter, a foot or so. It's been accurate, it displays transmitted frequency and PL or DPL tone. I'll try it tomorrow with my key fob and see what it displays.

Well I finally got around to testing the fob/freq counter...All I got was a constantly rolling display as long as I held the button, probably wouldn't help at all..

 

My endeavor in the beginning was to be able to repair a key FOB and then make sure it was able to send a signal. I needed a way to see if any signal was being sent. That part is answered and I thank you all for your replies. I am able to press the buttons and see the LED flash indicating something is being sent. I will no try to refine that signal to reduce the noise that it is picking up as well. I will let you know where this goes.

 

Interestingly enough, with the circuit I provided, I can see on the oscilloscope the digital code being transmitted. It is phase encoded. It would not be difficult to clone the key fob. The RF carrier frequency of the key fob I tested is 315MHz.

 

Interestingly enough, with the circuit I provided, I can see on the oscilloscope the digital code being transmitted. It is phase encoded. It would not be difficult to clone the key fob. The RF carrier frequency of the key fob I tested is 315MHz.

Quite a long time ago, one of the magazines (possibly EPE) published a "learning" IR remote - it was based around a static ROM and battery backup, so it was bulky and little practical use other than intercepting the IR beam from a neigbours key fob and nicking their car next time they park it.

All modern car fobs use "rolling code" - the car remembers what code it received last time, and expects the next code to be different by a preset increment.

 

Maybe. I've been clicking my key fob in the test lab. How does the car know which code is next? I see the same code on the scope.
Does the key fob receive a confirmation back from the car?

 

Maybe. I've been clicking my key fob in the test lab. How does the car know which code is next? I see the same code on the scope.
Does the key fob receive a confirmation back from the car?

Haven't the faintest idea - there's various appnotes on the interwotsit if you can be bothered searching and reading them.

Remember the even older ultrasonic car fobs? - many years ago I had a buzzy little 2-stroke, it had a cracked mirror that seemed to resonate as I thrashed it up the hill away from my flat - most times I passed a particular parked car; it bleeped and all 4 indicators flashed.

After a few months of that - the car was just gone.

 

To make a long story short, the encoder and decoder are both factory shipped with a 40 bit security code stored in an EEPROM. Each time an event is activated, the security code gets encrypted, a data frame is attached and a hopping-code algorithm is attached to it and then it is stored back to the EEPROM as it is sent out. The security code is then changed for the next transmission. On the decoder side, when the signal is sent, the encrypted message and it's code are decrypted and a hopping-code algorithm is applied to measureit's validity. If it is accepted, the function is carried out and the next security code is lined up for the next function.
If you want the long story, I can send it to you.
Very scary though that hackers are already able to hack this and steal a car. who said thieves were dumb?

 

Okay I hooked it up. 5VDC power and ground and the LED to data and ground. Power on and the LED comes on and flickers a bit (noise). I tried three different key FOBS and between the 315MHZ and 433MHz, I got all of them to flash through the LED. Any thoughts on removing the random flickering when a button is not activated? You can clearly distinguish between when it is random and when the key is pressed - solid flashing as opposed to random.

This is a pretty old thread but I am having the same problem you were and am in need of testing my car key fob, had already bought the receiver/transmitter before coming across this thread for that purpose but after setting up my small circuit I just get a faded LED light, i ahve tried testing maybe 3 key fobs on it and get no difference at output.

 

There is an arduino circuit you can build for test purposes and allows you to watch the code as it transmits. Much better visual than blinking LED. Youtube it and you will find what I am referring to.