FIGURE 6. Power Supply
left side of the bottom edge of M-bot’s
breadboard assembly. Leave a small
amount of slack in the wire in case you
want to reposition it later.
Follow the same procedure to
connect the positive lead from the
six-volt battery pack to the right side of
the bottom edge of the breadboard
assembly, again leaving a little slack.
Finally, connect the two ground
leads from the battery packs together;
solder them to a six inch piece of #22
solid wire and insulate the junction
with heat-shrink tubing. (Again, you
may want to add an additional ground
connection for possible future use with
additional bottom-base circuitry.) Strip
about 1/4 inch of insulation from the
other end of the solid wire, route it up
through the slot in the top base, and
insert it near the positive wire from
the nine-volt battery (on the left side of
the bottom edge of the breadboard
M-bot’s power supply is
very simple (see Figures 6
and 7). We will be using the
LM7805 voltage regulator,
which is capable of supplying
a full amp or more with an adequate
heatsink. Because M-bot’s motors are
powered by their own battery pack, we
don’t need the heatsink, but if you
modify M-bot to include considerable
additional circuitry, you may want to
add one later on.
With the left-side switch in the off
position, follow Figures 6 and 7 to assemble the regulator circuit on the far left
side of the bottom one-half breadboard.
Figure 8 presents the LM7805 pinout —
use it to properly orient the chip when
you insert it into the breadboard. When I
first constructed M-bot, I used an
LM78L05 regulator (100 mA maximum
output), which is still visible in Figure 7.
After testing M-bot for a while, the
LM78L05 was getting overly warm, so I
switched to the more powerful LM7805.
At that point, I somehow managed
to insert the LM7805 backwards!
Surprisingly, none of M-bot’s circuitry
was damaged, even though it took me a
while to figure out what I had done.
Once the power supply circuit is
assembled, turn on the left-side switch
and use a multimeter to check for a
stable + 5 volts on the output pin of the
regulator. When you are sure the circuit
is functioning properly, turn the
switch off again, and use jumper
wires to connect the ground wire
from both battery packs to the
power rail ground on the left side
of the breadboard assembly. Then,
working around the perimeter
of the breadboard area, connect
all of the ground rails together.
Be sure not to form a complete
circle, which would create a ground
loop and potential power problems —
just connect each ground rail to the
next until they are all connected. If you
are unfamiliar with the phenomena of
ground loops, Google for “ground
loop” to obtain a wealth of information.
Similarly, connect the positive output of the regulator circuit to the + 5 volt
rail on the left side of the breadboard
assembly, and then connect each + 5 volt
rail around the perimeter (again without
forming a complete circle). At least at
first, you may want to include an LED
and current-limiting resistor between the
+ 5 volt and ground rails to indicate when
power is applied to the breadboards.
Finally, you may want to include
a couple of 0.01 µF decoupling capacitors between the + 5 volt and ground
rails to minimize voltage transients. The
separate battery pack for the motor
supply should also help considerably in
this regard. When you have completed
the power supply and power rail
wiring, flip the left-side switch to the
on position and double-check that each
of the power rails properly supplies + 5
volts and ground.
Contact the Author
You can reach Ron via email
at Ron@JRHackett.net or visit his
website at www.JRHackett.net.
At this point, we have completed
the chassis assembly and power supply
wiring for M-bot’s breadboard area.
Next month, we will take a detailed
look at M-bot’s circuitry. In addition,
we will present two simple software
routines that will enable M-bot to avoid
obstacles, and to respond to visible
light levels in its environment. Finally,
we will suggest a couple of possible
modifications and improvements you
might want to consider as you carry
out your own experiments with M-bot.
Readers who feel ready to explore
some of this on their
own may want to download M-bot’s complete
schematic diagram from
the SERVO website to
get started with M-bot’s
FIGURE 7. Assembled
54 SERVO 09.2007
FIGURE 8. LM7805 Pinout.