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CHAPTER 9 TWO-WIRE POWERED SENSORS
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Figure 9-12. Melt the tip of the bulb You can see where the tip of the bulb has turned into a hole in Figure 9-13. It makes a pop sound when it does this.
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Figure 9-13. The hole in the glass bulb
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CHAPTER 9 TWO-WIRE POWERED SENSORS
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The next step is to break the bulb by pinching it in a vise or C-clamp, as shown in Figure 9-14. The bulb has been wrapped in a paper towel to contain the small pieces of glass.
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Figure 9-14. Break the bulb with a C-clamp Do not touch the filament or support structure because they re extremely fragile. Remove any remaining pieces of glass (see Figure 9-15) with needle-nose pliers; then carefully screw the bulb into its socket.
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Figure 9-15. Remove the remaining glass with needle-nose pliers
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CHAPTER 9 TWO-WIRE POWERED SENSORS
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Whistler Construction
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The finished electronic whistler is shown in Figure 9-16. Mounted to the side of the NXT, the filament is exposed so you can blow air on it.
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Figure 9-16. Finished electronic whistler The Whistler program (see Figure 9-17) reads the sensor, displays the value, and produces a whistle tone whose frequency is proportional to the value. As you blow on the filament, the pitch of the tone will increase. Just waving air at the sensor with your hand or moving it around will change the tone.
Figure 9-17. Whistler program
CHAPTER 9 TWO-WIRE POWERED SENSORS
Pressure Sensor
Back in 6 you made an elementary pressure sensor using LEGO pneumatics. Now that you can make op-amp circuits, you can make a more accurate sensor using the bridge pressure transducer shown in Figure 9-18. This is a Model 1230-030D-3L PC board mountable pressure transducer from Measurement Specialties (see Table 9-3 for the full bill of materials). It can measure up to 30psi (2,068hPa or 2 Atmospheres) and it reads the difference in pressure between the two ports.
Caution You can use this sensor only to measure the pressure of a gas. Do not try to measure the pressure of
liquids or let any moisture get into it it will ruin the sensor element.
Figure 9-18. Bridge pressure transducer
CHAPTER 9 TWO-WIRE POWERED SENSORS
Table 9-3. Bill of Materials
Component
U1 D1 D2 R1 R2, R4 R3 R5 R6 P1 C1
Part Number
LM324 1N4148 1N4733A 100k 10k 1k 1meg 470 1230-030D-3L 22uF
Description
Quad OpAmp Small Signal Diode 5.1V 1W Zener Diode 1/4 W 5% Carbon Film Resistor (brown-black-yellow-gold) 1/4 W 5% Carbon Film Resistor (brown-black-orange-gold) 1/4 W 5% Carbon Film Resistor (brown-black-red-gold) 1/4 W 5% Carbon Film Resistor (brown-black-green-gold) 1/4 W 5% Carbon Film Resistor (yellow-violet-brown-gold) Measurement Specialties 16V or Higher Electrolytic Capacitor
Radio Shack
276-1711 276-1122 276-565 271-312 For All Resistors See R1 See R1 See R1 See R1 Digi-Key MSP6822-ND 272-1014
The pressure transducer is component P1 in the circuit shown in Figure 9-19. Internally, it is four resistive elements in an arrangement known as a Wheatstone bridge. With 5V applied to the top of the bridge, the measurement of interest is the difference of the two voltage dividers, and this difference is only 100mV full-scale. Op-amp followers U1B and U1D condition the two voltage-divider values so the differential amplifier made with U1A can subtract and then amplify the difference by 10.
Figure 9-19. Pressure sensor circuit
CHAPTER 9 TWO-WIRE POWERED SENSORS
Diode D2 is a Zener-type diode that creates a constant 5V for the pressure transducer. Resistor R5 offsets the output of U1A so that the sensor can measure negative pressures. Figure 9-20 shows the pressure sensor built on a solderless breadboard.
Figure 9-20. Pressure sensor on solderless breadboard Table 9-4 shows the step-by-step construction of the whole sensor. Table 9-4. Component Placement
Component
U1 pin 1 C1 + J1 J2 D1 anode cathode J3 J4 D2 anode cathode R2
Start
F4 Y1 Y7 X7 J2 X3 C4 C3 D4
X1 J7 B7 Y2 B3 C5 F3 G5
CHAPTER 9 TWO-WIRE POWERED SENSORS
Table 9-4. Component Placement (continued)
Component
R3 R4 R1 R6 R5 J5 P1 pin 1 J6 J7 J8 J9 NXT white black
Start
G2 F10 J5 Y3 H3 H9 F13 X10 D6 I3 J8 H2
G4 F6 J4 J3 H6 H10
G14 H15 I16 J13 X2
After you build the pressure sensor, it needs to be calibrated. With nothing connected to the transducer ports, the pressure reading should be zero because the pressure difference is zero. However, the sensor reading is deliberately offset by a Raw value of approximately 580 so it can measure negative pressures. You can use Boyle s Law to calculate the slope of the relationship between Raw value and pressure. Boyle s Law says that the product of the pressure and volume of a gas is a constant. In other words, if you halve the volume of a gas, you double its pressure. You can get disposable 10ml oral syringes such as the one shown in Figure 9-21 from pharmacies. Set the syringe for exactly 10ml and connect it to the lower pressure port with a very short piece of LEGO pneumatic or other tubing. There s about 0.5ml of air in the tubing and sensor that is not measured by the syringe, so you need to compress the syringe to 3ml to cut the volume to exactly one-third.
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