UNDERSTANDING, DESIGNING & CONSTRUCTING ROBOTS & ROBOTIC SYSTEMS
■ BY VERN GRANER
THE HALLOWEEN PROP DROPPER
After all these years dispensing treats, wouldn't it be fun to deliver a few
tricks this year? The darkened porch is dimly lit by the glow of a single
Jack-o-lantern. Riding a sugar buzz, the costumed trick-or-treaters swarm
up the stairs and ring the bell. You open the door and they shout TRICK OR
TREAT! You look at them gravely, then slowly raise your eyes to the ceiling
above them. Puzzled, they look up just in time to see a big black bat come
swooping down towards them! They shriek in surprise, then laugh and
giggle when they see it’s just a rubber bat that is now winding its way back
up to the ceiling. You hand out candy and listen to the kids talk about how
cool and scary that bat was and who they're gonna bring to see it. You smile
slyly as you think to yourself "This is gonna be a fun night!" then push the
door shut to await your next victims.
DROP WHAT YOU’RE DOING!
How fun would it be to drop a Spooky Spider, a
Ghoulish Ghost, or a Scary Skeleton right out of "thin air"
above unsuspecting trick or treaters? The Prop Dropper is
a "quick and dirty" Halloween project you can probably
put together in a single night with just a small microcontroller, a couple of servo motors, and a few bits and
pieces from the ol' junk box. The Prop Dropper is
designed to detect the presence of a person, rapid-deploy
a small prop, display it for a moment, then wind it back up
out of view, ready for its next victim!
WHERE THE HECK
DO YOU GET
It's not uncommon for
folks to ask where the
inspiration for this type of
project comes from.
Sometimes it’s just a matter of
necessity guiding the development. Other times, it’s simply
an accident. The idea for this
■ FIGURE 1. If a spool is turned
sideways, the wire or string on
it tends to fall off the end.
particular project was sparked when I was carrying a small
spool of hookup wire. I shifted the spool from one hand
to the other as I reached to open a door and the spool
turned sideways, dumping a length of wire right off the
spool to the floor (similar to the photo shown in Figure 1).
As I was grumbling and winding the wire back up, it
occurred to me that it might be possible to make a purpose-built device that would do this very same "drop" effect. If
there hadn't been a flange on the downward facing spool
edge, it seemed likely that the entire spool would have
emptied. Based on this observation, I guessed I could put
together something that would use this method to quickly
drop an item into view and then retrieve it.
STOP, DROP, AND SCREAM!
As I've been involved in putting on various private
and commercial haunted houses in the last decade (see
Resources), I've seen lots of mechanisms used to pop an
item into view. Most of them incorporate sophisticated
designs using pneumatic systems or mechanical linkages
(Figure 2). I wanted something that would perform a
similar deploy/retrieve function, but I really wanted it to be
simple in operation and easy to build.
After sketching a number of designs, I finally settled
on using one continuous-rotation modified servo as a
winch and one standard servo to position the winch servo.
To drive them, I chose the EFX-TEK Prop-1 controller board
that features our old friend the Parallax BASIC Stamp