CHAPTER 27 ■ ENCOREI can’t imagine the part will work while the robot is moving, since the ball probably bounces around a lot.
But, it might be functional when the robot pauses.
Temperature Sensors
A thermistor is a variable resistor, just like a potentiometer, but one that changes resistance with changes in
temperature. For example, a negative temperature coefficient thermistor decreases in resistance when you
hold it in your hand or against a light bulb. It increases in resistance when dipped in ice water.
Thermistors are very small and fairly inexpensive (see Figure 27-12). They often sit inconspicuously on
circuit boards, within battery packs, or in probes.
Figure 27-12. Various thermistors for measuring temperature. The first two are plain, the LM34 includes
built-in electronics (center right), and the far right side shows a strip of tiny surface-mount thermistors
A thermistor is as easy to use as a cadmium-sulfide photoresistor. Hook the thermistor up in a series
with a standard resistor and test the voltage at the point where they connect. A comparator would work well
with a thermistor. In fact, you could replace the photoresistors in the line-following robot with thermistors,
and you’d end up with a really lame heat-seeking robot (thermistors change values too slowly and they’d all
be measuring nearly the same local air temperature.)
For the best results, a thermistor should be read by a smart part, like a microcontroller. The
microcontroller could contain a table of resistances with associated actions (shut down the battery charger
at a certain temperature) or the numeric temperature values to display (like degrees Celsius).
Touch Sensors
Sandwich has no method of detecting a wall, and thus smacks into it. But there are some basic sensors that
can provide robots with a crude sense of touch.