BY PETER J. STONARD
user interface, RTC, and power supply.
Fewer I/O boards can be employed, if
desired, down to the minimum of one,
giving eight channels.
This makes the basic design
attractive for smaller displays or
indoors to spice up a Christmas tree.
The use of smaller PCBs also makes
the packaging a bit easier, and the
entire system is built into a NEMA
raintight steel enclosure found at any
local home improvement store.
■ FIGURE 2
Once the system is completed
and installed, it requires no further
attention. The RTC will turn it on and
off each evening, which leaves just
the task of setting it up initially.
There is a push switch on the PCB to
reset the µC (but not destroy the
RTC settings), should a reboot be needed. We have eliminated the power on-off switch and the long AC cord that
may be plugged in away from the cabinet and therefore inconvenient to reset.
An LCD port is included on the controller PCB for connection to an industry
standard module, such as the popular
16 x 1 type. This is not needed when
the system is running outdoors, so I
didn’t add it to the cabinet hardware. It
does come in handy for field resetting of
the RTC, which can also be done by
downloading a new firmware image
over the AVR ISP programming port.
But, this option is not as attractive when
the system is installed outdoors.
■ FIGURE 3
indicate DC power is available over the
ribbon cable data bus. I found these to
be invaluable for field installation and
testing as one might be doing the
installation outdoors on the lawn or up
a ladder in the winter weather.
Refer to Figures 2 and 3. The I/O
card consists of eight identical opto-isolated AC channels and a decoder IC
that links to the other cards and controller
over an I2C (eye-squared-cee) data bus. A
DIP switch on each I/O card selects the
address of that card and, if desired, two
or more cards can have the same address
(if a large display needs more current that
a single card’s limit of five amps).
Each output channel has a monitor
LED (yellow), plus another LED (green) to
I/O CIRCUIT DESCRIPTION
The I/O PCB contains eight identical chan- fire the triac. Not a big problem as the early
nels of AC load control using triacs, which are firing of the triac represents full brightness
fed via fuse F1 with 120V AC from CN4. The of the controlled lamps. If the lamps operate
triac trigger circuit was made as simple as at, say, 97% power instead of 100%, no one
possible, and uses an opto-coupler to isolate will notice. At the end of each AC half-cycle,
the hot AC circuits from the ground referenced the voltage reverses and releases any triacs
DC control circuits. Component references for that were turned on earlier in the half-cycle.
each channel are offset by 100, so R101, R201, Current for each opto-coupler LED is
R301 represent “R1” in channels 1, 2, and 3, supplied by the I2C decoder, IC1. It has eight
respectively. We’ll use Rx01, Rx02, etc., below channels of open drain outputs, and each
to refer to all eight channels. sinks current for the opto-coupler and the
The triac, Qx01, has a TO-220 package monitor LED for that channel. Current is
and is attached to the bracket as a heatsink. limited for the opto-coupler by Rx01 and
Because each I/O card is power limited to the LED by Rx02, respectively.
controlling about 600 watts (limit of 5A at The decoder IC receives data over the
fuse, F1), the triac does not need heatsink- I2C bus via connectors CN1 and CN2 which
ing, unless one channel carries the full load are in parallel and serve to daisy-chain the
power. Note that electrically isolated tab I2C bus to other cards. Resistors R5 and R6
triacs are required, so the devices can be isolate the decoder IC from the bus, and
attached directly to the metal mounting termination resistors (R7-R10) were added
screw and metal bracket (which is ground- but are not needed.
ed through the cabinet). There is a DIP switch on each card to
Each opto-coupler, ICx01, has a photo shift the address range for the I2C decoder IC.
diac, which can conduct in either direction All I2C components listen to all messages
when the associated internal LED is turned on the bus, but only correctly addressed
on. This allows the triac to fire on both polar- messages are acted upon. I/O cards
ities of the AC waveform. Current to fire the require setting of the DIP switches prior
triac is limited by the 330Ω resistor, Rx03, on to installation in the system, and all eight
the hot side. The downside to this scheme is possibilities (three binary address lines) were
that the AC waveform must rise above or included so the same I/O card design can be
below zero before there is enough current to reused for other projects.
November 2007 51