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CHAPTER 9 TWO-WIRE POWERED SENSORS
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Figure 9-3. Two-wire power supply
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NXT-G has a block for the RCX-style Light Sensor called the Legacy Light* block. Instructions for importing blocks like this were given back in 2. You will use it for all two-wire powered sensors (see Figure 9-4). There s an output from the block with the usual 0 100 light value, but we re more interested in the Raw value output that has a wider operating range.
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Figure 9-4. NXT-G Legacy Light* block
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CHAPTER 9 TWO-WIRE POWERED SENSORS
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The little NXT-G program shown in Figure 9-5 just reads the Raw value and puts it on the NXT display. Because the amplifier has a gain of 1, a 5V input will display 1023. However, because of limitations previously mentioned, the Raw value will go down to only about 200 when the input is 0V. There is a plot showing the complete relationship between input voltage and Raw values in Appendix A.
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Figure 9-5. NXT-G Read Raw Program
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Half-Volt Sensor
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Figure 9-6 shows the circuit for a voltage sensor that reads input voltages between 0.5V and +0.5V. Diode D2 has been added to the basic Figure 9-2 design to provide an offset voltage. When diodes conduct, they behave a little like a 0.58V battery.
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Figure 9-6. Half-Volt Sensor circuit The positive input on the op-amp in this circuit with the components listed in Table 9-1 will have a gain of 1+R1/R2, 1+330/100, or 4.3. With no voltage on the negative input, the output of the op-amp will be offset by 0.58 4.3 or 2.49V. This offset will allow you to measure negative voltages (as well as positive ones).
CHAPTER 9 TWO-WIRE POWERED SENSORS
Table 9-1. Bill of Materials
Component
U1 D1, D2 R1 R3 R2 R4 C1
Part Number
LM324 1N4148 330k 1k 100k 10k 22uF
Description
Quad op-amp Small signal diodes 1/4 W 5% Carbon film resistor (orange-orange-yellow-gold) 1/4 W 5% Carbon film resistor (brown-black-red-gold) 1/4 W 5% Carbon film resistor (brown-black-yellow-gold) 1/4 W 5% Carbon film resistor (brown-black-orange-gold) 16V or higher electrolytic capacitor
Radio Shack
276-1711 276-1122 271-312 For all resistors See R1 See R1 See R1 272-1014
The gain of the negative input is R1/R2 or 3.3. The negative input inverts the voltage so a positive input subtracts from the output voltage. For example, with an input of +0.5V, the op-amp output will be the offset 2.49V minus 1.65V, which equals 0.84V. Notice that the more positive the input voltage, the smaller the op-amp output voltage, the smaller the NXT input voltage, and the smaller the Raw value. The usable range of the voltage sensor is limited by the low-voltage capability of the op-amp and how weak the NXT batteries are. It should give excellent results, between 0.5V and +0.5V, with the relationship of input voltage to Raw value shown in Figure 9-7. After all the circuit gains and offsets, the 0V input is offset by about 560, and the slope is 1.6mV per Raw value over a range of +/ 0.6V.
Figure 9-7. Input voltage versus Raw value for Half-Volt Sensor
CHAPTER 9 TWO-WIRE POWERED SENSORS
Construction of the sensor follows the same techniques described in Appendix A. Figure 9-8 shows the circuit built on a solderless breadboard.
Figure 9-8. Half-Volt Sensor built on solderless breadboard Table 9-2 shows the step-by-step instructions for connecting the parts.
CHAPTER 9 TWO-WIRE POWERED SENSORS
Table 9-2. Component Placement
Component
U1 pin 1 C1 + J1 J2 D1 anode cathode (the cathode is the end with the line) D2 anode cathode J3 R3 (brown-black-red-gold) R1 (orange-orange-yellow-gold) R2 (brown-black-yellow-gold) R4 (brown-black-orange-gold) NXT black-white Input + Figure 9-9 shows the sensor moved to a PC board.
Start
F4 Y1 Y7 X7 J2 F3 G6 I2 H4 I5 H6 X2 G11
X1 J7 B7 Y2 X3 G3 I4 H5 I11 H7 H2 X11
CHAPTER 9 TWO-WIRE POWERED SENSORS
Figure 9-9. Half-Volt Sensor on PC board measuring a Solar cell output The voltmeter program (see Figure 9-10) converts the Raw sensor value into mV and displays the result on the NXT display. Because of variations in electrical components, you might need to adjust the
CHAPTER 9 TWO-WIRE POWERED SENSORS
exact offset and scale values for your sensor to get the best zero and voltage calibration. To keep precision with the NXT integer math, the scale value is 10 times larger than it should be to start with, and the result is divided by 10 to make up for it. If you added 5 before you divided, then it would be the same as rounding the number. Of course, second-generation NXT software works with floating-point numbers, and you don t need to be as careful when scaling calculations like this.
Figure 9-10. Voltmeter program An interesting use for the Half-Volt Sensor is characterizing the output voltage of a solar cell. A single silicon solar cell (Radio Shack 276-124 for example) has an output voltage approaching 600mV in bright sunlight. However, with dimmer light or with any kind of load on its output, the voltage will fall within the range easily measured with the sensor.
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