FIGURE 7. Close-up of the power
terminal. The L298 board provides its
own regulated five volts for the
circuitry on the breadboard.
cells. I prefer NiMH batteries
because they’re a little more
environmentally friendly. Though
they initially cost more than alkaline
cells, when you factor in recharging
them hundreds of times, they’re a
very good deal. (Be sure to use a
recharger made for NiMH batteries.)
When using NiMH or NiCd
batteries, the six-cell pack delivers a
nominal 7. 2 volts. When using
alkaline batteries, the pack delivers
nine volts. Either voltage is
acceptable. However, be sure to
recharge your NiMH or NiCd
batteries when the pack voltage falls
under seven volts.
of the wired power terminal. I’ve used a small cable tie to
keep everything together.
Use a small flat-bladed screwdriver to tighten the
terminals so the wires remain snug. You DO NOT want any
of these wires to accidentally come loose, or else they could
cause a short circuit and damage to the L298 module.
Attach the battery holder to the Beginner Bot base
using two 1” Velcro™ squares as shown in Figure 8. This
arrangement allows you to pull the holder off the base to
replace or recharge the batteries.
For the prototype Beginner Bot, I used a set of six AA
size nickel-metal hydride (NiMH) cells. You can use AAA size
if you wish, and substitute nickel-cadmium (NiCd) or alkaline
Add the Solderless
Augmenting the L298 motor module is a small
collection of circuitry to provide the Beginner Bot with
sensitivity to light. Rather than solder together this circuitry,
you can use a small solderless breadboard. A mini
breadboard with 170 contact points will do.
The control circuit uses two cadmium sulfide (CdS)
photocells, an inexpensive and readily available 74HC14
integrated circuit, a couple of resistors, and some wire. The
solderless breadboard connects with the L298 module via
the two power wires you provided
earlier, plus a pair of 10” length three-conductor male-to-female R/C servo
At the heart of the control circuit
is a CdS photocell, also called a light
sensitive resistor. It’s connected as
shown in Figure 9 with a 22 kΩ
resistor to form a voltage divider. The
output of a photocell is a varying
resistance — the darker it is, the
higher the resistance. With the resistor
added and a connection point in
between, the output becomes a
voltage that varies between zero and
five volts, depending on the
brightness of the light.
The 22 kΩ resistor helps establish
the sensitivity of the light sensor. The
FIGURE 8. The Beginner Bot with
L298 board and batteries installed,
ready to go. The battery holder is held
in place with Velcro.
40 SERVO 09.2011