■ FIGURE 3. Rabbit controller board schematic.
magnetic field is holding the relay’s switch contacts closed.
When the coil current is cut off, that magnetic field
collapses. As it does, it induces current in the coil which
causes a reverse voltage to appear across the relay’s coil.
The magnitude of that voltage is proportional to how
quickly the field collapses, and can be quite large. The
diode provides a low impedance path into which the
magnetic field’s induced current can discharge.
The Wi-Fi Module Interface
There are 50 pins in the Rabbit module’s connector. In
the schematic in Figure 3, I chose to bundle all 50 into one
bus — the “RCM5450_bus” — for the sake of easy and clear
One thing to beware: For some reason, Rabbit labels
all the pins on one side of the module connector as even,
and the other side carries all the odd pins. If you’re used to
normal IC package numberings, you’ll be expecting the
numbers to start with pin 1 at one corner, and sequentially
number the rest around the package in a counterclockwise
direction (looking down at the part).
Per Rabbit’s documentation, there is a 3V battery
backup connected through a 2.2K resistor. U51 is a
standard decoupling capacitor. S1 is a momentary contact
reset switch, and R34 pulls the reset pin to an inactive high
level when S1 is not active. Two corner pins — 2 and 50 —
46 January 2011
are grounds, and pin 1 supplies + 3.3V to the module.
If you are not familiar with “bus-style” schematics, that
thick blue line that all of the pins seem to “connect to”
does not imply that all of the signals are somehow
connected together into one big fat (useless) blue wire.
That thick blue line is just a convenient way of grouping all
of the signals at the interface of the RCM5450W module
so that we can pick out subsets of them for various uses in
other schematic pages.
Interfacing to the
Earlier I said we would take advantage of the fact that
we will never want more than one zone on at a time.
Here’s where we do that — in the circuit that translates the
module’s port outputs into the 16 transistor relay driver
The CD4515 chip is a 4: 16 decoder with an output
inhibit, plus a four-bit latch on its inputs. The 4515’s four
select inputs — A, B, C, and D — pick one of the 16 Y
output pins to assert. I’m going to call this four-bit input
“DCBA,” because D is the most significant bit, and A is the
least significant. If only the B bit is high and the other three
are low, I’ll show that as 0010, and it corresponds to a
binary 2, meaning the Y2 output would be asserted. All
non-selected outputs will remain high. Note that the Y