■ FIGURE 10. Dremel drill press used
to drill holes for speaker grill.
■ FIGURE 11. Speaker grill after PCB
drill template is removed.
■ FIGURE 12. Speaker hot-glued to the
bottom side of the jar lid.
precise drilling in order to get a nice
professional look. Since there was going
to be a speaker in the jar lid, there
would have to be holes to let the sound
out. Though I could have just randomly
drilled a lot of holes, I didn’t want the
unit to look quite that rough. So, to
get nice, even spacing, I took an old
piece of .100” perf board and hot-melt
glued it to the top of the jar (Figure
9). I then placed the lid on my
Dremel drill press and used the perf
board to guide the bit as I punched
holes to make a nice circular grid
(Figure 10). When I was done, I
popped the old PCB off the top leaving a nice speaker grill (Figure 11).
Next, I hot-melt glued the speaker
that came with the 20 second sound
module to the underside of the jar lid
(Figure 12). I placed the pushbutton
back into its hole (Figure 13) and then
did a test-fit on the top of the jar
(Figure 14). I put the main PCB with
the servo on it down inside the jar
and marked four dots on the bottom
that corresponded to the locations of
the four standoffs. I drilled four holes
and ran in a couple of screws to hold
the PCB in place to see how I was
for clearance from the servo horn to
the pushbutton body (Figure 15). It fit
with room to spare! Now that I had a
button and a speaker, it was time to
get the sound module working.
The RadioShack 20 second
sound module is a cute little device
that really lends itself to hacking.
Though the sound quality isn’t terrific,
it’s usually good enough to playback
voice quality recordings. Strangely,
the external pushbutton switch is the
one used for recording, the playback
button is located on-board (seems
sorta backwards to me!). Through
some trial and error, I discovered that
the onboard playback pushbutton
can be remotely operated by adding
a wire to the top resistor pad of R3
as shown circled in red in Figure 16.
When this wire is taken to ground
and then released, the unit will begin
to play back its stored sound.
I hot-melt glued the sound module
to the opposite side of the servo motor
on the PCB and then ran the power
■ FIGURE 13. Pushbutton test-fit next
MONSTERS DETECTED! RUN FOR YOUR LIFE!
Inevitably, when I’ve described the operation of the detector to friends and
colleagues, someone always asks “can you set it so that once out of every three or
four activations, the unit sounds a siren and yells ‘MONSTERS IN THE CLOSET!
RUN FOR YOUR LIFE!!”
Though we all got a chuckle out of imagining the result of such a scan, in
reality I have never considered actually doing this, even as a joke. Some folks also
asked “Could you have the unit periodically declare a monster is present on the first
pass and then, after the second pass, declare the monster has left?”
I considered the dialog very carefully and even went so far as to consult some
friends in child care and child development to insure that the dialog had the
intended purpose. In the end, I decided that the Peanut Butter Monster Detector
should never detect a monster. If it ever gave a “positive” reading, that would tend
to lend credence to the concept that imaginary monsters exist OR that the detector
could be “fooled”. If you think about it, giving a false positive leads to the question,
“Could it give a false negative?”
For maximum reassurance, not only does the unit NOT detect a monster on the
first pass, it then double-checks to make sure that the first pass was not mistaken.
Not only is this doubly reassuring, this also causes the unit to make sounds and
perform actions for about 20 seconds or so, helping the child focus on the device
and forget about whatever noise or feeling awoke them in the first place.
So far, this approach has worked very well to provide reassurance and to
empower my child by providing a way for her to take charge of her own fears and
learn to help herself sleep through the night.
So, if you were thinking of writing to ask me about this, no you’re not the only
person who’s thought of it.
Also along these lines, when you are recording the dialog for your detector,
make sure you are alone and that any older siblings aren’t around to see how you
do it. Recording over your reassuring words with “GIANT MONSTER DETECTED
UNDER THE BED! RUN FOR YOUR LIFE!” is just the sort of trick older siblings might
love to play on the younger ones! You have been warned!
October 2008 71