CIRCUITS
B
After 20 minutes pawing through one of my junk
boxes in the attic, I found an old pass card from a
previous employer. It carried a different code, so it
wouldn’t work at my current office, but its coil was
intact. What’s more, the heat in the attic had dried
out the glue — the card practically fell apart in my
hands (Figure B). Plan C was the way to go!
I used a sharp craft knife to pry away the old glue
that held the coil and mini PCB to the plastic shell.
I took great care, as I couldn’t afford any breakages.
After 10 minutes, the salvaged coil was free.
I unsoldered the donor coil from its original PCB
using a fine-tipped soldering iron, holding the PCB
with a pair of locking tweezers. This was easy, as
I didn’t have to worry about overheating and possibly damaging the old IC. After removing the coil,
I tested its continuity with a multimeter. Success!
The coil was intact.
The tricky part was attaching the new PCB. This
time, I had to be careful to keep the iron on the joint
for as little time as possible, since the PCB was
small and could quickly overheat, killing it forever.
Things went well, though, and because the ends
of the coil were already tinned where I removed
them from the old board, the joints were easy to
make. The coil from my old card was now connected
to the mini PCB from my new card.
Now it was time to look at the housing. My old
card’s shell was filled with dried-out glue and covered with ground-in grime. Fifteen minutes later the
case was clean, thanks to some isopropyl alcohol, a
soft cotton rag, and a bit of elbow grease.
The last step was to position the coil and PCB in
the old case and hold it in place with a few drops of
glue (Figure C). The card’s thin plastic backing had
suffered from the cleaning process, so I elected to
replace it with a piece of clear, self-adhesive book
film trimmed in place with a craft knife. (White
book film would have been fine, but with clear
film, the card made a great talking point with my
co-workers.) The results looked good, but the acid
test would have to wait until the next morning at work.
At 8: 36 a.m. I waltzed up to the front door and
waved my pass at the security panel. “Click!” went the
door. “Woo hoo!” I shouted. The guys in HR couldn’t
understand why I was smiling so much that day.
A
C
Fig. A: Removing part of the shell reveals the tiny
circuit board and damaged coil. Fig. B: The old, donor
card. Its coil is intact, but its chip contains the wrong
code. Fig. C: The good chip soldered to the donor coil,
in its case.
Graham Cattley has been obsessed by electronics since he
was 4, and has published numerous technical electronics
articles in Australia over the past 15 years.
RFID Tag: How Does It Work?
There’s a lot of misinformation out there about how
RFID works. Here’s a typical explanation:
The reading unit (attached to the door) transmits
radio energy. The card receives this energy, converts
it to electrical energy, and then uses it to transmit
its serial number back to the reading unit, which
recognizes the card and opens the door.
While this is neat and simple, it’s also wrong.
Yes, the card’s coil antenna absorbs the RF energy,
and this is used to power the chip, but technically
speaking, the card doesn’t transmit anything at all!
Without going into the gory details (look up
“load modulation” if you want them), you can better
explain RFID with the following analogy:
Let’s say you’re out on a boat, and you want to
use a mirror to send information to a lighthouse. You
can encode it in a binary format and then transmit
one bit each time the lighthouse beam sweeps by,
where reflect means 1 and not reflect means 0.
Security pass cards use the same principle, but
instead of one sweep every 10 seconds or so, the
reader transmits at 125kHz. The card’s chip communicates its data by selectively shorting out its coil
over successive cycles of the 125kHz transmission.
A shorted coil doesn’t absorb any of the RF energy,
while a non-shorted coil does, so the reader then
distinguishes 1 from 0 by measuring the peak voltage on its antenna to see if it’s high or low. In this
way, the card communicates its 24-bit serial number, along with synchronizing and checksum data.
122 Make: Volume 19
