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CHAPTER 5 RESISTIVE SENSORS
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Figure 5-33. Finished homebrew Light Sensor
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Comparison of LEGO and CdS Light Sensor
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The resistance of a CdS LDR drops with light level, just like the NTC thermistor resistance changes with temperature, and for the same reason. Probably not all LDRs work this way, but you can model the resistance of the type 8P sensors with a simple equation. For the 8001, the value of K is 10,000:
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K Lux
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Humans experience light levels over a huge range in the course of a day. Table 5-1 shows that light variation from starlight to sunlight is a variation from 0.001 to 100,000 lux. Table 5-1. Typical Light Levels
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Light
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Daylight Hazy Cloudy bright Store windows
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50,000 100,000 25,000 50,000 10,000 25,000 1,000 5,000
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CHAPTER 5 RESISTIVE SENSORS
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Table 5-1. Typical Light Levels (continued)
Light
Office Living room Hallway Street lights Full moon Starlight
200 500 50 200 50 100 1 20 0.01 0.2 0.001
The resistance of the CdS sensor over a somewhat more limited range is shown in the plot in Figure 5-34. The light level axis has to be plotted with a logarithmic scale to allow for the wide range of values.
Figure 5-34. Plot of LDR resistance versus light intensity
CHAPTER 5 RESISTIVE SENSORS
Using the resistance equation for the 8001 LDR, a Raw value can be predicted for the sensor at various light levels and plotted in Figure 5-35. Unlike the NXT Light Sensor, the Raw value decreases with increasing light. The Raw values vary over a range from almost 0 to more than 900. The CdS sensor also varies more at low light level than the NXT, and continues to vary after the NXT sensor hits 100 in bright light. The biggest disadvantage of the CdS sensor over the NXT sensor is that it changes value much more slowly.
Figure 5-35. CdS and NXT Light Sensor comparison
Theremin
The theremin was named after L on Theremin, who invented it in 1919. It was probably the first allelectronic musical instrument, and Figure 5-36 shows L on playing it. It was played by a musician merely waving his hands around the instrument without ever touching it. The distance of the right hand to a vertical antenna controlled the pitch or frequency of the note, while the distance of the left hand to a loop antenna controlled the volume. It produces an eerie monotone that was popular in 1950s science fiction films.
CHAPTER 5 RESISTIVE SENSORS
Figure 5-36. Inventor L on Theremin playing his theremin The original theremin used radio waves to detect the placement of the musician s hands. The NXT theremin uses the amount of light falling on two homebrew Light Sensors. The sensors are mounted on arms extending from the sides of the NXT, as shown in Figure 5-37. While playing, you need to arrange the light so your hands cast shadows onto the sensors. The closer you hold your hand to the sensor, the darker the shadow.
Figure 5-37. NXT theremin The NXT-G program shown in Figure 5-38 couldn t be much simpler. The Raw value from the volume sensor is read in on port 1. To make sure that the value reaches 0 or less, the first math block subtracts 400 from the Raw value. The next Math block scales the value to the range of the volume input on the Sound block. You might need to adjust these values for your particular sensors and lighting
CHAPTER 5 RESISTIVE SENSORS
conditions. The Sound block volume input range is 0 to 100, but it actually has only 5 volume levels: 100, 75, 50, 25, and 0 for mute. Unfortunately, this coarse volume control limits the vibrato effects the original theremin is famous for. The tone or pitch is read in on port 2, and can be fed directly to the tone input of the Sound block.
Figure 5-38. NXT-G theremin program Moving your hand toward the volume Light Sensor increases the volume, while moving your other hand toward the tone Light Sensor increases the frequency of the note. A unique feature of a theremin is that it can hold a note indefinitely if you don t move either hand.
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