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Creator QR Code in Font TWO-WIRE POWERED SENSORS

Figure 9-21. Calibrating the pressure sensor with Boyle s Law Boyle s Law says the pressure is now three times as high as it was initially. The initial pressure was atmospheric, which is about 14.7psi (1,014hPa or 1atm), so now it is 44.1psi (3,041hPa or 3atm). The pressure transducer reads the difference between the two ports, and the other port is still seeing atmospheric pressure. So, the pressure sensor should be reading 29.4psi (2,027hPa or 2atm). This should lead to a slope of approximately 6.2 Raw values to 1psi. When you re done calibrating the sensor, you should be able to generate a plot like the one shown in Figure 9-22.

Figure 9-22. Plot of pressure versus Raw value Figure 9-23 is a NXT-G program that automatically converts the Raw values to psi and displays the value on the NXT. It will work with any generation NXT software because it scales the value before the divide.

Back in 5 we showed you how to build your own light sensors with CdS Light Dependent Resistors (LDRs). However, they lacked the capability to generate light like the LEGO sensor does. This capability is particularly useful for sensing objects by reflecting light off of them instead of just measuring the ambient light level. In this chapter we will show you how to add the same LED illumination control as the LEGO Light sensor.

Pin 5 of the NXT sensor input port, or the yellow wire in cable, controls the state of the LED in the Light Sensor. Unfortunately, the strength of the signal from the NXT is just not strong enough to directly power an LED. We need to add a simple transistor amplifier for that (Figure 10-1 shows the circuit for a CdS LDR Light Sensor that has LED control).

Figure 10-1. LED control circuit The black triangle with little arrows in the circuit is the electronic symbol for an LED. You will really get to know LEDs when we show you how to connect them to outputs in 12. Figure 10-2 shows the typical package of a light emitting diode and the corresponding symbol. The symbol is the same as the diode discussed in 9, except for the added arrows that represent the light being emitted. Instead of a painted line on the body of a convention diode that signifies the cathode end, the LED has a flat space in the flange directly next to that lead. The cathode lead is also a little shorter than the anode lead.

Figure 10-2. LED outline and circuit symbol You already learned about PNP transistors in 8, where they were used as a buffer to isolate sensors from the NXT measurement circuitry. In this circuit, Q1 is a 2N3904 NPN type transistor. The NPN symbol (see Figure 10-3) is identical to the PNP, except that the little arrow on the emitter leg points away from the base. The physical pin outs are exactly the same, but the parts are not interchangeable.

Figure 10-3. NPN transistor symbol and 2N3904 pin out The transistor circuit is generically known as a grounded emitter amplifier. Ground is just an old electronics term for the part of the circuit that is at zero volts. If the voltage on the base of Q1 becomes positive enough, the collector will become effectively connected to the emitter. It is much like closing a switch. LED and R1 are connected in series to the 4.3V power supply and the collector of Q1. When Q1 switches on, it completes the circuit to ground and the LED lights up. Q1 needs only a small amount of current from the NXT into its base to turn on, and R2 and R3 are there to limit that current. Table 10-1 has the complete bill of materials for the CdS sensor with LED control.