My Remote Keyless Entry fob mod - what did I do wrong?

[ncdave4life]

What did I do wrong?

Two things were driving me nuts about my '97 Buick's Remote Keyless Entry key fob (transmitter):

1) The darn car alarm (honking horn) kept going off, whenever the "panic" button on the RKE transmitter bumped something else in my pocket, and

2) The trunk kept popping open without my knowledge, whenever THAT button bumped something in my pocket.

After spending hours bailing the water out of the trunk of my car following a rainstorm, and drying it out with lots and lots of towels, fans, etc., I'd had enough. I resolved to fix the darn RKE fob, once and for all.

So I rewired the fob: I disconnected the panic button, and put it in series with the trunk-open button. When I was done, there was no way to activate the panic alarm, and to open the trunk required holding the trunk button and (what used to be) the panic button at the same time.

It worked great, except that the range of my RKE fob was less than half what it used to be (why?). Still, I was happy with the result, for about 5 months. Then it quit working completely.

My first guess was that the battery had given out, but that's not the problem.

Can anyone give me a clue about what went wrong?

Q #1: What was the range so greatly reduced? Was the circuit trace that I changed acting as an antenna, and I ruined the antenna design?

Q #2: Where IS the antenna in this thing, anyhow?

Q #3: Why did it eventually cease working completely?

Q #4: If I want to make this modification to my remaining (unmodified, and still working) RKE fob, what should I do differently? I didn't try to preserve the layout of the circuit traces that I cut, I just made sure they were connected together -- was that a mistake?

Here are some photos:

http://www.burtonsys.com/RKE/

Thanks in advance!

BTW, I'm sorry if these are dumb questions. I'm an EKN member, but that was a long time ago, and I was never an RF guy. These days I'm a computer geek.

-Dave Burton
Cary, NC
dave482 at burtonsys dot com but please no spam

 

[biff44]

The antenna is the big loop that goes 3/4 of the way around the circumfrence on the "pushbutton" side of the board.

About the only thing that I can see wrong is that you draped the red wire directly over where the antenna goes. This might have mistuned the oscillator, which is tightly coupled to the antenna.

I would get a slightly larger piece of wire, and replace the red wire, routing it towards the inside of the fob (near the battery) rather than the outside. I would replace the battery with a new one. Then I would see if it worked.

There are basically only a couple things that could go wrong with one of these.

1) you can screw up the way the oscillator oscillates. That is commensurate with your report of poor range. In other words, you can pull the frequency of the oscillator slightly, making it hard for the receiver to see it. You can screw up the turn-on time of the oscillator (by mistuning the antenna), which can cause bit errors). Or you can screw up the output power (again by mistuning the antenna). There is usually a resistor, between the digital output of the microprocessor chip and the base of the RF transistor, that ~ controls output power. You could try making that resistor a little smaller to increase output power.

In any event, the best way to work on these is with a spectrum analyzer to see if the frequency is correct, and with a antenna/detector diode/oscilloscope to see if the turn-on turn-off of the RF signals is correct (this is a pulsed amplitude modulated system).

2) you can screw up the microprocessor. When you disconnect power, the micro may go into a weird mode. When the battery voltage dips, the micro can also do some weird stuff. Push the button, and see if there is a data stream coming out of the micro.

 

[MacGyverS2000]

Swap the rubber keypad membrane from your working unit to your modified one and replace the battery. If it works again, suspect the membrane... the carbon has probably been worn off of the pad bottoms.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[bmnet]

From the schematics, I think your antennas are the four interlocking traces on the reverse side of the pcb. Looking at the modifications that you made, you shorted out two of them and that would explain the range reduction.
The terminations resulting from the above would also act as a source of noise which would swamp down your output signal and also be the source of damage to microprocessor (power reflections due to an open connection and mismatch).

With the remaining fob, I would recommend that you modify at the touch button only (cut out the thing), but any modification of the circuit will mis-tune everything unless you've got the required instruments to check the stability as you go along.

 

[MikeHalloran]

OR, solve the problem mechanically, e.g.:

1. Build up a dam around each offending button so you need something pointy to push it.

2. Cover each offending button with a solid plate having only a small hole in it, so you need something _really_ pointy to push it.

3. Carry the fob on a carabiner on a belt loop.

4. Leave the fob at home and use the key.





Mike Halloran
Pembroke Pines, FL, USA