FIGURE 9. Basic
wiring diagram for a
higher the value, the more sensitive it is — but the less
linear the photocell’s response to light. Different photocells
exhibit different output behaviors, so you need to select a
resistor value that best matches the photocell and the light
conditions you expect.
Gordon McComb is the author of
Robot Builder’s Bonanza, now in its
fourth edition. Greatly expanded and
updated, this best selling book
covers the latest trends in amateur
robotics, and comes with 10 all
new robot construction projects,
plus more ideas for building
robots from found parts. Look
for Robot Builder’s Bonanza,
4th Ed in the SERVO Webstore
at http://store.servo
magazine.com.
Gordon may be reached at rbb@robotoid.
FIGURE 10. Alternative
hookup scheme for a
CdS photocell, using a
potentiometer to adjust
for sensitivity.
running at five volts, the positive-going threshold voltage is
around 2. 4 volts; the negative-going threshold voltage is
about 1.5 volts.) In total darkness, the output of the
photocell is zero volts (more or less). As the 74HC14 is an
inverting buffer, it will take this zero volts, and the output
will be five volts. As the light increases, the buffer does not
Sources
Precut and predrilled
Beginner Bot base, with all
construction hardware:
Budget Robotics
www.budgetrobotics.com
Seeed Studio L298 Dual
H-bridge module, IC:
Seeed Studio
(item MOT103B1M)
www.seeedstudio.com
Robotshop
(item RB-See-103)
www.robotshop.com
Trossen Robotics
(item SEEED-MOT103B1M)
www.trossenrobotics.com
Compatible L298 H-bridge
modules (selected
listing only):
Hobby Engineering
www.hobbyengineering.com
Pololu
www.pololu.com
SparkFun
www.sparkfun.com
Solarbotics
www.solarbotics.com
Replacement FA-130 size high
efficiency Mabuchi motors:
Pololu
item #1117
www.pololu.com
Plus, you need (available from
many online and local
retailers):
Six-cell AAA (preferred) or
AA battery holder with nine
volt battery clip terminals;
nine volt battery clip; 22
gauge solid and stranded
conductor hookup wire;
Velcro™ squares; assorted
assembly hardware as per the
text; 170 contact point mini
breadboard; two 22 kΩ
resistors; two large CdS
photocells; 74HC14 hex
inverter IC; and heat shrink
tubing.
SERVO 09.2011 41
