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CHAPTER 5 RESISTIVE SENSORS
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Figure 5-7. Measuring the salinity of water Repeat this process till you have a plot of resistance and concentration, as shown in Figure 5-8. Then you can use the plot backward to determine the salinity given the resistance of an unknown sample.
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Figure 5-8. Resistance of water with varying amounts of salt
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Legacy Temperature Sensor
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Temperature is one of the most useful measurements you can make with your NXT. You can use the NXT as a digital thermometer to monitor the temperature of an experiment continuously, or you can log the temperature into a file to see how the value has changed over a long time period. You can even use the value in a control loop that regulates temperature. LEGO makes a Temperature Sensor, as pictured in Figure 5-9 (9V Temperature Sensor PN#W979889). It is a legacy sensor from the RCX and requires the NXT conversion cable (PN#770323) or one you make yourself with instructions in 11.
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Figure 5-9. LEGO Legacy Temperature Sensor It s a little expensive, considering that you can build your own for a fraction of the price. The LEGO sensor isn t suitable for every sensing application anyway. For example, I wouldn t use it to monitor the temperature inside a hamster cage the hamster might mistake it for a chew toy. The temperature-measuring range of the NXT is from 4 F to 158 F ( 20 C to 70 C). LEGO probably didn t want you to boil your temperature probe. Although limited, it still represents a decent range of temperatures that you re likely to encounter. Using the sensor in a NXT-G program requires importing the Legacy Temperature block shown in Figure 5-10. (We covered importing NXT-G blocks like this back in 2.)
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Figure 5-10. Legacy Temperature block The configuration window for the block, in which you decide whether the reading is in Celsius or Fahrenheit, is shown in Figure 5-11. An important thing to know about the block is that the temperature output is ten times the actual temperature. That allowed you to have 1/10 degree accuracy with only the integer arithmetic of the original NXT software.
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Figure 5-11. Configuration window
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Thermistors
The LEGO Legacy Temperature Sensor is based on an electronic component called a thermistor, like those illustrated in Figure 5-12. A thermistor is an unusual resistor that changes resistance value with temperature. (The name thermistor is just the combination of the two words thermal and resistor.) The particular thermistor used with the NXT decreases in resistance with an increase in temperature. Because the slope of the relationship between temperature and resistance is negative, the type is known as a Negative Temperature Coefficient (NTC) thermistor.
CHAPTER 5 RESISTIVE SENSORS
Figure 5-12. Examples of thermistors, with the GE RL0503-5820-97-MS on the right A thermistor is manufactured by attaching two wires to a tiny pellet of semiconducting material, which is usually a metallic oxide (see Figure 5-13). When the temperature of the pellet increases, more electrons in the semiconductor are made available to conduct electricity, so the resistance goes down. Usually the whole thing is coated with epoxy to seal it from moisture and other contamination. Variations in the room temperature resistance and the amount that the resistance changes with temperature create hundreds of different thermistors to choose from. Additionally, size, packaging, and accuracy take the selection well into the thousands.
Figure 5-13. Semiconducting pellet inside a thermistor
CHAPTER 5 RESISTIVE SENSORS
Reverse Engineering the NXT
How do we find a thermistor compatible with the NXT We start by reverse engineering the NXT Raw-totemperature-conversion equation. The plot in Figure 5-14 was made by recording both the temperature and Raw values simultaneously. As you can see, the NXT conversion equation isn t a simple offset or multiplier.
Figure 5-14. Plot of temperature and Raw value You already know how to convert a Raw value into a resistance measurement from the NXT Ohmmeter project. Feeding Raw values into the equation and calculating R values allows you to make a plot of temperature and sensor resistance, as in Figure 5-15. You must match this plot for a NXTcompatible Temperature Sensor.
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