code supplied at the article link, the Prop Dropper 2 will
drop both props, wait three seconds, and then wind them
up again while playing an evil laughing sound. This serves
two purposes. First, it makes sure the props are both
properly initialized in case the power was shut off in the
middle of a wind up cycle. Second, it makes sure your
sounds are playing properly.
Once the props are back in position, a Parallax
PING))) ultrasonic range finder is used to detect when a
person enters the target area. Once they do, the first prop
is dropped and an accompanying sound file is played.
When the person starts to move forward again, a second
prop is dropped and that sound file is played. After the
person leaves, the props reset and the system is ready to
scare the next victim.
While a PIR sensor can be shrouded to focus the
detection area to be more limited, I opted to use the
PING))) sensor. This gives me the ability to see not only
when someone is in range to drop the prop on, but I can
detect their distance from the sensor. This means I can
control which prop drops and when.
Like Vern's original Prop Dropper, this unit uses two
servos for each prop we're going to drop. Each pair of
servos consists of a tilt (standard) servo and a wind-up
(continuous rotation) servo joined together. The tilt servo
is responsible for tipping the wind-up servo, which
inherently tilts the spool that holds the string the prop
dangles from. By tipping the spool vertically, the string
unravels all at once resulting in the prop dropping
suddenly until it reaches the end of the string and
stops. Figure 1 shows a completed dropper assembly
installed on a board.
When we want to reset the prop, we simply move
the tilt servo to set the wind-up servo (and therefore
the spool) back to a horizontal position and then
rotate the wind-up servo to raise the prop back up,
ready to drop again.
You will need to copy sound (. WAV) files to the
microSD card from your computer. A card reader or
adapter may be required to connect the microSD card
to your computer. I have tested the WAV player object
with 11 kHz and 22 kHz mono WAV files in both an
eight-bit and 16-bit format. I am unclear if other
formats are supported by the object used. You can
use a free audio utility such as Audacity to record,
edit, and modify sound files.
In the Parts List, you will find the main Parallax
components I used in this project, as well as other
items used in the build. The first thing I did was set
everything up on my desk as a prototype so I could
make sure I had all the required objects for the
hardware, and so I could center/calibrate the
continuous rotation servos. In Figure 2, you can see
Prototyping is essential for hashing out designs
like this because you have software and hardware
components mixed together. In this case, I needed to
synchronize the motions of several servos (both standard
and continuous). I also tested each section independently
before connecting everything together to make it easier to
debug later. Servos are inherently slightly different from
each other, so you have to test to find the right pulse
values to make each servo do what you want.
October 2014 47
FIGURE 2. Prototyping the Prop Dropper 2.
All connections are in relation to the
Propeller Activity board.
Servo voltage jumper set to 5V.
All connections use servo headers.
Components shown are not to scale.