This afternoon I pushed the unlock button on the remote
keyless entry (RKE) fob for my Chevrolet Cobalt - no response. Minutes earlier it worked when
I locked the doors with it as I left to go into WalMart. Strange, I thought, that it should
fail so abruptly if it was a battery issue. Never in the 2½ years that I've owned the Cobalt
has the fob ever been dropped, banged, or in any other way abused, so I certainly did not
suspect what I found to be the problem. After getting home, I retreated to the workbench and
pried open the soft case. Inside, the battery and its spring steel holder was floating freely
in the available space.
The image to the left is how it should have looked, with the battery holder firmly soldered
in place on the PCB. The main electrical contact is the large tab at the top center, but there
are also small fingers protruding from the bottom on both sides of the fingers that solder
into holes in the PCB. Those two tabs connect to the same point in the circuit electrically,
so for this device to fail, all three solder joints needed to fail. They did. To the right
is what the assembly (or should I say disassembly) looked like.
A quick check with the DMM showed that the CR-2032 battery still had a charge of 3.20 volts
after all this time - impressive.
I removed the PCB
from the fob casing and removed the failed solder with some wick and a 40 W soldering iron,
then cleaned the area with isopropyl alcohol. The battery holder was then re-inserted into
the through holes and soldered at all three points. Another cleaning with alcohol and a trip
to the garage to verify that it works again completed the job. The case simply snaps back
The cause of the poor soldering job is most likely the large thermal inertia of the metal
battery holder prevented it from attaining proper solder wetting temperature during the reflow
process, so a cold solder joint resulted. My re-soldering job was effortless since the tinning
on the metal was quite sufficient to facilitate wetting.
Out of curiosity, I popped open Melanie's RKE fob and inspected the solder joints on hers.
Sure enough, one of the through-hole fingers had a cracked solder joint. I repaired that,
too. If I had taken my fob to the Chevy dealer, you can bet he would have wanted to charge
me $100 for a new one when clearly it was a manufacturing defect.
Alright, as long as the beast was apart, I felt compelled to take an inventory of the components
used. As you can see, the parts count is very low.
The first thing I do is
up the FCC ID code that by law is printed on all wireless devices in the USA. Many times
there will be very useful photos and data available for public viewing that was submitted
by the laboratories that performed the qualification testing. Not so on this device (FCC ID:
KOBGT04A), which is registered to Lear Corporation, in Southfield, Michigan, near Motor City,
A visual inspection (click on
image above for closer look) shows that there is an 8-pin SOIC that is the RFID IC brains.
Its package marking is 0218-002/PR0619/3PS, and is made by
Microchip. It appears to be a Code Hopping
Encoder similar to the
but I cannot positively identify the exact model. Note the two diodes used for B4 (the 5th
button), which jibes with the two diodes seen on the PCB.
The UHF oscillator circuit looks to be a single transistor that is interfaced directly
to the antenna. Its frequency determining device is likely the oval lidded package which is
probably either a ceramic or SAW resonator (it bears a muRata logo and the package marking
of 31kV). The oscillator circuit is gated on and off by the DATA output of the HCS device.
So in all, the
discrete component parts lists is comprised of 1 integrated circuit, 1 resonator, 3 resistors,
5 capacitors, 2 diodes, and 1 three-pin transistor. Add in a 2-layer PCB, a plastic housing,
rubber switch pad, and a battery, and that is the totality of it.
The printed antenna is clearly visible along the top perimeter of the PCB.
These two photos
show the keypad membrane and the switch contacts on the back side of the PCB. Note that it
is set up to accommodate 7 button positions, but this only uses 5. Ingenuously simple really,
but the soldering sucks. My somewhat educated guess is that the total manufacturing cost is