Mounting of the battery, power switch, and PIR sensor.
The finished back panel of the MiniBot (voltmeter, power
switch, USB port, and volume control).
It is connected to the output of the power switch, so it
displays the battery voltage whenever the robot is on. The
meter is mounted on the back of the robot next to the
power switch. On the robot with the black trash can body,
just cut out a rectangular opening so you can slide the
voltmeter through and secure it with a drop of glue.
On the frosted white/clear version, you can set the
meter inside the can and secure it with a pair of screws.
This provides a great looking display since the readings will
show right through the body.
With the meter installed, it is easy to tell how the
battery is doing at all times, and it provides a cool display. It
shows the load on the battery increase whenever the servos
are moving and when other circuits are active that draw a
noticeable amount of current.
These meters are so inexpensive now, I install them on
all my projects.
50 SERVO 05.2013
Mounting Sensors and Devices
on the Body
Now that the base has the main drive and power
system all installed, it is time to move on to some sensors
and miscellaneous items. Since we may want to have a
MiniBot perform guard duty, one of the first sensors to add
is a PIR sensor. There are some inexpensive adjustable ones
available from Propeller Powered. Another good option is
the newer Parallax PIR module which will light up when it
detects motion. This shows whenever it detects something,
so you can tell when it is triggered. The sensor is mounted
directly on the front of the robot from the inside. I drilled a
small pilot hole about 2” down from the rim. Then, I drilled
a hole large enough so the domed lens can just fit through
using a stepped drill (Unibit) which is an ideal tool for this.
It can make large holes in plastic without cracking it. Once
the hole is in place, I use a bit of adhesive (GOOP) to hold
the sensor there.
Since these robots use a speech chip, they need a
speaker installed. The best location for the speaker seemed
to be on the front of the robot about 4. 5” down from the
rim under the PIR module. I drilled a large hole and added
a perforated cover so the sound would project through. A
series of smaller holes drilled into the front would work just
as well. If you go that route, try to align them in a good
looking pattern. The speaker itself was mounted to the
inside of the body directly behind this opening using two
NOTE: When drilling into plastic, you need to be very
careful not to crack it. To start, just use a small drill to make
a pilot hole. Any drill bit 1/8” or less will do. Then, follow
up with a Unibit or stepped drill to enlarge the hole. The
stepped drills are much less likely to catch and crack the
plastic. If you try using a larger drill bit, it will more often
than not break the plastic instead of making a proper hole.
Preparing the Head Assembly
One of the most interesting features of the MiniBot is
the automatic control of the swing lid and the panning
control of the sonar module on top. Not only can you aim
the sonar from side to side, but you can also tilt it up and
down to help build up a nice map on what may really be in
front of the robot.
Since this is a small robot, we need to use small servos
for the task. I used a pair of inexpensive 9 g servos from
eBay that were close to $3 each shipped. Standard sized
servos are just too large to fit in the lid.
Getting the servo installed for tilting the swing lid can
be tricky and is probably the most difficult part of the build.
Start by popping out the swing lid from the top cover. You
should then see two thick pins on either side of the top
cover. The servo can be mounted on either side, but I