Making a Smaller Passive Key Fob

[Libertas]

The idea:

The key fob is huge and I really don't need the buttons. I can start the car and lock/unlock the doors and open the trunk - all without using the buttons. If you want the buttons, feel free to stop reading, this isn't for you.

I want a key fob small enough to fit in my wallet so I can just forget I even have it.

First steps:

I purchased an extra third key on eBay for about $50. http://www.ebay.com/itm/331853048343 I then used the programming procedure in the manual (page 60ish) to program the third key. (You have to hold the start button in the very last step for a couple of seconds - the manual doesn't say this, but it was the only way it worked for me.)

I then disassembled the new key down to the circuit board. There are two batteries in the key, but I immediately discovered only one was necessary.

The huge plastic case provides the connection from the top (positive) side of the battery to the circuit board and those four fingered spring things provide the bottom (negative) connection. So, I used an inch of old speaker wire and some black electrical tape to connect the wire to the positive side of the battery and to the rectangular part of the board to complete the circuit. I wrapped it all in copious amounts of electrical tape and - sure enough - it worked!

The problem:

The buttons still are operational and if this is in my wallet, I will constantly be hitting the buttons. My solution is to find some appropriately sized nylon washers to place around the buttons (circled below in red) so they can't be pushed.

Next Steps:

1) find correctly sized nylon washers (or - more daring - just pull the buttons off of the circuit board - can this be done without breaking passive functionality)
2) experiment with thinner battery (battery is the fat point)
3) find someone who knows something about circuit boards (I know nothing) to tell me which parts can just be cut off to make this smaller.

Stay tuned. . .

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[Libertas]

Need some help from someone with actual knowledge - -

Each of the four buttons seems to be attached by six points - three on top and three on bottom. Can I just used a solder gun to melt these six points and remove the button without damaging passive functionality?

 

[QRK]

I like this idea! I don't know jack about circuit boards and wiring (I'm pretty convinced it all just works through magic) but I hope this works out because I'd like to do the same.

 

[Coaster]

Need some help from someone with actual knowledge - -

Each of the four buttons seems to be attached by six points - three on top and three on bottom. Can I just used a solder gun to melt these six points and remove the button without damaging passive functionality?

You could probably just remove the buttons by desoldering and be fine. You need to google "Surface Mount desoldering".

 

[Jay-rod427]

Could probably just pull the rubber actual button out. Without those there is nothing to press the actual switch that is recessed down inside.

 

[HISSMAN]

Buy solder wick. It will keep you from contaminating other parts of the board with solder when removing it. If it doesn't work, you can always reattach them.

[ame]

 

[Dadsreward]

I would recommend using heat shrink on the finished product instead of electrical tape. Preferably, environmental heat shrink.

 

[zuki_dan]

Interesting idea. Keep us updated with how it works out. I might just have to try this. About the only thing I press the buttons for is to roll the windows down as I am walking up on a hot day.

 

[GT 50]

I like this idea! I am getting so tired of constantly setting of my car alarm.

 

[mustang1]

I recently set off my car alarm leaning against the car to check the oil. I may attempt to de-solder that button.

 

[PJR202]

This is in no way official technical advice. I have a two year in electronics, but other than a hobby project here and there I haven't used it in my career in the 17 years since school.


Unless you're just trying to isolate only the part that the car senses, there isn't much you can cut away. There's not a lot of excess board there. If they're designed the way I think, then your buttons are just an open circuit that's cut through (completed) when the button is pressed, which will create a pulse to activate whatever is downstream. Since it's already an open, then cutting them off would still only leave the circuit open. As far as rerouting or soldering, that board is tiny. Your chances of pulling that off with no knowledge of circuitry and what I presume are little to no soldering experience or the fine equipment necessary to remove and/or make the connections is very small.

 

[PJR202]

Really, if you can just rig up the power source you could just find and/or alter slim profile case of some sort and put it all in there. The thickness of it is a bigger problem than the length (that's what she said).

 

[7jay]

From Libertas
1) find correctly sized nylon washers (or - more daring - just pull the buttons off of the circuit board - can this be done without breaking passive functionality)
2) experiment with thinner battery (battery is the fat point)
3) find someone who knows something about circuit boards (I know nothing) to tell me which parts can just be cut off to make this smaller.

To protect the switches, try using a 3-d pen See 3d pen on Amazon or Youtube.
Actually, you should be able to make a case enclosing the circuit board with the material. The pen works like a 3-d printer, but is cheap.. handheld.

Nice explanation. Thank you

 

[groundhog]

stop wearing skinny jeans. problem solved

 

[Libertas]

This is in no way official technical advice. I have a two year in electronics, but other than a hobby project here and there I haven't used it in my career in the 17 years since school.


Unless you're just trying to isolate only the part that the car senses, there isn't much you can cut away. There's not a lot of excess board there. If they're designed the way I think, then your buttons are just an open circuit that's cut through (completed) when the button is pressed, which will create a pulse to activate whatever is downstream. Since it's already an open, then cutting them off would still only leave the circuit open. As far as rerouting or soldering, that board is tiny. Your chances of pulling that off with no knowledge of circuitry and what I presume are little to no soldering experience or the fine equipment necessary to remove and/or make the connections is very small.

Thanks for this response. I guess you are right that I will probably not have the skill to desolder the buttons. And - even if I did - they might be more complex than just closing a circuit.

I am guessing that the antenna is the line around the perimeter where the key hole is. Everything else seems functional, so no real way to make it smaller. Wish I could get slimmer.

As for a case, this will live in my wallet, so I really don't care what it looks like and a good electrical tape wrap is probably the thinnest way to go.

I'll head to the hardware store to look for nylon washers this weekend and update with a photo and status.

With all of the cars out there that have push button start and touch door handles for lock/unlock, I'm surprised there is not a more elegant solution out there.

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