These are the instructions for building the LEGO Bionicle Rahkshi Vorahk that was released in 2003. BI 8591 [4.43 Mb]8591 Vorahk is a BIONICLE Rahkshi set released in 2003. Stealthy and ruthless in equal measure, Vorahk use their Staffs of Hunger to suck the strength away from their enemies, and use it for themselves. It is this method of draining that makes them as fierce as they are. They are also the greatest hunters of the Rahkshi. It has 45 pieces. No power can resist Vorahk! From the exciting new direct-to-video movie BIONICLE: The Mask of Light, here come the Toa Nuva's greatest foes: the Rahkshi. These "sons of Makuta" have been unleashed on the island to find the Mask of Light and stop the coming of the Seventh Toa! Vorahk's staff can drain the energy of any foe -- and add it to the might of this Rahkshi! Not even the powerful Onua Nuva can stand against him. Will the day of the Seventh Toa end before it has even begun?This page describes my second I2C
For a general overview of I2C and for basic information on how to use I2C devices from a NXT program, see the first part of this series. This part shows how to construct a pressure sensor for the NXT, one that can measure the pressure in a Lego pneumatic system. sensor allows the NXT when to turn a pneumatic compressor on and off. For a lot of information on Lego pneumatics, see C. S. Soh'sA commercial pressure sensor for the NXT is it is an analog sensor with a pressure range similar to that of my chip that we'll use is an analog-to-digitalAn analog-to-digital converter chip allows you to connect an analog sensor (one that converts changes in some physical quantity to changes in voltage) to a digital communication channel, like the I2C channel of the NXT's input ports. Here we'll use an ADC chip to interface a pressure sensor to the NXT. The pressure sensor runs on 5.1 Volt supply, higher than the 4.3 Volts that the NXT supplies to sensor ports.
Therefore, we'll need to use the 9 Volts supply line and step it down to 5 Volts. Warning: connecting your NXT to any home-made one described here) can damage it. The ADC chip that I used the PCF8591 from Philips. The chip has four analog inputs that it can measure and one analog output (so the chip is both an ADC and a digital-to-analog converter, aThe chip runs on anything from 2.5 to 6 Volts, so it can easily run on the NXT's 4.3V supply. The chip has four analog inputs that can independently measure voltages between two reference voltages called VagndThese voltages (and the inputs) should not be exceed the supply voltages by more than 0.5v. You can also configure the chip (via I2C programming) to use some of the inputs as differential inputs, relative to one another and not relative to theWe won't use differential inputs here. mode, the chip divides the voltage range into 256 equal intervals and represents each measurement using an unsigned byte (in the differential
setting, the byte is signed, telling you which input is higher and by The chip also has one output that you can set to 256 different voltages between Vagnd and Vref. We won't use it here. To use the chip, the IC master addresses it (the four high bits are 1001 and the three lower ones are configurable via pins) and then sends a control byte. The control byte tells the chip how you want to configure the inputs (absolute, differential, or a combination), whether you want to use the analog output, and which input you want to read next. You can also tell the chip to send several input values (from different ADC channels) sequentially. the circuit to use only one input, number 2, I didn't use the analog output, and I only wanted one measurement in each ICTherefore, in every transaction I sent the chip the two-byte sequence 0x90 0x02 and received one byte back, representing the voltage at input 2. The pressure sensor that I used is the MPX4250A.
It is made by Freescale, and it is designed primarily for automotice applications. measures absolute pressure, it contains on-chip circuitry for temperature compensation, and it comes calibrated. The chip comes on four different packags: two for soldering onto a printed-circuit board (PCB) and two that plug into a socket and are inteded to be secured with some kind of screws; two with a port (a little nozzle for connecting a hose) and two without a port. applications, we need the versions with a port. for soldering into a PCB comes in a package called "small outline" which is inconvenient for prototyping (and difficult to solder by hand), so I used the bigger package type, called Unibody. In principle, this chip is easy to use. Only 3 of its pins areOne should be connected to ground, one to suply voltage around 5.1 Volts (between 4.85 and 5.35), and the third is the output pin: its voltage indicates the pressure at the port.
The output voltage and the pressure are related by the following formula: Vout = Vs* (0.004 x P-0.04), where Vs is the supply voltage and P is the pressure in kPa (kilo Pascals). 101325 Pascals are one atmosphere, and the sensor measures between 0 and 250 kPa, so it can measure pressure up to 2.5 times the normal pressure at sea level. To use this chip in an I2C sensor, we need to hook its output pin to the input pin of an analog-to-digial converter like the PCF8591, and to make sure that the reference voltage that the A/D converter uses is the supply voltage of the analog sensorThe PCF8591 has a voltage refence pin that I usedIf the A/D chip uses its supply voltage as the reference voltage, then we would need to use the same supply voltage for the sensor and for the A/D chip. The MPX4250A needs supply voltage between 4.85 andThe NXT supplies 4.3 Volts to all the ports. low for the pressure sensor. My measurements showed that the NXT
that this voltage ranges between 4.1 V and 4.7 V, so it definitely mayEven if the sensor would not completely fail when powered by this supply, it may function inaccurately; accuracy, and caliberation is only guaranteed for 4.85 to 5.35 Volts. Therefore, we need to somehow generate regulated 5.1 Volts (or 5V, which is good enough here) from the sensor port. We will do this by using pin 1 of the sensor port. called ANA (analog), which is primarily used for analog sensing by the NXT (it is used with the NXT's light, sound, and touch sensors, as well as with the RCX sensors). When the sensor port is configured for I2C, you can tell the NXT to powerTo do so, you configure the port to IN_TYPE_LOWSPEED_9V. I measured the voltage on this pin in this configuration, I got aroundWe will use a simple inear voltage regulator to step down and regulate this voltage to 5V. In the first prototype I used a standard low-power linearIt worked, but it is not a good idea.
requires 7V as input; in my example it did work with 6.75, but it mayThis regulator, the 78L05, is a pretty old design. are newer chips called low-drop linear regulator that need an input voltage that is just a bit higher than the regulated output. regulators should work more reliably here; I plan to try a regulator from ST Microelectronics, the L4949V5. Another consideration when using the ANA pin of sensor ports is the low amount of current that they can supply, 18mA. than enough for the PCF8591 and the MPX4250A (which need 1mA and 10mA max, respectively), but it means that the regulator itself must be pretty efficient (the L4949V5 consumes 5-8mA at low loads, so it is The 18mA current limit also has advantages: since the NXT limits the current to this pin, it cannot damage devices by providing too much power to them. 5.1V Zener diode can serve as a 5V regulator in this case. I didn't draw this circuit but rather built it directy from
the data sheets onto the breadboard. It uses only three chips: the MPX4250A pressure sensor, the PCF8591 to do the analog-to-digital conversion and I2C interfacing, and a 5V voltageapart from these, it only uses two 82k pullup resistors (required on the I2C lines according to the NXT documentation) and three bypass capacitors, a 0.22μF on the input side of the regulator, and two 100nF capacitors on the 4.3V and I built the circuit on a simple breadboard. connection to the NXT from my 8574 prototype: I soldered the 6 wires in the NXT cable to 6 header pins (standard 0.1" pins, like those of DIP chips), which allows me to plug the NXT cable directly onto theIt's still a little ugly, but mechanically and electrically You can see the wire that goes to the NXT (the little proto-board on the left), the two 82k pullup resistors and the PCF8591 chip (in the center). The pressure sensor is in the center, and above it you can see the voltage regulator and the