up the sidewalk and it might be fun to
have the candles change as they
approach. The trigger circuit shown in
Figure 3 accommodates a Parallax PIR
sensor (via a three-pin header X1) or a
normally open contact (usually a mat
switch) connected to TB1. The voltage
for the PIR and the normally open
contact is 5V, so a 2.2K series resistor
is used to limit the current into the
Propeller (which operates at 3.3V).
Construction is point-to-point and
as it can be difficult to see these
connections in a photo, I created the
graphic in Figure 4 as a guide. Note
that the resistors between P0-P7 and the ULN2803A are
stood up on end. Also note that while it looks like the
wires go on top, they are actually soldered to the bottom.
Finally, the colored dots on the PIR header show cable
orientation (ground is at the top of the board). Figure 5
shows my completed prototype.
With a few items from the hardware store, we can
turn the simple LED circuit into a nice looking flame. The
first thing we have to do is modify the LED. I’ve never
been able to find bright LEDs that are diffused; the bright
ones tend to come in a water-clear package. This makes
the output very beam-like, as the convex end of the LED
acts as a lens. We can fix that with a file or a bit of
sandpaper. By grinding off the dome at the end of the
LED, we will get more light shining from the side of the
package. Figure 6 shows unmodified and modified LEDs.
The next step is to solder the resistor to the LED, add
leads, and then protect everything with heat shrink tubing.
In addition to protecting the connections, the heat shrink
adds a little stiffness that will be useful in the final step.
Figure 7 shows a wick in process and one that is completed.
I tend to solder the resistor to the anode side of the LED.
If you want to create a flameless faux candle, you can
use the wick as is; simply install it in a candle body, recessed
enough to provide the illumination created by a hidden flame.
You can actually take real candles (pillar type), burn them down
■ FIGURE 2.
■ FIGURE 3. Trigger input.
a bit, and then drill a hole through the center of the body.
By installing the LED in the wax candle, the light will shine
through the translucent paraffin and create a beautiful effect.
To create a “flame” for the wick, I use clear silicone —
the type used to seal seams around bathtubs and sinks.
The problem with silicone is that it takes an annoyingly
long time to fully cure. This can be fixed by adding two
drops of water and two drops of glycerin into two ounces
of silicone. In order to ensure a complete mix, I dip the
end of a match stick into yellow acrylic paint and then
dab that into the silicone. (A tiny bit of color goes a long
way.) The idea is not to color the silicone so much to
change it, but to provide a visual indication that the water
and glycerin are completely mixed in. By adding the water
and glycerin, the wicks will be ready in a couple hours
versus the 24 normally required for the silicone to cure.
After mixing the silicone in a small cup, dip the LED
into it, swirl it around a bit, and then slowly pull it out to
create a natural, wispy flame shape. You must keep the LED
in the upside-down position until the silicone is completely
cured (give it two hours if you mixed in the water and
glycerin). Figure 8 shows a completed wick/flame.
For many projects, I build faux candles from PVC
pipe. Simply cut it, clean it, and apply hot glue drips to the
top to finish the body. The wick is installed and secured
with hot glue; at this point, I have a candle ready for a
■ FIGURE 4. Wiring diagram.
■ FIGURE 5. Candle
October 2010 39