Build Your Own Combat Robot

246 Build Your Own Combat Robot
FIGURE 11-4
Implementation of
the thermistor
using a voltage
divider circuit.
the resistance of the sensor, the temperature can be calculated. Measuring the sensor’s
resistance is accomplished by using a voltage divider circuit. Figure 11-4
shows a simple schematic drawing of this type of sensor.
The voltage to be measured is the V ou
, which is defined in the following equation,
where R sensor is the thermistor resistance, R is some other resistor used in the
1
circuit, and V in
is the input voltage:
Because V out is being measured and the thermistor’s resistance, R sensor , is unknown,
equation 1 can be solved for the resistance of the sensor.
Equation 2 shows this new relationship:
After the thermistor’s resistance is measured, the temperature can be calculated
using the Steinhart-Hart Equation, which describes how the resistance changes
with temperature in semiconductor thermistors. The basic form of the equation is
shown in equation 3, where constants A, B, and C are thermistor-specific constants
that are obtained from the manufacturer of the thermistor, or they can be
determined experimentally. T K is the temperature in degrees Kelvin.
A more useful Equation is shown in equation 4, where the temperature, T C ,isin
degrees Celsius:
11.1
11.2
11.3
11.4