CHAPTER 25 ■ LAUNCHING THE LINE-FOLLOWERThe results of these experiments are not applicable if you lift up the robot from standard course
position. You want to test what the robot is experiencing under racing conditions.
Expected Voltages at the Sensor Test Points
After balancing the sensors and adjusting headlights, what follows are the voltages I measured on Sandwich.
Compare your voltages to mine. Recognize that depending on how fresh or drained your battery is, your
robot is unlikely to have the same voltages that Sandwich did. Simply scale the numbers to determine if your
robot is roughly similar.
■ Note I covered Sandwich with a cardboard box during testing to avoid deviations due to ambient lighting
and shadows. During these tests, Sandwich is actually supplied power from a wall transformer, because a
dropping battery voltage would skew the values of the last tests performed.
Positive power bus: 9.07 V
Be sure to measure the voltage at the power bus, not at the battery. A battery disconnected from a
circuit produces higher voltages than it does under load. Also, if a diode is added to the line-following circuit
to protect from reverse installation of the battery, then a bit more voltage is dropped before reaching the rest
of the circuits.
TP1 and TP2 on white paper: 2.036 V and 2.034 V
TP1 and TP2 on black paper: 7.54 V and 7.40 V
Sandwich’s brightness-balancing trimpot (R2) was adjusted on white paper. Notice how the static value
fails to balance the sensors at darker levels. The white-paper test-point voltages are similar to each other, but
the black-paper test-point voltages are not as similar to each other. No big deal. Just remember to rebalance
the sensors on the course at race day under local lighting and surface conditions.
1-inch wide blue masking tape beneath right sensors on black paper,
TP1 and TP2: 4.43 V and 7.13 V
1-inch wide ordinary masking tape beneath right sensors on black paper,
TP1 and TP2: 3.098 and 7.02 V
1-inch wide strip of shiny aluminum foil beneath right sensors on black paper,
TP1 and TP2: 2.374 and 7.21 V
1-inch wide strip of white paper beneath right sensors on black paper,
TP1 and TP2: 2.134 and 6.88 V
A couple of interesting results:- The robot can see the difference between blue tape and black flooring. The blue tape
is a little above the middle of the robot’s brightness range. - Ordinary masking tape (slightly tan) is less bright than white paper.
- Aluminum foil doesn’t diffuse as much light as white paper.