A Lego car controlled by a homemade remote featuring a 3-axis accelerometer. Using an accelerometer(Freescale MMA7261QT), a microcontroller (Analog Devieces Microconvertor 832 and 842) and some wireless modules from Hacom (HAC-UM96) we started building a car in lego. We made a multifunctional PCB for the ADuC microcontroller which featured a MAX3221 level convertor for the serial communication, a 7805 voltage regulator and a small 3.3V regulator (Ti TPS71533) for the accelerometer.
Both the transmitter and the receiver use the same PCBs and microcontrollers. The receiver has an extra H-bridge (National Semiconductor LMD18200) to control the motor while the sender was the only one equiped with the accelerometer.
The microcontroller is a bit overpowered, the ADuC 842 which is used in the transmitter can run at speeds up to 16 MIPS while the 832, used in the car, can run at a maximum speed of 1.3 MIPS. All we actually needed was a microcontroller with a serial port and some I/O ports, not eight 12 bit ADC’s like the ADuC’s. But since we had acces to those IC’s and already knew how to program this 8051-based controller we choose to use the ADuC’s. The Freescale MMA7261QT accelerometer is an easy to use but hard to solder IC. The footprint, a QFN16, made it very hard to handle, but once soldered on, it was very easy to communicate with. The 3-axis accelerometer has 3 analog outputs which are 0V at maximal negative G-force, supply voltage at maximum positive G-force and half the supply voltage at 0G. The IC works at 3.3V and only consumes 0.5mA when working, since we didn’t need a high-current 3.3V power source, we went for the TPS71533 from Texas Instruments which is capable of delivering 50mA and is packed in a very small SOT-23 casing.
