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Magical Sensor Box

During my placement year, I wanted to play with as much hardware as I could. So after paycheck #2 came in, I went and spent some money!

Most of the items that I bought were related to the RC car idea that I had for my final year project (which unfortunately never happened, but something much better did!). The general idea behind the box was to get some experience with designing and building a more complex system than I had previously, with a view to then using some of the features on the RC car. It has 3-axis accelerometers and gyros, a bank of 4x I2C memory chips (128kB each), a light sensor, LEDs, an XBee module and an ATMega328p in the form of an Arduino Pro Mini.

Shopping List

The finished board 
Nearly everything was ordered from SparkFun. They are a fantastic bunch, it's only a shame they are in the US, not England (import Tax...). The important bits are listed below: The main components came to about £120.

Analog I/O

All 3 axis of the gyros are connected directly to the Arduino's analog pins 0, 1 and 2. The light sensor is connected to analog pin 4.

Digital I/O

The red, green and blue are controlled by 3 of the Arduino PWM outputs on the digital lines 9, 10 and 11. 3 of the 4 white LEDs are controllable via the other PWM outputs of the Arduino 3, 5 and 6. The fourth white LED is controlled by an XBee PWM output.

I2C

@ 0x1D- 3-axis accelerometer
@ 0x20- I/O Expander
@ 0x50- EEPROM 0
@ 0x51- EEPROM 1
@ 0x52- EEPROM 2
@ 0x53- EEPROM 3

I/O Expander

As the IC can only really sink current, a LOW output turns the LEDs on. The outputs are in 3-state at power on, with a pull-up resistor taking the outputs high. Outputs should be set to high before they are used as an input. If a logic 1 is applied to an I/O pin that was written to low, a large current flows to GND - this is something to watch out for when working with the acceerometer interrupts (I managed to cook one).

Side 0 (first byte)

@ 00- Edge LED (0 is on)
@ 01- LED (0 is on)
@ 02- LED (0 is on)
@ 03- LED (0 is on)
@ 04- LED (0 is on)
@ 05- LED (0 is on)
@ 06- LED (0 is on)
@ 07- LED (0 is on)

Side 1 (second byte)

@ 10- EEPROM 0 Write Protect (1 = protect)
@ 11- EEPROM 1 Write Protect (1 = protect)
@ 12- EEPROM 2 Write Protect (1 = protect)
@ 13- EEPROM 3 Write Protect (1 = protect)
@ 14- 3-axis Accelerometer INT1 (1 = interrupt, never set to output)
@ 15- 3-axis Accelerometer INT2 (1 = interrupt, never set to output)
@ 16- Pitch / Roll Gyro power down (1 = power down)
@ 17- Yaw Gyro power down (1 = power down)

Results

I got everything running nicely, one bit of software relayed the gyro outputs to the RGB LED, another showed the accelerometer values. I managed to use the XBee to successfully wirelessly program the AVR (thanks ladyada) - I needed to set T3 to 1 on the XBee inside my toy in order to get this working. I started developing a filesystem that I was going to implement on the onboard EEPROM, but never completed it. A future project was always to get an AHRS going using the sensors, but I never got to this.

And finally, here is a picture of it with the lid on, and all the lights shining! (well... nearly)