PI ZERO

HUNT THE SNARK
Tutorial
MODIFYING THE REMOTE
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>STEP-01
Prepare the stripboard
Cut a piece of stripboard, 20 columns of 11 hole tracks,
and break the track at the places shown. Do this with
a scalpel so that the minimum amount of copper on
each side of the hole is removed. Then, using solder
braid, remove the four-position slider switch from the
remote’s circuit board.
>STEP-02
Mount the components
Solder the 3K3 resistors across the breaks in the tracks and add the
transistors. The pinout for the transistors shown is the most common
one; check that the transistors you’re using match this. Finally, solder the
double row pin-header socket between the track breaks.
Pin 1 Pin 2 Pin 15 Pin 3 Pin 4
was to understand what was happening in the circuit,
so it was out with the screwdrivers to prise the two
halves of the case apart. This was the hardest part of
the project and the result is shown in Fig 1 (overleaf).
Inside was a single-chip encoder SC5262, and a crystal
oscillator which acts as the 433.92MHz transmitter.
Remote controllers have a small grid of tracks covered
in dark material, and carbon pills on a membrane are
pressed down to short these tracks together. This
remote, however, was a little unconventional, in that
there were three sets of wires that were all shorted
together when a button was pushed. We traced the
wiring and, with reference to the encoder’s data
sheet, worked out what was happening.
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The design
Basically, for each channel there’s an on and off
button. Pressing either shorts one of four data
lines (one for each channel) to ground and at the same
time shorts out the transmit-enable line or another
data line, depending on whether it’s the on or off
column of the channel. The key to understanding the
operation was to spot that there’s a diode that also
pulls the transmit-enable line down when that data
line is pulled down. While the remote runs off a very
small 12V battery, the only signals changed by the
key presses are to either let a pin float, or connect
it to ground. This makes the interface circuit very
simple indeed. All that’s needed is an open collector
Pin 6 Pin 5 Pin 9
Pin 16 Pin 7 Pin 8
>STEP-03
Wire it into the remote
Add the wires to the board. There are two sorts: those that connect
between points on the board, and those that go from the board to the
pins of the encoder chip on the remote’s main board. Use thin wires for
this; we used 28 AWG solid wire for the point-to-point ones, and very
thin multi-stranded flexible wires from board to chip. Mark the lowerleft
socket with a white marker pen; this is the ground connection.
raspberrypi.org/magpi February 2016 53