CANDLES WITH THE
If you’re like me, October is a very busy month, mostly
trying to finish building props and decorations (some from
last year!) for Halloween. Last minute ideas abound, but
most just don’t fall into the time constraints I usually find
myself boxed in by.
A really great way to add atmosphere to a Halloween display
is with candles. The real ones, though, can be dangerous, go
out if the wind blows, and cannot be controlled with any
sort of [practical] automation. There are electronic candles available in retail
and specialty stores, but most are not terribly bright and — like their real
counterparts — cannot be controlled (without some major hacking).
There is good news. With some simple parts, you can whip up a custom candle controller that will run eight
outputs, and can easily be expanded for more. In addition
to candle outputs, there is a trigger input to control
behavior, and at the end I’ll show you how a few more
components can be added for additional control; for
example, playing audio from an external device.
Building a Candle Controller
My processor of choice is the Parallax Propeller chip.
Having worked with it almost daily for the past 18 months,
I have a real comfort level and enjoy developing code for
■ FIGURE 1. ULN output.
it. It’s high level language, Spin, is fairly easy to use and
the interactive nature of the IDE simplifies and expedites
experimenting with new code.
In order to make this an “afternoon project,” I hand-wired
the circuit to a Propeller proto board. These boards are an
inexpensive way to get started with the Propeller. If you’re
going to do this as a one-off and never use the Propeller again,
you can get a USB version of the proto board. If you think
the Propeller is in your future, then you can go with the
cheaper (non-USB) version and use a Prop-Clip to program it.
Let’s look at the circuitry first. The candle outputs are
driven by a ULN2803A; this lets us use anything from
simple LEDs to some medium current monsters. With
eight outputs running, you should limit the current to
about 180 mA through each ULN channel. When it
comes to LEDs, that’s a LOT of current. Figure 1 shows
the ULN connections to the Propeller. Advanced users will
note that resistors R1-R8 are, in fact, optional but having
seen ULNs melt internally (usually from an oversized
inductive load) and do bad things on the processor side,
these resistors are cheap insurance.
There’s nothing to the “wick” circuit shown in Figure 2.
I’m using a 12 volt supply and the resistor shown allows about
20 mA through the LED. The LED, in fact, is the trickiest part
of the project. Finding one with the right color and brightness
can be a chore. I found that the NTE30039 has the right
color (nice warm orange) and is very bright ( 7,000 mcd).
We could stop there and have a lot of fun, but why
stop? On Halloween, the To Ts (Trick-or-Treaters) will wander